Linux Hack - index-of.co.ukindex-of.co.uk › Hacking › Linux Hacker by Mr Ajay Kumar Tiwari.pdf · Linux Hack Ajay Kumar Tiwari Foreword Another collection of hacks? Yes! If you

LinuxHack

LinuxHackAjayKumarTiwari

TableofContentsIntroduction

Foreword

Version

Chapter1:PowerfulCDCommandHacksUseCDPATHtodefinethebasedirectoryforcdcommand

Usecdaliastonavigateupthedirectoryeffectively

Performmkdirandcdusingasinglecommand

Use“cd-”totogglebetweenthelasttwodirectories

Usedirs,pushdandpopdtomanipulatedirectorystack

Use“shopt-scdspell”toautomaticallycorrectmistyped

directorynamesoncd

Chapter2:DateManipulationSetSystemDateandTime

SetHardwareDateandTime

DisplayCurrentDateandTimeinaSpecificFormat

DisplayPastDateandTime

DisplayFutureDateandTime

Chapter3:SSHClientCommandsIdentifySSHClientVersion

LogintoRemoteHostusingSSH

DebugSSHClientSession

ToggleSSHSessionusingSSHEscapeCharacter

SSHSessionStatisticsusingSSHEscapeCharacter

Chapter4:EssentialLinuxCommandsGrepCommand

2


FindCommand

SuppressStandardOutputandErrorMessage

JoinCommand

ChangetheCase

XargsCommand

SortCommand

UniqCommand

CutCommand

StatCommand

DiffCommand

Displaytotalconnecttimeofusers

Chapter5:PS1,PS2,PS3,PS4andPROMPT_COMMANDPS1-DefaultInteractionPrompt

PS2-ContinuationInteractivePrompt

PS3-Promptusedby“select”insideshellscript

PS4-Usedby“set-x”toprefixtracingoutput

PROMPT_COMMAND

Chapter6:ColorfulandFunctionalShellPromptUsingPS1Displayusername,hostnameandbasenameofdirectoryin

theprompt

Displaycurrenttimeintheprompt

Displayoutputofanycommandintheprompt

Changeforegroundcoloroftheprompt

Changebackgroundcoloroftheprompt

Displaymultiplecolorsintheprompt

Changethepromptcolorusingtput

CreateyourownpromptusingtheavailablecodesforPS1

variable

UsebashshellfunctioninsidePS1variable

UseshellscriptinsidePS1variable

Chapter7:ArchiveandCompression3


Zipcommandbasics

Advancedcompressionusingzipcommand.

PasswordProtectionofZipfiles

Validateaziparchive

TarCommandBasics

Combinegzip,bzip2withtar

Chapter8:CommandLineHistoryDisplayTIMESTAMPinhistoryusingHISTTIMEFORMAT

SearchthehistoryusingControl+R

Repeatpreviouscommandquicklyusing4differentmethods

Executeaspecificcommandfromhistory

Executepreviouscommandthatstartswithaspecificword

Controlthetotalnumberoflinesinthehistoryusing

HISTSIZE

ChangethehistoryfilenameusingHISTFILE

Eliminatethecontinuousrepeatedentryfromhistoryusing

HISTCONTROL

Eraseduplicatesacrossthewholehistoryusing

HISTCONTROL

Forcehistorynottorememberaparticularcommandusing

HISTCONTROL

Clearalltheprevioushistoryusingoption-c

Substitutewordsfromhistorycommands

Substituteaspecificargumentforaspecificcommand

DisabletheusageofhistoryusingHISTSIZE

IgnorespecificcommandsfromthehistoryusingHISTIGNORE

Chapter9:SystemAdministrationTasksPartitionusingfdisk

Formatapartitionusingmke2fsk

Mountthepartition

4


Finetunethepartitionusingtune2fs

Createaswapfilesystem.

Createanewuser

Createanewgroupandassigntoanuser

SetupSSHpasswordlesslogininOpenSSH

Usessh-copy-idalongwithssh-agent

Crontab

SafeRebootOfLinuxUsingMagicSysRqKey

Chapter10:ApachectlandHttpdExamplesPassdifferenthttpd.conffilenametoapachectl

UseatemporaryDocumentRootwithoutmodifying

httpd.conf

IncreasetheLogLeveltemporarily

DisplaythemodulesinsideApache

Showallaccepteddirectivesinsidehttpd.conf

Validatethehttpd.confaftermakingchanges

Displaythehttpdbuildparameters

Loadaspecificmoduleonlyondemand

Chapter11:BashScriptingExecutionSequenceof.bash_*files

Howtogeneraterandomnumberinbashshell

Debugashellscript

Quoting

Readdatafilefieldsinsideashellscript

Chapter12:SystemMonitoringandPerformanceFreecommand

TopCommand

PsCommand

DfCommand

KillCommand

DuCommand

5


SarCommand

vmstatCommand

NetstatCommand

SysctlCommand

NiceCommand

ReniceCommand

6

lsofcommands.


Introduction”Thereareonly10typesofpeopleintheworld—thosewhounderstand

binary,thosewhodon’t,andthosewhounderstandgraycode”

—Geek

I’mAjayKumarTiwari,authorofthisBook

Ihavedoneintensiveprogrammingonseveral

languagesandCismyfavorite.Ihavedonelotofwork

ontheinfrastructuresideincludingLinuxsystem

administration,DBA,Networking,Hardwareand

Storage(EMC).

Copyright©2015–AjayKumarTiwari

7


ForewordAnothercollectionofhacks?Yes!Ifyouhavejustcompletedyourfirstadmin

courseorlookingforbetterwaystogetthejobdonethe“LinuxHack”

eBookisagoodpointtostart.Theseusefultipsareconcise,wellwrittenand

easytoread.

Welldone-IwillrecommendthiseBooktomystudents.

—Prof.Dr.FritzMehner,FHSüdwestfalen,Germany

(AuthorofseveralVimplugins,includingbash-supportvimplugin)

VersionVersion

1.0

Date

25-Mar-2015

Revisions

FirstEdition

8


Chapter1:PowerfulCDCommandHackscdisoneofthemostfrequentlyusedcommandsduringaUNIXsession.The6

cdcommandhacksmentionedinthischapterwillboostyourproductivity

instantlyandmakeiteasiertonavigatethedirectorystructurefrom

commandline.

Hack1.UseCDPATHtodefinethebasedirectoryforcdcommandIfyouarefrequentlyperformingcdtosubdirectoriesofaspecificparent

directory,youcansettheCDPATHtotheparentdirectoryandperformcdto

thesubdirectorieswithoutgivingtheparentdirectorypathasexplained

below.

[ramesh@dev-db~]# pwd

/home/ramesh

[ramesh@dev-db~]# cdmail

-bash:cd:mail:Nosuchfileordirectory

[Note:Thisislookingformaildirectoryundercurrent

directory]

[ramesh@dev-db~]# exportCDPATH=/etc

[ramesh@dev-db~]# cdmail

/etc/mail

[Note:Thisislookingformailunder/etcandnot

undercurrentdirectory]

[ramesh@dev-db/etc/mail]# pwd

/etc/mail

Tomakethischangepermanent,addexportCDPATH=/etctoyour

~/.bash_profile

9


SimilartothePATHvariable,youcanaddmorethanonedirectoryentryin

theCDPATHvariable,separatingthemwith:,asshownbelow.

exportCDPATH=.:~:/etc:/var

Thishackcanbeveryhelpfulunderthefollowingsituations:

o

OracleDBAsfrequentlyworkingunder$ORACLE_HOME,canset

theCDPATHvariabletotheoraclehome

Unixsysadminsfrequentlyworkingunder/etc,cansetthe

CDPATHvariableto/etc

Developersfrequentlyworkingunderprojectdirectory

/home/projects,cansettheCDPATHvariableto/home/projects

End-usersfrequentlyaccessingthesubdirectoriesundertheir

homedirectory,cansettheCDPATHvariableto~(home

directory)

o

o

o

Hack2.UsecdaliastonavigateupthedirectoryeffectivelyWhenyouarenavigatingupaverylongdirectorystructure,youmaybeusing

cd....\withmultiple..\’sdependingonhowmanydirectoriesyouwanttogo

upasshownbelow.#mkdir-p

/tmp/very/long/directory/structure/that/is/too/deep

#cd/tmp/very/long/directory/structure/that/is/too/deep

#pwd


#cd../../../../

10

LinuxHack#pwd

/tmp/very/long/directory/structure

AjayKumarTiwari

Insteadofexecutingcd../../../..tonavigatefourlevelsup,useoneofthe

followingthreealiasmethods:

Method1:Navigateupthedirectoryusing“..n”Intheexamplebelow,..4isusedtogoup4directorylevel,..3togoup3

directorylevel,..2togoup2directorylevel.Addthefollowingaliastoyour

~/.bash_profileandre-login.

alias

alias

alias

alias

alias

..=“cd..”

..2=“cd../..”

..3=“cd../../..”

..4=“cd../../../..”

..5=“cd../../../../..”

#cd


#..4

[Note:use..4togoup4directorylevel]

#pwd

/tmp/very/long/directory/structure/

Method2:NavigateupthedirectoryusingonlydotsIntheexamplebelow,…..(fivedots)isusedtogoup4directorylevel.

Typing5dotstogoup4directorystructureisreallyeasytoremember,as

whenyoutypethefirsttwodots,youarethinking“goinguponedirectory”,

afterthateveryadditionaldot,istogoonelevelup.So,use….(fourdots)to

goup3directoryleveland..(twodots)togoup1directorylevel.Addthe

followingaliastoyour~/.bash_profileandre-loginforthe…..(fivedots)to

workproperly.

alias..=“cd..”

11

LinuxHackalias

alias

alias

alias

…=“cd../..”

….=“cd../../..”

…..=“cd../../../..”

……=“cd../../../../..”

AjayKumarTiwari#cd/tmp/very/long/directory/structure/that/is/too/deep

#…..

[Note:use…..(fivedots)togoup4directorylevel]

#pwd

/tmp/very/long/directory/structure/

Method3:Navigateupthedirectoryusingcdfollowedby

consecutivedotsIntheexamplebelow,cd…..(cdfollowedbyfivedots)isusedtogoup4

directorylevel.Makingit5dotstogoup4directorystructureisreallyeasyto

remember,aswhenyoutypethefirsttwodots,youarethinking“goingup

onedirectory”,afterthateveryadditionaldot,istogoonelevelup.So,use

cd….(cdfollowedbyfourdots)togoup3directorylevelandcd…(cd

followedbythreedots)togoup2directorylevel.Addthefollowingaliasto

your~/.bash_profileandre-loginfortheabovecd…..(fivedots)towork

properly.

alias

alias

alias

alias

alias

cd..=“cd..”

cd…=“cd../..”

cd….=“cd../../..”

cd…..=“cd../../../..”

cd……=“cd../../../../..”#cd/tmp/very/long/directory/structure/that/is/too/deep

#cd…..

[Note:usecd…..togoup4directorylevel]

#pwd

/tmp/very/long/directory/structure

12


Method5:NavigateupthedirectoryusingcdfollowedbynumberIntheexamplebelow,cd4(cdfollowedbynumber4)isusedtogoup4

directorylevel.

alias

alias

alias

alias

alias

cd1=“cd

cd2=“cd

cd3=“cd

cd4=“cd

cd5=“cd

..”

../..”

../../..”

../../../..”

../../../../..”

Hack3.PerformmkdirandcdusingasinglecommandSometimeswhenyoucreateanewdirectory,youmaycdtothenewdirectory

immediatelytoperformsomeworkasshownbelow.

#mkdir-p/tmp/subdir1/subdir2/subdir3

#cd/tmp/subdir1/subdir2/subdir3

#pwd

/tmp/subdir1/subdir2/subdir3

Wouldn’titbenicetocombinebothmkdirandcdinasinglecommand?Add

thefollowingtothe.bash_profileandre-login.

$vi.bash_profile

functionmkdircd(){mkdir-p“$@”&&evalcd

“\”\$$#\””;}

Now,performbothmkdirandcdatthesametimeusingasinglecommandas

shownbelow:

13

LinuxHack#mkdircd/tmp/subdir1/subdir2/subdir3

AjayKumarTiwari

[Note:Thiscreatesthedirectoryandcdtoit

automatically]

#pwd


Hack4.Use“cd-”totogglebetweenthelasttwodirectoriesYoucantogglebetweenthelasttwocurrentdirectoriesusingcd-asshown

below.#cd/tmp/very/long/directory/structure/that/is/too/deep

#cd/tmp/subdir1/subdir2/subdir3

#cd-

#pwd


# cd-

#pwd


# cd-

#pwd


Hack5.Usedirs,pushdandpopdtomanipulatedirectorystackYoucanusedirectorystacktopushdirectoriesintoitandlaterpopdirectory

fromthestack.Followingthreecommandsareusedinthisexample.

14

LinuxHacko

o

o

dirs:Displaythedirectorystack

pushd:Pushdirectoryintothestack

AjayKumarTiwari

popd:Popdirectoryfromthestackandcdtoit

Dirswillalwaysprintthecurrentdirectoryfollowedbythecontentofthe

stack.Evenwhenthedirectorystackisempty,dirscommandwillstillprint

onlythecurrentdirectoryasshownbelow.

#popd

-bash:popd:directorystackempty

#dirs

~

#pwd

/home/ramesh

Howtousepushdandpopd?Letusfirstcreatesometemporarydirectories

andpushthemtothedirectorystackasshownbelow.

#

#

#

#

mkdir

mkdir

mkdir

mkdir

/tmp/dir1

/tmp/dir2

/tmp/dir3

/tmp/dir4

#cd/tmp/dir1

#pushd.

#cd/tmp/dir2

#pushd.

#cd/tmp/dir3

#pushd.

#cd/tmp/dir4

#pushd.

15


#dirs

/tmp/dir4/tmp/dir4/tmp/dir3/tmp/dir2/tmp/dir1

[Note:Thefirstdirectory(/tmp/dir4)ofthedir

commandoutputisalwaysthecurrentdirectoryandnot

thecontentfromthestack.]

Atthisstage,thedirectorystackcontainsthefollowingdirectories:

/tmp/dir4

/tmp/dir3

/tmp/dir2

/tmp/dir1

Thelastdirectorythatwaspushedtothestackwillbeatthetop.Whenyou

performpopd,itwillcdtothetopdirectoryentryinthestackandremoveit

fromthestack.Asshownabove,thelastdirectorythatwaspushedintothe

stackis/tmp/dir4.So,whenwedoapopd,itwillcdtothe/tmp/dir4and

removeitfromthedirectorystackasshownbelow.

#popd

#pwd

/tmp/dir4

[Note:Aftertheabovepopd,directoryStackContains:

/tmp/dir3

/tmp/dir2

/tmp/dir1]

#popd

#pwd

/tmp/dir3


/tmp/dir2

/tmp/dir1]

#popd

16

LinuxHack#pwd

/tmp/dir2

AjayKumarTiwari


/tmp/dir1]

#popd

#pwd

/tmp/dir1

[Note:Aftertheabovepopd,directoryStackisempty!]

#popd

-bash:popd:directorystackempty

Hack6.Use“shopt-scdspell”toautomaticallycorrectmistypeddirectorynamesoncdUseshopt-scdspelltocorrectthetyposinthecdcommandautomaticallyas

shownbelow.Ifyouarenotgoodattypingandmakelotofmistakes,thiswill

beveryhelpful.

#cd/etc/mall

-bash:cd:/etc/mall:Nosuchfileordirectory

# shopt-scdspell

#cd/etc/mall

#pwd

/etc/mail

[Note:Bymistake,whenItypedmallinsteadofmail,

cdcorrecteditautomatically]

17


Chapter2:DateManipulationHack7.SetSystemDateandTimeTochangethesystemdateuse:

#date{mmddhhmiyyyy.ss}

o

o

o

o

o

o

mm–Month

dd–Date

hh–24hourformat

mi–Minutes

yyyy–Year

ss–seconds

Forexample,tosetsystemdatetoJan31st2008,10:19p.m,53seconds

#date013122192009.53

Youcanalsochangesystemdateusingsetargumentasshownbelow.

#date013122192009.53

#date+%Y%m%d-s“20090131”

#date-s“01/31/200922:19:53”

#date-s“31JAN200922:19:53”

#dateset=“31JAN200922:19:53”

Tosetthetimeonly:

#date+%T-s“22:19:53”

18

LinuxHack#date+%T%p-s“10:19:53PM”

AjayKumarTiwari

Hack8.SetHardwareDateandTimeBeforesettingthehardwaredateandtime,makesuretheOSdateandtime

issetappropriatelyasshowninthehack#7.

Setthehardwaredateandtimebasedonthesystemdateasshownbelow:

#hwclock–systohc

#hwclock—systohc–utc

Usehwclockwithoutanyparameter,toviewthecurrenthardwaredateand

time:

#hwclock

ChecktheclockfiletoverifywhetherthesystemissetforUTC:

#cat/etc/sysconfig/clock

ZONE=“America/Los_Angeles”

UTC=false

ARC=false

Hack9.DisplayCurrentDateandTimeinaSpecificFormatFollowingaredifferentwaysofdisplayingthecurrentdateandtimein

variousformats:

19

LinuxHack$date

ThuJan

AjayKumarTiwari

108:19:23PST2009

$date—date=“now”

ThuJan108:20:05PST2009

$date—date=“today”


$date—date=‘1970-01-0100:00:01UTC+5hours’+%s

18001

$date‘+CurrentDate:%m/%d/%y%nCurrentTime:%H:%M:%S’

CurrentDate:01/01/09

CurrentTime:08:21:41

$date+”%d-%m-%Y”

01-01-2009

$date+”%d/%m/%Y”

01/01/2009

$date+”%A,%B%d%Y”

Thursday,January012009

Followingarethedifferentformatoptionsyoucanpasstothedate

command:

o

o

o

o

o

o

%Ddate(mm/dd/yy)

%ddayofmonth(01..31)

%mmonth(01..12)

%ylasttwodigitsofyear(00..99)

%alocale’sabbreviatedweekdayname(Sun..Sat)

%Alocale’sfullweekdayname,variablelength

(Sunday..Saturday)

%blocale’sabbreviatedmonthname(Jan..Dec)

o

20

LinuxHacko

AjayKumarTiwari

%Blocale’sfullmonthname,variablelength

(January..December)

%Hhour(00..23)

%Ihour(01..12)

%Yyear(1970…)

o

o

o

Hack10.DisplayPastDateandTimeFollowingarevariouswaystodisplayapastdateandtime:

$date—date=‘3secondsago’


$date—date=“1dayago”

WedDec3108:27:13PST2008

$date—date=“1daysago”

WedDec3108:27:18PST2008

$date—date=“1monthago”

MonDec108:27:23PST2008

$date—date=“1yearago”

TueJan108:27:28PST2008

$date—date=“yesterday”

WedDec3108:27:34PST2008

$date—date=“10months2dayago”

ThuFeb2808:27:41PST2008

21


Hack11.DisplayFutureDateandTimeFollowingexamplesshowshowtodisplayafuturedateandtime.

$date

ThuJan

108:30:07PST2009

$date—date=‘3seconds’


$date—date=‘4hours’


$date—date=‘tomorrow’

FriJan208:30:25PST2009

$date—date=“1day”


$date—date=“1days”


$date—date=“2days”

SatJan308:30:43PST2009

$date—date=‘1month’

SunFeb108:30:48PST2009

$date—date=‘1week’


$date—date=“2months”

SunMar108:30:58PST2009

$date—date=“2years”

SatJan108:31:03PST2011

$date—date=“nextday”


22

LinuxHack$date—date=”-1daysago”


$date—date=“thisWednesday”

WedJan700:00:00PST2009

AjayKumarTiwari

23


Chapter3:SSHClientCommandsHack12.IdentifySSHClientVersionSometimesitmaybenecessarytoidentifytheSSHclientthatyouare

currentlyrunningandit’scorrespondingversionnumber.Usessh–Vto

identifytheversionnumber.PleasenotethatLinuxcomeswithOpenSSH.

ThefollowingexampleindicatesthatthisparticularsystemisusingOpenSSH:

$ssh-V

OpenSSH_3.9p1,OpenSSL0.9.7aFeb192003

ThefollowingexampleindicatesthatthisparticularsystemisusingSSH2:

$ssh-V

ssh:SSHSecureShell3.2.9.1(non-commercialversion)

oni686-pc-linux-gnu

Hack13.LogintoRemoteHostusingSSHTheFirsttimewhenyoulogintoaremotehostfromalocalhost,itwilldisplay

thehostkeynotfoundmessageandyoucangive“yes”tocontinue.Thehost

keyoftheremotehostwillbeaddedunder.ssh2/hostkeysdirectoryofyour

homedirectory,asshownbelow.

localhost$ ssh-ljsmithremotehost.example.comHostkeynotfoundfromdatabase.

Keyfingerprint:

xabie-dezbc-manud-bartd-satsy-limit-nexiu-jambl-title-jarde-

tuxum

Youcangetapublickey’sfingerprintbyrunning

%ssh-keygen-Fpublickey.pub

24

LinuxHackAjayKumarTiwarionthekeyfile.

Areyousureyouwanttocontinueconnecting(yes/no)? Yes

Hostkeysavedto

/home/jsmith/.ssh2/hostkeys/key_22_remotehost.example.com.pub

hostkeyforremotehost.example.com,acceptedbyjsmithMon

May26200816:06:50-0700

[email protected]:

remotehost.example.com$

TheSecondtimewhenyoulogintotheremotehostfromthelocalhost,itwill

promptonlyforthepasswordastheremotehostkeyisalreadyaddedtothe

knownhostslistofthesshclient.

localhost$ ssh-ljsmithremotehost.example.com

[email protected]:

remotehost.example.com$

Forsomereason,ifthehostkeyoftheremotehostischangedafteryou

loggedinforthefirsttime,youmaygetawarningmessageasshownbelow.

Thiscouldbebecauseofvariousreasonssuchas:

o

o

Sysadminupgraded/reinstalledtheSSHserverontheremotehost

Someoneisdoingmaliciousactivityetc.,

Thebestpossibleactiontotakebeforesaying“yes”tothemessagebelow,is

tocallyoursysadminandidentifywhyyougotthehostkeychangedmessage

andverifywhetheritisthecorrecthostkeyornot.

localhost$ ssh-ljsmithremotehost.example.com@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@

@WARNING:HOSTIDENTIFICATIONHASCHANGED!@

@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@

ITISPOSSIBLETHATSOMEONEISDOINGSOMETHINGNASTY!

Someonecouldbeeavesdroppingonyourightnow(man-in-the-

25

LinuxHackAjayKumarTiwarimiddleattack)!

Itisalsopossiblethatthehostkeyhasjustbeenchanged.

Pleasecontactyoursystemadministrator.

Addcorrecthostkeyto

“/home/jsmith/.ssh2/hostkeys/key_22_remotehost.example.com.pu

b”

togetridofthismessage.

Receivedserverkey’sfingerprint:

xabie-dezbc-manud-bartd-satsy-limit-nexiu-jambl-title-arde-

tuxum

Youcangetapublickey’sfingerprintbyrunning

%ssh-keygen-Fpublickey.pub

onthekeyfile.

Agentforwardingisdisabledtoavoidattacksbycorrupted

servers.

Areyousureyouwanttocontinueconnecting(yes/no)? yes

Doyouwanttochangethehostkeyondisk(yes/no)? yes

Agentforwardingre-enabled.

Hostkeysavedto

/home/jsmith/.ssh2/hostkeys/key_22_remotehost.example.com.pub

hostkeyforremotehost.example.com,acceptedbyjsmithMon

May26200816:17:31-0700

[email protected]’spassword:

remotehost$

Hack14.DebugSSHClientSessionSometimesitisnecessarytoviewdebugmessagestotroubleshootanySSH

connectionissues.pass-v(lowercasev)optiontothesshasshownbelowto

viewthesshdebugmessages.

ExamplewithoutSSHclientdebugmessage:localhost$ ssh-ljsmithremotehost.example.com

warning:Connectingtoremotehost.example.comfailed:

Noaddressassociatedtothename

26


ExamplewithSSHclientdebugmessage:locaclhost$ ssh-v-ljsmithremotehost.example.com

debug:

SshConfig/sshconfig.c:2838/ssh2_parse_config_ext:

Metaconfigparsingstoppedatline3.

debug:

SshConfig/sshconfig.c:637/ssh_config_set_param_verbose:

Settingvariable‘VerboseMode’to‘FALSE’.

debug:

SshConfig/sshconfig.c:3130/ssh_config_read_file_ext:

Read17paramsfromconfigfile.

debug:Ssh2/ssh2.c:1707/main:Userconfigfilenot

found,usingdefaults.(Lookedfor

‘/home/jsmith/.ssh2/ssh2_config’)

debug:Connectingtoremotehost.example.com,port22…

(SOCKSnotused)

warning:Connectingtoremotehost.example.comfailed:

Noaddressassociatedtothename

Hack15.ToggleSSHSessionusingSSHEscapeCharacterWhenyou’veloggedontotheremotehostusingsshfromthelocalhost,you

maywanttocomebacktothelocalhosttoperformsomeactivityandgoback

toremotehostagain.Inthiscase,youdon’tneedtodisconnectthessh

sessiontotheremotehost.Instead,followthestepsbelow.

1.Logintoremotehostfromlocalhost:

localhost$ssh-ljsmithremotehost

27


2.Nowyouareconnectedtotheremotehost:

remotehost$

3.Tocomebacktothelocalhosttemporarily,typetheescapecharacter~

andControl-Z.

Whenyoutype~youwillnotseethatimmediatelyonthescreenuntilyou

press<Control-Z>andpressenter.So,ontheremotehostinanewlineenter

thefollowingkeystrokesforthebelowtowork:~<Control-Z>

remotehost$ ~^Z

[1]+Stoppedssh-ljsmithremotehost

localhost$

4.Nowyouarebacktothelocalhostandthesshremotehostclientsession

runsasatypicalUNIXbackgroundjob,whichyoucancheckasshownbelow:

localhost$ jobs

[1]+Stoppedssh-ljsmithremotehost

5.Youcangobacktotheremotehostsshwithoutenteringthepassword

againbybringingthebackgroundsshremotehostsessionjobtoforegroundon

thelocalhost.

localhost$ fg%1

ssh-ljsmithremotehost

remotehost$

28


Hack16.SSHSessionStatisticsusingSSHEscapeCharacterTogetsomeusefulstatisticsaboutthecurrentsshsession,dothefollowing.

ThisworksonlyonSSH2client.

1.Logintoremotehostfromlocalhost.

localhost$ssh-ljsmithremotehost

2.Ontheremotehost,typesshescapecharacter~followedbysasshown

below.ThiswilldisplaylotofusefulstatisticsaboutthecurrentSSH

connection.

remotehost$[Note:The~sisnotvisibleonthe

commandlinewhenyoutype.]

remotehost:remotehost

localhost:localhost

remoteversion:SSH-1.99-OpenSSH_3.9p1

localversion:SSH-2.0-3.2.9.1SSHSecure

Shell(non-commercial)

compressedbytesin:1506

uncompressedbytesin:1622

compressedbytesout:4997

uncompressedbytesout:5118

packetsin:15

packetsout:24

rekeys:0

Algorithms:

Chosenkeyexchangealgorithm:diffie-hellman-

group1-sha1

Chosenhostkeyalgorithm:ssh-dss

Commonhostkeyalgorithms:ssh-dss,ssh-rsa

Algorithmsclienttoserver:

Cipher:aes128-cbc

MAC:hmac-sha1

Compression:zlib

29

LinuxHackAlgorithmsservertoclient:

Cipher:aes128-cbc

MAC:hmac-sha1

Compression:zlib

localhost$

AjayKumarTiwari

AdditionalSSHInfoOnasidenote,tosetupSSHkeybasedauthentication,referopenSSHand

SSH2tutorials.

30


Chapter4:EssentialLinuxCommandsHack17.GrepCommandgrepcommandisusedtosearchfilesforaspecifictext.Thisisincredibly

powerfulcommandwithlotofoptions.

Syntax:grep[options]pattern[files]

HowcanIfindalllinesmatchingaspecifickeywordonafile?Inthisexample,greplooksforthetextJohninside/etc/passwdfileand

displaysallthematchinglines.

#grepJohn/etc/passwd

jsmith:x:1082:1082:JohnSmith:/home/jsmith:/bin/bash

jdoe:x:1083:1083:JohnDoe:/home/jdoe:/bin/bash

Option-v,willdisplayallthelinesexceptthematch.Intheexamplebelow,

itdisplaysalltherecordsfrom/etc/passwordthatdoesn’tmatchJohn.

Note: Thereareseverallinesinthe/etc/passwordthatdoesn’tcontainthe

wordJohn.Onlythefirstlineoftheoutputisshownbelow.

#grep-vJohn/etc/passwdjbourne:x:1084:1084:JasonBourne:/home/jbourne:/bin/bash

Howmanylinesmatchedthetextpatterninaparticularfile?Intheexamplebelow,itdisplaysthetotalnumberoflinesthatcontainsthe

textJohnin/etc/passwdfile.

31


#grep-cJohn/etc/passwd

2

Youcanalsogetthetotalnumberoflinesthatdidnotmatchthespecific

patternbypassingoption-cv.

#grep-cvJohn/etc/passwd

39

Howtosearchatextbyignoringthecase?Passtheoption-i(ignorecase),whichwillignorethecasewhilesearching.

#grep-ijohn/etc/passwd

jsmith:x:1082:1082:JohnSmith:/home/jsmith:/bin/bash

jdoe:x:1083:1083:JohnDoe:/home/jdoe:/bin/bash

HowdoIsearchallsubdirectoriesforatextmatchingaspecific

pattern?Useoption-r(recursive)forthispurpose.Intheexamplebelow,itwillsearch

forthetext“John”byignoringthecaseinsideallthesubdirectoriesunder

/home/users.

Thiswilldisplaytheoutputintheformatof“filename:linethatmatchingthe

pattern”.Youcanalsopasstheoption-l,whichwilldisplayonlythenameof

thefilethatmatchesthepattern.

#grep-rijohn/home/users

/home/users/subdir1/letter.txt:John,Thanksforyour

contribution.

/home/users/name_list.txt:JohnSmith

32

LinuxHack/home/users/name_list.txt:JohnDoe

#grep-riljohn/root

/home/users/subdir1/letter.txt

/home/users/name_list.txt

AjayKumarTiwari

Hack18.FindCommandfindisfrequentlyusedcommandtofindfilesintheUNIXfilesystembasedon

numerousconditions.Letusreviewsomepracticeexamplesoffindcommand.

Syntax:find[pathnames][conditions]

Howtofindfilescontainingaspecificwordinitsname?Thefollowingcommandlooksforallthefilesunder/etcdirectorywithmail

inthefilename.

#find/etc-name“*mail*”

Howtofindallthefilesgreaterthancertainsize?Thefollowingcommandwilllistallthefilesinthesystemgreaterthan

100MB.

#find/-typef-size+100M

Howtofindfilesthatarenotmodifiedinthelastxnumberofdays?Thefollowingcommandwilllistallthefilesthatweremodifiedmorethan60

daysagounderthecurrentdirectory.

33

LinuxHack#find.-mtime+60

AjayKumarTiwari

Howtofindfilesthataremodifiedinthelastxnumberofdays?Thefollowingcommandwilllistallthefilesthatweremodifiedinthelast

twodaysunderthecurrentdirectory.

#find.–mtime-2

Howtodeleteallthearchivefileswithextension*.tar.gzand

greaterthan100MB?Pleasebecarefulwhileexecutingthefollowingcommandasyoudon’twant

todeletethefilesbymistake.Thebestpracticeistoexecutethesame

commandwithls–ltomakesureyouknowwhichfileswillgetdeletedwhen

youexecutethecommandwithrm.#find/-typef-name*.tar.gz-size+100M-execls-l{}\;

#find/-typef-name*.tar.gz-size+100M-execrm-f{}\;

Howtoarchiveallthefilesthatarenotmodifiedinthelastx

numberofdays?Thefollowingcommandfindsallthefilesnotmodifiedinthelast60days

under/home/jsmithdirectoryandcreatesanarchivefilesunder/tmpinthe

formatofddmmyyyy_archive.tar.

#find/home/jsmith-typef-mtime+60|xargstar-cvf

/tmp/`date‘+%d%m%Y’_archive.tar`

Onasidenote,youcanperformlotoffilerelatedactivities(includingfinding

files)usingmidnightcommanderGUI,apowerfultextbasedfilemanagerfor

Unix.

34


Hack19.SuppressStandardOutputandErrorMessageSometimewhiledebuggingashellscript,youmaynotwanttoseeeitherthe

standardoutputorstandarderrormessage.Use/dev/nullasshownbelowfor

suppressingtheoutput.

Suppressstandardoutputusing>/dev/nullThiswillbeveryhelpfulwhenyouaredebuggingshellscripts,whereyou

don’twanttodisplaytheechostatementandinterestedinonlylookingat

theerrormessages.

#catfile.txt>/dev/null

#./shell-script.sh>/dev/null

Suppressstandarderrorusing2>/dev/nullThisisalsohelpfulwhenyouareinterestedinviewingonlythestandard

outputanddon’twanttoviewtheerrormessages.

#catinvalid-file-name.txt2>/dev/null

#./shell-script.sh2>/dev/null

Hack20.JoinCommandJoincommandcombineslinesfromtwofilesbasedonacommonfield.

Intheexamplebelow,wehavetwofiles–employee.txtandsalary.txt.Both

haveemployee-idascommonfield.So,wecanusejoincommandtocombine

35


thedatafromthesetwofilesusingemployee-idasshownbelow.

$catemployee.txt

100

200

300

400

JasonSmith

JohnDoe

SanjayGupta

AshokSharma

$catbonus.txt

100

200

300

400

$5,000

$500

$3,000

$1,250

$ joinemployee.txtbonus.txt

100

200

300

400

JasonSmith$5,000

JohnDoe$500

SanjayGupta$3,000

AshokSharma$1,250

Hack21.ChangetheCase

Convertafiletoallupper-case$catemployee.txt

100

200

300

400

JasonSmith

JohnDoe

SanjayGupta

AshokSharma

$ tra-zA-Z<employee.txt

100JASONSMITH

200JOHNDOE

300SANJAYGUPTA

36

LinuxHack400ASHOKSHARMA

AjayKumarTiwari

Convertafiletoalllower-case$catdepartment.txt

100

200

300

400

FINANCE

MARKETING

PRODUCTDEVELOPMENT

SALES

$ trA-Za-z<department.txt

100

200

300

400

finance

marketing

productdevelopment

sales

Hack22.XargsCommandxargsisaverypowerfulcommandthattakesoutputofacommandandpassit

asargumentofanothercommand.Followingaresomepracticalexampleson

howtousexargseffectively.

1.Whenyouaretryingtodeletetoomanyfilesusingrm,youmaygeterror

message:/bin/rmArgumentlisttoolong–Linux.Usexargstoavoidthis

problem.

find~-name‘*.log’-print0|xargs-0rm-f

2.Getalistofallthe*.conffileunder/etc/.Therearedifferentwaystoget

thesameresult.Followingexampleisonlytodemonstratetheuseofxargs.

Theoutputofthefindcommandinthisexampleispassedtothels–loneby

oneusingxargs.

37


#find/etc-name“*.conf”|xargsls–l

3.IfyouhaveafilewithlistofURLsthatyouwouldliketodownload,youcan

usexargsasshownbelow.

#caturl-list.txt|xargswget–c

4.Findoutallthejpgimagesandarchiveit.

#find/-name*.jpg-typef-print|xargstar-cvzf

images.tar.gz

5.Copyalltheimagestoanexternalhard-drive.

#ls*.jpg|xargs-n1-icp{}/external-hard-

drive/directory

Hack23.SortCommandSortcommandsortsthelinesofatextfile.Followingareseveralpractical

examplesonhowtousethesortcommandbasedonthefollowingsampletext

filethathasemployeeinformationintheformat:

employee_name:employee_id:department_name.

$ catnames.txt

EmmaThomas:100:Marketing

AlexJason:200:Sales

MadisonRandy:300:ProductDevelopment

SanjayGupta:400:Support

NishaSingh:500:Sales

38


Sortatextfileinascendingorder$ sortnames.txt

AlexJason:200:Sales





Sortatextfileindescendingorder$ sort-rnames.txt





AlexJason:200:Sales

Sortacolondelimitedtextfileon2ndfield(employee_id)$ sort-t:-k2names.txt


AlexJason:200:Sales




Sortatabdelimitedtextfileon3rdfield(department_name)and

suppressduplicates$ sort-t:-u-k3names.txt

39

LinuxHackEmmaThomas:100:Marketing


AlexJason:200:Sales


AjayKumarTiwari

Sortthepasswdfilebythe3rdfield(numericuserid)$ sort-t:-k3n/etc/passwd|more

root:x:0:0:root:/root:/bin/bash

bin:x:1:1:bin:/bin:/sbin/nologin

daemon:x:2:2:daemon:/sbin:/sbin/nologin

adm:x:3:4:adm:/var/adm:/sbin/nologin

lp:x:4:7:lp:/var/spool/lpd:/sbin/nologin

Sort/etc/hostsfilebyip-addres$ sort-t.-k1,1n-k2,2n-k3,3n-k4,4n/etc/hosts

127.0.0.1localhost.localdomainlocalhost

192.168.100.101dev-db.thegeekstuff.comdev-db

192.168.100.102prod-db.thegeekstuff.comprod-db

192.168.101.20dev-web.thegeekstuff.comdev-web

192.168.101.21prod-web.thegeekstuff.comprod-web

Combinesortwithothercommandso

o

ps–ef|sort :Sorttheoutputofprocesslist

ls-al|sort+4n :Listthefilesintheascendingorderofthefile-

size.i.esortedby5thfiledanddisplayingsmallestfilesfirst.

ls-al|sort+4nr :Listthefilesinthedescendingorderofthe

file-size.i.esortedby5thfiledanddisplayinglargestfilesfirst.

o

40


Hack24.UniqCommandUniqcommandismostlyusedincombinationwithsortcommand,asuniq

removesduplicatesonlyfromasortedfile.i.eInorderforuniqtowork,all

theduplicateentriesshouldbeintheadjacentlines.Followingaresome

commonexamples.

1.Whenyouhaveanemployeefilewithduplicateentries,youcandothe

followingtoremoveduplicates.

$sortnamesd.txt|uniq

$sort–unamesd.txt

2.Ifyouwanttoknowhowmanylinesareduplicates,dothefollowing.The

firstfieldinthefollowingexamplesindicateshowmanyduplicateswhere

foundforthatparticularline.So,inthisexamplethelinesbeginningwith

AlexandEmmawerefoundtwiceinthenamesd.txtfile.

$ sortnamesd.txt|uniq–c

2

2

1

1

1

AlexJason:200:Sales





3.Thefollowingdisplaysonlytheentriesthatareduplicates.

$ sortnamesd.txt|uniq–cd

2AlexJason:200:Sales

2EmmaThomas:100:Marketing

41


Hack25.CutCommandCutcommandcanbeusedtodisplayonlyspecificcolumnsfromatextfileor

othercommandoutputs.

Followingaresomeoftheexamples.

Displaythe1stfield(employeename)fromacolondelimitedfile$ cut-d:-f1names.txt

EmmaThomas

AlexJason

MadisonRandy

SanjayGupta

NishaSingh

Display1stand3rdfieldfromacolondelimitedfile$ cut-d:-f1,3names.txt

EmmaThomas:Marketing

AlexJason:Sales

MadisonRandy:ProductDevelopment

SanjayGupta:Support

NishaSingh:Sales

Displayonlythefirst8charactersofeverylineinafile$ cut-c1-8names.txt

EmmaTho

AlexJas

Madison

SanjayG

NishaSi

42


MiscCutcommandexampleso

cut-d:-f1/etc/passwd Displaystheunixloginnamesforalltheusers

inthesystem.

free|tr-s‘‘|sed‘/^Mem/!d’|cut-d””-f2 Displaysthetotal

memoryavailableonthesystem.

o

Hack26.StatCommandStatcommandcanbeusedeithertocheckthestatus/propertiesofasingle

fileorthefilesystem.

Displaystatisticsofafileordirectory.$ stat/etc/my.cnf

File:

Size:

Device:

Access:

(0/

Access:

Modify:

Change:

`/etc/my.cnf’

346Blocks:16IOBlock:4096regularfile

801h/2049dInode:279856Links:1

(0644/-rw-r—r—)Uid:(0/root)Gid:

root)

2009-01-0102:58:30.000000000-0800

2006-06-0120:42:27.000000000-0700

2007-02-0214:17:27.000000000-0800

$ stat/home/ramesh

File:`/home/ramesh’

Size:4096Blocks:8IOBlock:

4096directory

Device:803h/2051dInode:5521409Links:7

Access:(0755/drwxr-xr-x)Uid:(401/ramesh)Gid:(

401/ramesh)

Access:2009-01-0112:17:42.000000000-0800

Modify:2009-01-0112:07:33.000000000-0800

Change:2009-01-0912:07:33.000000000-0800

43


Displaythestatusofthefilesystemusingoption–f$ stat-f/

File:

ID:

Blocks:

1876998

Inodes:

“/”

0Namelen:255Type:ext2/ext3

Total:2579457Free:2008027Available:

Size:4096

Total:1310720Free:1215892

Hack27.DiffCommanddiffcommandcomparestwodifferentfilesandreportsthedifference.The

outputisverycrypticandnotstraightforwardtoread.

Syntax:diff[options]file1file2

Whatwasmodifiedinmynewfilewhencomparetomyoldfile?Theoption-winthediffcommandwillignorethewhitespacewhile

performingthecomparison.

Inthefollowingdiffoutput:

o

Thelinesabove–,indicatesthechangeshappenedinfirstfileinthe

diffcommand(i.ename_list.txt).

Thelinesbelow–,indicatesthechangeshappenedtothesecond

fileinthediffcommand(i.ename_list_new.txt).Thelinesthat

belongtothefirstfilestartswith<andthelinesofsecondfilestarts

with>.

o

# diff-wname_list.txtname_list_new.txt

44

LinuxHack2c2,3

<JohnDoe

–

>JohnMDoe

>JasonBourne

AjayKumarTiwari

Hack28.DisplaytotalconnecttimeofusersAccommandwilldisplaythestatisticsabouttheuser’sconnecttime.

ConnecttimeforthecurrentloggedinuserWiththeoption–d,itwillbreakdowntheoutputfortheindividualdays.In

thisexample,I’vebeenloggedintothesystemformorethan6hourstoday.

OnDec1st,Iwasloggedinforabout1hour.

$ ac–d

Dec1

Dec2

Dec3

Dec4

Today

total

total

total

total

total

1.08

0.99

3.39

4.50

6.10

ConnecttimeforalltheusersTodisplayconnecttimeforalltheusersuse–passhownbelow.Pleasenote

thatthisindicatesthecumulativeconnecttimefortheindividualusers.

$ ac-p

john

madison

sanjay

nisha

3.64

0.06

88.17

105.92

45

LinuxHackramesh

total309.21

AjayKumarTiwari

111.42

ConnecttimeforaspecificuserTogetaconnecttimereportforaspecificuser,executethefollowing:

$ ac-dsanjay

Jul2

Aug25

Sep3

Sep4

Dec24

Dec29

Today

total

total

total

total

total

total

total

12.85

5.05

1.03

5.37

8.15

1.42

2.95

46


Chapter5:PS1,PS2,PS3,PS4andPROMPT_COMMANDHack29.PS1-DefaultInteractionPromptThedefaultinteractivepromptonyourLinuxcanbemodifiedasshownbelow

tosomethingusefulandinformative.Inthefollowingexample,thedefault

PS1was“\s-\v\$”,whichdisplaystheshellnameandtheversionnumber.Let

uschangethisdefaultbehaviortodisplaytheusername,hostnameand

currentworkingdirectorynameasshownbelow.

-bash-3.2$

exportPS1=”\u@\h\w>“

ramesh@dev-db~>cd/etc/mail

ramesh@dev-db/etc/mail>

[Note:Promptchangedto“username@hostnamecurrent-

dir>”format]

FollowingPS1codesareusedinthisexample:

o

o

o

\u–Username

\h–Hostname

\w-Fullpathnameofcurrentdirectory.Pleasenotethatwhen

youareinthehomedirectory,thiswilldisplayonly~asshown

above

NotethatthereisaspaceattheendinthevalueofPS1.Personally,Iprefer

aspaceattheendofthepromptforbetterreadability.

MakethissettingpermanentbyaddingexportPS1=”\u@\h\w>”toeither

.bash_profile(or).bashrcasshownbelow.

47


ramesh@dev-db~>vi~/.bash_profile

ramesh@dev-db~>vi~/.bashrc

[Note:AddexportPS1=”\u@\h\w>”tooneoftheabove

files]

RefertothenextchapterforseveralpracticalexamplesofPS1usagein

detail.

Hack30.PS2-ContinuationInteractivePromptAverylongcommandcanbebrokendowntomultiplelinesbygiving\atthe

endoftheline.Thedefaultinteractivepromptforamulti-linecommandis

“>“.Letuschangethisdefaultbehaviortodisplay“continue->”byusingPS2

environmentvariableasshownbelow.

ramesh@dev-db~> myisamchk—silent—force—fast—

update-state\

> —key_buffer_size=512M—sort_buffer_size=512M\

> —read_buffer_size=4M—write_buffer_size=4M\

> /var/lib/mysql/bugs/*.MYI

[Note:Thisusesthedefault“>”forcontinuation

prompt]

ramesh@dev-db~> exportPS2=“continue->“

ramesh@dev-db~> myisamchk—silent—force—fast—

update-state\

continue-> —key_buffer_size=512M—

sort_buffer_size=512M\

continue-> —read_buffer_size=4M—write_buffer_size=4M

\

continue-> /var/lib/mysql/bugs/*.MYI

[Note:Thisusesthemodified“continue->”for

48

LinuxHackcontinuationprompt]

AjayKumarTiwari

Ifounditveryhelpfulandeasytoread,whenIbreakmylongcommandsinto

multiplelinesusing.Ihavealsoseenotherswhodon’tliketobreak-uplong

commands.

Hack31.PS3-Promptusedby“select”insideshellscriptYoucandefineacustompromptfortheselectloopinsideashellscript,using

thePS3environmentvariable,asexplainedbelow.

ShellscriptandoutputWITHOUTPS3:ramesh@dev-db~> catps3.sh

selectiinmontuewedexit

do

case$iin

mon)echo“Monday”;;

tue)echo“Tuesday”;;

wed)echo“Wednesday”;;

exit)exit;;

esac

done

ramesh@dev-db~> ./ps3.sh

1)mon

2)tue

3)wed

4)exit

#?1

Monday

#?4

[Note:Thisdisplaysthedefault“#?”forselect

commandprompt]

49


ShellscriptandoutputWITHPS3:ramesh@dev-db~> catps3.sh

PS3=“Selectaday(1-4):“

selectiinmontuewedexit

do

case$iin

mon)echo“Monday”;;

tue)echo“Tuesday”;;

wed)echo“Wednesday”;;

exit)exit;;

esac

done


1)mon

2)tue

3)wed

4)exit

Selectaday(1-4):1

Monday

Selectaday(1-4):4

[Note:Thisdisplaysthemodified“Selectaday(1-4):”

forselectcommandprompt]

Hack32.PS4-Usedby“set-x”toprefixtracingoutputThePS4shellvariabledefinesthepromptthatgetsdisplayed,whenyou

executeashellscriptindebugmodeasshownbelow.

ShellscriptandoutputWITHOUTPS4:50

LinuxHackramesh@dev-db~> catps4.sh

set-x

echo“PS4demoscript”

ls-l/etc/|wc-l

du-sh~


++echo‘PS4demoscript’

PS4demoscript

++ls-l/etc/

++wc-l

243

++du-sh/home/ramesh

48K/home/ramesh

AjayKumarTiwari

[Note:Thisdisplaysthedefault“++”whiletracingthe

outputusingset-x]

ShellscriptandoutputWITHPS4:ThePS4definedbelowintheps4.shhasthefollowingtwocodes:

o

o

$0-indicatesthenameofscript

$LINENO-displaysthecurrentlinenumberwithinthescript

ramesh@dev-db~> catps4.sh

exportPS4=’$0.$LINENO+‘

set-x

echo“PS4demoscript”

ls-l/etc/|wc-l

du-sh~


../ps4.sh.3+echo‘PS4demoscript’

PS4demoscript

../ps4.sh.4+ls-l/etc/

51

LinuxHack../ps4.sh.4+wc-l

243

../ps4.sh.5+du-sh/home/ramesh

48K/home/ramesh

AjayKumarTiwari

[Note:Thisdisplaysthemodified“{script-name}.{line-

number}+”whiletracingtheoutputusingset-x]

Hack33.PROMPT_COMMANDBashshellexecutesthecontentofthePROMPT_COMMANDjustbefore

displayingthePS1variable.

ramesh@dev-db~> exportPROMPT_COMMAND=“date+%k:%m:%S”

22:08:42

ramesh@dev-db~>

[Note:ThisdisplaysthePROMPT_COMMANDandPS1output

ondifferentlines]

IfyouwanttodisplaythevalueofPROMPT_COMMANDinthesamelineasthe

PS1,usetheecho-nasshownbelow.

ramesh@dev-db~> exportPROMPT_COMMAND=“echo-n[$(date

+%k:%m:%S)]”

[22:08:51]ramesh@dev-db~>

[Note:ThisdisplaysthePROMPT_COMMANDandPS1output

onthesameline]

52


Chapter6:ColorfulandFunctionalShellPromptUsingPS1Hack34.Displayusername,hostnameandbasenameofdirectoryinthepromptThePS1inthisexampledisplaysfollowingthreeinformationintheprompt:

o

o

o

\u–Username

\h–Hostname

\W–Basenameofthecurrentworkingdirectory

-bash-3.2$ exportPS1=”\u@\h\W>“

ramesh@dev-db~>cd/etc/mail

ramesh@dev-dbmail>

Hack35.DisplaycurrenttimeinthepromptInthePS1environmentvariable,youcandirectlyexecuteanyLinux

command,byspecifyingintheformat$(linux_command).Inthefollowing

example,thecommand$(date)isexecutedtodisplaythecurrenttimeinside

theprompt.

ramesh@dev-db~> exportPS1=”\u@\h[\$(date

+%k:%m:%S)]>“

ramesh@dev-db[11:09:56]>

53


Youcanalsouse\ttodisplaythecurrenttimeinthehh:mm:ssformatas

shownbelow:

ramesh@dev-db~> exportPS1=”\u@\h[\t]>“


Youcanalsouse\@todisplaythecurrenttimein12-houram/pmformatas

shownbelow:

ramesh@dev-db~> exportPS1=”[\@]\u@\h>“

[04:12PM]ramesh@dev-db>

Hack36.DisplayoutputofanycommandinthepromptYoucandisplayoutputofanyLinuxcommandintheprompt.Thefollowing

exampledisplaysthreeitemsseparatedby|(pipe)inthecommandprompt:

o

o

o

\!:Thehistorynumberofthecommand

\h:hostname

$kernel_version:Theoutputoftheuname-rcommandfrom

$kernel_versionvariable

\$?:Statusofthelastcommand

o

ramesh@dev-db~> kernel_version=$(uname-r)ramesh@dev-db~> exportPS1=”\!|\h|$kernel_version|\$?>“

473|dev-db|2.6.25-14.fc9.i686|0>

54


Hack37.ChangeforegroundcolorofthepromptDisplaypromptinbluecolor,alongwithusername,hostandcurrentdirectory

information

$ exportPS1=”\e[0;34m\u@\h\w>\e[m“

[Note:Thisisforlightblueprompt]

$ exportPS1=”\e[1;34m\u@\h\w>\e[m“

[Note:Thisisfordarkblueprompt]

\e[-Indicatesthebeginningofcolorprompt

x;ym-Indicatescolorcode.Usethecolorcodevaluesmentioned

below.

\e[m-indicatestheendofcolorprompt

o

o

o

ColorCodeTable:Black0;30

Blue0;34

Green0;32

Cyan0;36

Red0;31

Purple0;35

Brown0;33

[Note:Replace0with1fordarkcolor]

Makethecolorchangepermanentbyaddingthefollowinglinesyour

~/.bash_profileor~/.bashrc

$vi~/.bash_profile

55


STARTCOLOR=’\e[0;34m’;

ENDCOLOR=”\e[0m”

exportPS1=”$STARTCOLOR\u@\h\w>$ENDCOLOR”

Hack38.ChangebackgroundcolorofthepromptChangethebackgroundcolorbyspecifying\e[{code}minthePS1promptas

shownbelow.

$ exportPS1=”\e[47m\u@\h\w>\e[m“

[Note:ThisisforLightGraybackground]

Combinationofbackgroundandforeground.

$ exportPS1=”\e[0;34m\e[47m\u@\h\w>\e[m“

[Note:ThisisforLightBlueforegroundandLightGray

background]

Addthefollowingtoyour~/.bash_profileor~/.bashrctomaketheabove

backgroundandforegroundcolorpermanent.

$vi~/.bash_profile

STARTFGCOLOR=’\e[0;34m’;

STARTBGCOLOR=”\e[47m”

ENDCOLOR=”\e[0m”

exportPS1=”$STARTFGCOLOR$STARTBGCOLOR\u@\h\w>

$ENDCOLOR”

Playaroundbyusingthefollowingbackgroundcolorandchoosetheonethat

matchyourtaste:

o

\e[40m

56

LinuxHacko

o

o

o

o

o

o

\e[41m

\e[42m

\e[43m

\e[44m

\e[45m

\e[46m

\e[47m

AjayKumarTiwari

Hack39.DisplaymultiplecolorsinthepromptYoucanalsodisplaymultiplecolorsinthesameprompt.Addthefollowing

functiontoyour~/.bash_profile

functionprompt{

localBLUE=”\[\033[0;34m\]”

localDARK_BLUE=”\[\033[1;34m\]”

localRED=”\[\033[0;31m\]”

localDARK_RED=”\[\033[1;31m\]”

localNO_COLOR=”\[\033[0m\]”

case$TERMin

xterm*|rxvt*)

TITLEBAR=’\[\033]0;\u@\h:\w\007\]’

;;

*)

TITLEBAR=””

;;

esac

PS1=”\u@\h[\t]>”

PS1=”${TITLEBAR}\

$BLUE\u@\h$RED[\t]>$NO_COLOR”

PS2=’continue->‘

PS4=’$0.$LINENO+‘

}

57


Youcanre-loginforthechangestotakeeffectorsourcethe.bash_profileas

shownbelow.

$../.bash_profile

$ prompt


Hack40.ChangethepromptcolorusingtputYoucanalsochangecolorofthePS1promptusingtputasshownbelow:

$exportPS1=”\[$(tputbold)$(tputsetb4)$(tputsetaf

7)\]\u@\h:\w$\[$(tputsgr0)\]“

tputColorCapabilities:

o

o

o

o

tputsetab[1-7]-SetabackgroundcolorusingANSIescape

tputsetb[1-7]-Setabackgroundcolor

tputsetaf[1-7]-SetaforegroundcolorusingANSIescape

tputsetf[1-7]-Setaforegroundcolor

tputTextModeCapabilities:

o

o

o

o

o

tputbold-Setboldmode

tputdim-turnonhalf-brightmode

tputsmul-beginunderlinemode

tputrmul-exitunderlinemode

tputrev-Turnonreversemode

58

LinuxHacko

o

o

AjayKumarTiwari

tputsmso-Enterstandoutmode(boldonrxvt)

tputrmso-Exitstandoutmode

tputsgr0-Turnoffallattributes

ColorCodefortput:

o

o

o

o

o

o

o

o

0–Black

1–Red

2–Green

3–Yellow

4–Blue

5–Magenta

6–Cyan

7-White

Hack41.CreateyourownpromptusingtheavailablecodesforPS1variableUsethefollowingcodesandcreateyourownpersonalPS1Linuxpromptthat

isfunctionalandsuitesyourtaste.

o

o

\aanASCIIbellcharacter(07)

\dthedatein“WeekdayMonthDate”format(e.g.,“TueMay

26″)

\D{format}-theformatispassedtostrftime(3)andtheresultis

insertedintothepromptstring;anemptyformatresultsina

locale-specifictimerepresentation.Thebracesarerequired

\eanASCIIescapecharacter(033)

o

o

59

LinuxHacko

o

o

o

o

o

o

\hthehostnameuptothefirstpart

\Hthehostname

AjayKumarTiwari

\jthenumberofjobscurrentlymanagedbytheshell

\lthebasenameoftheshell’sterminaldevicename

\nnewline

\rcarriagereturn

\sthenameoftheshell,thebasenameof$0(theportion

followingthefinalslash)

\tthecurrenttimein24-hourHH:MM:SSformat

\Tthecurrenttimein12-hourHH:MM:SSformat

\@thecurrenttimein12-houram/pmformat

\Athecurrenttimein24-hourHH:MMformat

\utheusernameofthecurrentuser

\vtheversionofbash(e.g.,2.00)

\Vthereleaseofbash,version+patchlevel(e.g.,2.00.0)

\wthecurrentworkingdirectory,with$HOMEabbreviatedwitha

tilde

\Wthebasenameofthecurrentworkingdirectory,with$HOME

abbreviatedwithatilde

\!thehistorynumberofthiscommand

\#thecommandnumberofthiscommand

\$iftheeffectiveUIDis0,a#,otherwisea$

\nnnthecharactercorrespondingtotheoctalnumbernnn

\abackslash

\[beginasequenceofnon-printingcharacters,whichcouldbe

usedtoembedaterminalcontrolsequenceintotheprompt

60

o

o

o

o

o

o

o

o

o

o

o

o

o

o

o

LinuxHacko

AjayKumarTiwari

\]endasequenceofnon-printingcharacter

Hack42.UsebashshellfunctioninsidePS1variableYoucanalsoinvokeabashshellfunctioninthePS1asshownbelow.

ramesh@dev-db~>functionhttpdcount{

>psaux|grephttpd|grep-vgrep|wc-l

>}

ramesh@dev-db~> exportPS1=”\u@\h[`httpdcount`]>“

ramesh@dev-db[12]>

[Note:Thisdisplaysthetotalnumberofrunninghttpd

processes]

Youcanaddthefollowinglinetoyour~/.bash_profileor~/.bashrctomake

thischangepermanent:

$vi.bash_profile

functionhttpdcount{

psaux|grephttpd|grep-vgrep|wc-l

}

exportPS1=’\u@\h[`httpdcount`]>‘

Hack43.UseshellscriptinsidePS1variableYoucanalsoinvokeashellscriptinsidethePS1variable.Intheexample

below,the~/bin/totalfilesize.sh,whichcalculatesthetotalfilesizeofthe

currentdirectory,isinvokedinsidethePS1variable.

ramesh@dev-db~>cat~/bin/totalfilesize.sh

61


forfilesizein$(ls-l.|grep“^-”|awk‘{print

$5}’)

do

lettotalsize=$totalsize+$filesize

done

echo-n“$totalsize”

ramesh@dev-db~>exportPATH=$PATH:~/bin

ramesh@dev-db~> exportPS1=”\u@\h

[\$(totalfilesize.sh)bytes]>“

ramesh@dev-db[534bytes]>cd/etc/mail

ramesh@dev-db[167997bytes]>

[Note:Thisexecutesthetotalfilesize.shtodisplay

thetotalfilesizeofthecurrentdirectoryinthePS1

prompt]

62


Chapter7:ArchiveandCompressionHack44.ZipcommandbasicsHowtozipmultiplefiles?syntax:zip{.zipfile-name}{file-names}

#zipvar-log-files.zip/var/log/*

adding:var/log/acpid(deflated81%)

adding:var/log/anaconda.log(deflated79%)

adding:var/log/anaconda.syslog(deflated73%)

adding:var/log/anaconda.xlog(deflated82%)

adding:var/log/audit/(stored0%)

adding:var/log/boot.log(stored0%)

adding:var/log/boot.log.1(deflated40%)




Howtozipadirectoryandit’sfilesrecursively?#zip-rvar-log-dir.zip/var/log/

updating:var/log/(stored0%)

adding:var/log/wtmp(deflated78%)

adding:var/log/scrollkeeper.log(deflated94%)

adding:var/log/rpmpkgs.3(deflated68%)

adding:var/log/spooler(stored0%)

adding:var/log/cron.2(deflated90%)

adding:var/log/spooler.1(stored0%)

adding:var/log/spooler.4(stored0%)

adding:var/log/httpd/(stored0%)

adding:var/log/rpmpkgs.1(deflated68%)

adding:var/log/anaconda.log(deflated79%)

adding:var/log/secure.2(deflated93%)

63

LinuxHack

Howtounzipa*.zipcompressedfile?#unzipvar-log.zip

Archive:var-log.zip

inflating:var/log/acpid

inflating:var/log/anaconda.log

inflating:var/log/anaconda.syslog

inflating:var/log/anaconda.xlog

creating:var/log/audit/

AjayKumarTiwari

Toseeadetailedoutputduringunzippassthe–voptionasshownbelow.# unzip-vvar-log.zip

Archive:var-log.zip

LengthMethodSizeRatioDateTime

Name

––—––––-––––-

-

1916Defl:N36981%02-08-0814:27

var/log/acpid

13546Defl:N290079%02-02-0714:25

var/log/anaconda.log

skip..

7680Defl:N411

var/log/wtmp.1

40981Defl:N7395

var/log/Xorg.0.log

–––––

–-

414069912809229

files

95%

82%

–

93%

12-30-0810:55

02-08-0814:28

fe876ee9

6386a95e

–

56

CRC-32

––

e2ffdc0c

34cc03a1

–

Howtolistacontentofzipfilewithuncompressingit?# unzip-lvar-log.zip

Archive:var-log.zip

LengthDateTime

–––––-

191602-08-0814:27

Name

–-

var/log/acpid

64

LinuxHack13546

..skip..

40981

40981

––—

41406991

02-08-0814:28

02-08-0714:56

02-02-0714:25

AjayKumarTiwari

var/log/anaconda.log

var/log/Xorg.0.log

var/log/Xorg.0.log.old

––-

56files

Hack45.Advancedcompressionusingzipcommand.Thereare10levelsofcompressionprovidedbyzipcommand.

o

Level0isthelowestlevel,whereitjustarchivesthefilewithout

anycompression.

Level1willperformlittlecompression.But,willbeveryfast.

Level6isthedefaultlevelofcompression.

Level9isthemaximumcompression.Thiswillbeslowerwhen

comparedtodefaultlevel.Inmyopinion,unlessyouare

compressingahugefile,youshouldalwaysuselevel9.

o

o

o

Intheexamplebelow,IusedLevel0,defaultLevel6,andLevel9

compressiononasamedirectory.Seethecompressedfilesizeyourself.

#zipvar-log-files-default.zip/var/log/*

#zip-0var-log-files-0.zip/var/log/*

#zip-9var-log-files-9.zip/var/log/*

#ls-ltr-rw-r—r—1rootroot

var-log-files-default.zip

-rw-r—r—1rootroot

var-log-files-0.zip

-rw-r—r—1rootroot

var-log-files-9.zip

2817248Jan

41415301Jan

2582610Jan

113:05

113:05

113:06

65


Hack46.PasswordProtectionofZipfilesPasstheoption–Ptothezipcommandtoassignapasswordtothezipfile.

#zip-Pmysecurepwdvar-log-protected.zip/var/log/*

Theaboveoptionisgoodifyouareusingthecommandinsideashell-script

forbackgroundjobs.However,whenyouareperformingthecompression

interactivelyonthecommand-line,youdon’twantthepasswordtobevisible

inthehistory.So,usetheoption–easshownbelowtoassignthepassword.

#zip-evar-log-protected.zip/var/log/*

Enterpassword:

Verifypassword:

updating:var/log/acpid(deflated81%)

updating:var/log/anaconda.log(deflated79%)

Whenyouareuncompressingapasswordprotectedfile,itwillaskforthe

passwordasshownbelow.

#unzipvar-log-protected.zip

Archive:var-log-protected.zip

[var-log-protected.zip]var/log/acpidpassword:

Hack47.ValidateaziparchiveSometimeyoumaywanttovalidateaziparchivewithoutextractingit.To

testthevalidityofthezipfile,passoption–tasshownbelow.

# unzip-tvar-log.zip

Archive:var-log.zip

testing:var/log/acpid

testing:var/log/anaconda.log

66

OK

OK

LinuxHacktesting:var/log/anaconda.syslog

skip…

testing:var/log/wtmp

testing:var/log/wtmp.1

testing:var/log/Xorg.0.log

AjayKumarTiwari

OK

OK

OK

OK

Noerrorsdetectedincompresseddataofvar-log.zip.

Hack48.TarCommandBasicstarcommand(tapearchive)isusedtoconvertagroupoffilesintoan

archive.

Syntax:tar[options][tar-archive-name][other-file-

names]

HowcanIcreateasinglebackupfileofallfilesandsubdirectories

undermyhomedirectory?Thefollowingcommandcreatesasinglearchivebackupfilecalled

my_home_directory.tarunder/tmp.Thisarchivewillcontainallthefilesand

subdirectoriesunder/home/jsmith.

o

o

Optionc,standsforcreateanarchive.

Optionvstandsforverbosemode,displaysadditionalinformation

whileexecutingthecommand.

Optionfindicatesthearchivefilenamementionedinthecommand.

o

#tarcvf/tmp/my_home_directory.tar/home/jsmith

67


HowdoIviewallthefilesinsidethetararchive?Optiontwilldisplayallthefilesfromthetararchive.

#tartvf/tmp/my_home_directory.tar

HowdoIextractallthefilesfromatararchive?Optionxwillextractthefilesfromthetararchiveasshownbelow.Thiswill

extractthecontenttothecurrentdirectorylocationfromwherethe

commandisexecuted.

#tarxvf/tmp/my_home_directory.tar

HowdoIextracttar.gzfilestoaspecificdirectory?#tarxvfz/tmp/my_home_directory.tar.gz–C

/home/ramesh

Hack49.Combinegzip,bzip2withtarHowtousegzipwithtar?Add optionz tothetarcommandwhendealingwithtar.gzcompressedfile.

#tarcvfz/tmp/my_home_directory.tar.gz/home/jsmith

#tarxvfz/tmp/my_home_directory.tar.gz

#tartvfz/tmp/my_home_directory.tar.gz

Note: Usinggzipisfasterwhencomparedtobzip2.

68


Howtousebzip2withtar?Add optionj tothetarcommandwhendealingwithtar.bz2compressedfile.

#tarcvfj/tmp/my_home_directory.tar.bz2/home/jsmith

#tarxvfj/tmp/my_home_directory.tar.bz2

#tartvfj/tmp/my_home_directory.tar.bz2

Note: Usingbizp2giveshigherlevelofcompressionwhencomparedtogzip.

69


Chapter8:CommandLineHistoryWhenyouareusingLinuxcommandlinefrequently,usingthehistory

effectivelycanbeamajorproductivityboost.Infact,onceyouhave

masteredthe15examplesthatI’veprovidedhere,you’llfindusingcommand

linemoreenjoyableandfun.

Hack50.DisplayTIMESTAMPinhistoryusingHISTTIMEFORMATTypicallywhenyoutypehistoryfromcommandline,itdisplaysthe

command#andthecommand.Forauditingpurpose,itmaybebeneficialto

displaythetimestampalongwiththecommandasshownbelow.

# exportHISTTIMEFORMAT=’%F%T‘

#history

1

2

3

4

|more

2008-08-05

2008-08-05

2008-08-05

2008-08-05

19:02:39

19:02:39

19:02:39

19:02:39

servicenetworkrestart

exit

id

cat/etc/redhat-release

Note: Youcanalsosetupthefollowingaliastoviewtherecenthistory

commands.

aliash1=‘history10’



Hack51.SearchthehistoryusingControl+RIstronglybelievethatthismaybeyourmostfrequentlyusedfeatureof

history.Whenyou’vealreadyexecutedaverylongcommand,youcansimply

70


searchhistoryusingakeywordandre-executethesamecommandwithout

havingtotypeitfully.PressControl+Randtypethekeyword.

Inthefollowingexample,Isearchedforred,whichdisplayedtheprevious

command“cat/etc/redhat-release”inthehistorythatcontainedtheword

red.

#[Note:PressCtrl+Rfromthecommandprompt,which

willdisplaythereverse-i-searchpromptasshown

below]

(reverse-i-search)`red‘:cat/etc/redhat-release

[Note:Pressenterwhenyouseeyourcommand,which

willexecutethecommandfromthehistory]

# cat/etc/redhat-release

Fedorarelease9(Sulphur)

Sometimesyouwanttoeditacommandfromhistorybeforeexecutingit.For

e.g.youcansearchforhttpd,whichwilldisplayservicehttpdstopfromthe

commandhistory,selectthiscommandandchangethestoptostartandre-

executeitagainasshownbelow.

#[Note:PressCtrl+Rfromthecommandprompt,which

willdisplaythereverse-i-searchprompt]

(reverse-i-search)`httpd‘:servicehttpdstop

[Note:Presseitherleftarroworrightarrowkeywhen

youseeyourcommand,whichwilldisplaythecommand

foryoutoedit,beforeexecutingit]

#servicehttpdstart

71


Hack52.Repeatpreviouscommandquicklyusing4differentmethodsSometimeyoumayenduprepeatingthepreviouscommandsforvarious

reasons.Followingarethe4differentwaystorepeatthelastexecuted

command.

1.Usethe uparrow toviewthepreviouscommandandpressenter

toexecuteit.

2. Type!! andpressenterfromthecommandline

3. Type!-1 andpressenterfromthecommandline.

4. PressControl+P willdisplaythepreviouscommand,pressenter

toexecuteit

Hack53.ExecuteaspecificcommandfromhistoryInthefollowingexample,Ifyouwanttorepeatthecommand#4,execute!4

asshownbelow.

#history

1

2

3

4

|more

servicenetworkrestart

exit

id


# !4


Fedorarelease9(Sulphur)

72


Hack54.ExecutepreviouscommandthatstartswithaspecificwordType!followedbythestartingfewlettersofthecommandthatyouwould

liketore-execute.Inthefollowingexample,typing!psandenter,executed

thepreviouscommandstartingwithps,whichis‘psaux|grepyp’.

# !ps

psaux|grepyp

root169470.0

Sl13:100:00ypbind

root175030.0

S+19:190:00grepyp

0.1

0.0

36516

4124

1264?

740pts/0

Hack55.ControlthetotalnumberoflinesinthehistoryusingHISTSIZEAppendthefollowingtwolinestothe.bash_profileandrelogintothebash

shellagaintoseethechange.Inthisexample,only450commandwillbe

storedinthebashhistory.

#vi~/.bash_profile

HISTSIZE=450

HISTFILESIZE=450

Hack56.ChangethehistoryfilenameusingHISTFILEBydefault,historyisstoredin~/.bash_historyfile.Addthefollowinglineto

the.bash_profileandrelogintothebashshell,tostorethehistorycommand

in.commandline_warriorfileinsteadof.bash_historyfile.I’myettofigure

outapracticaluseforthis.Icanseethisgettingusedwhenyouwanttotrack

commandsexecutedfromdifferentterminalsusingdifferenthistoryfile

name.

73


#vi~/.bash_profile

HISTFILE=/root/.commandline_warrior

Hack57.EliminatethecontinuousrepeatedentryfromhistoryusingHISTCONTROLInthefollowingexamplepwdwastypedthreetimes,whenyoudohistory,

youcanseeallthe3continuousoccurrencesofit.Toeliminateduplicates,

setHISTCONTROLtoignoredupsasshownbelow.

#pwd

#pwd

#pwd

#history|tail-4

44pwd

45pwd

46pwd

47history|tail-4

[Note:Therearethreepwdcommandsinhistory,after

executingpwd3timesasshownabove]

# exportHISTCONTROL=ignoredups

#pwd

#pwd

#pwd

#history|tail-3

56exportHISTCONTROL=ignoredups

57pwd

58history|tail-4

74


[Note:Thereisonlyonepwdcommandinthehistory,

evenafterexecutingpwd3timesasshownabove]

Hack58.EraseduplicatesacrossthewholehistoryusingHISTCONTROLTheignoredupsshownaboveremovesduplicatesonlyiftheyareconsecutive

commands.Toeliminateduplicatesacrossthewholehistory,setthe

HISTCONTROLtoerasedupsasshownbelow.

# exportHISTCONTROL=erasedups

#pwd

#servicehttpdstop

#history|tail-3

38pwd

39servicehttpdstop

40history|tail-3

#ls-ltr

#servicehttpdstop

#history|tail-6

35exportHISTCONTROL=erasedups

36pwd

37history|tail-3

38ls–ltr

39servicehttpdstop

40history|tail-6

[Note:Thepreviousservicehttpdstopafterpwdgot

erased]

75


Hack59.ForcehistorynottorememberaparticularcommandusingHISTCONTROLWhenyouexecuteacommand,youcaninstructhistorytoignorethe

commandbysettingHISTCONTROLtoignorespaceANDtypingaspaceinfront

ofthecommandasshownbelow.Icanseelotofjuniorsysadminsgetting

excitedaboutthis,astheycanhideacommandfromthehistory.

Itisgoodtounderstandhowignorespaceworks.But,asabestpractice,don’t

hidepurposefullyanythingfromhistory.

# exportHISTCONTROL=ignorespace

#ls–ltr

#pwd

#

servicehttpdstop

[Note:Thereisaspaceatthebeginningofservice,

toignorethiscommandfromhistory]

#history|tail-3

67ls–ltr

68pwd

69history|tail-3

Hack60.Clearalltheprevioushistoryusingoption-cSometimeyoumaywanttoclearalltheprevioushistory.Howeveryoumay

stillwanttokeepthehistorymovingforward.

# history-c

76


Hack61.SubstitutewordsfromhistorycommandsWhenyouaresearchingthroughhistory,youmaywanttoexecuteadifferent

commandbutusethesameparameterfromthecommandthatyou’vejust

searched.

Intheexamplebelow,the!!:$nexttothevicommandgetstheargument

fromthepreviouscommandtothecurrentcommand.

#lsanaconda-ks.cfg

anaconda-ks.cfg

#vi!!:$

vianaconda-ks.cfg

Intheexamplebelow,the!^nexttothevicommandgetsthefirstargument

fromthepreviouscommand(i.ecpcommand)tothecurrentcommand(i.evi

command).

#cpanaconda-ks.cfganaconda-ks.cfg.bak

anaconda-ks.cfg

#vi!^

vianaconda-ks.cfg

Hack62.SubstituteaspecificargumentforaspecificcommandIntheexamplebelow,!cp:2searchesforthepreviouscommandinhistory

thatstartswithcpandtakesthesecondargumentofcpandsubstitutesitfor

thels-lcommandasshownbelow.

77


#cp~/longname.txt/really/a/very/long/path/long-

filename.txt

#ls-l!cp:2

ls-l/really/a/very/long/path/long-filename.txt

Intheexamplebelow,!cp:$searchesforthepreviouscommandinhistory

thatstartswithcpandtakesthelastargument(inthiscase,whichisalsothe

secondargumentasshownabove)ofcpandsubstitutesitforthels-l

commandasshownbelow.

#ls-l!cp:$

ls-l/really/a/very/long/path/long-filename.txt

Hack63.DisabletheusageofhistoryusingHISTSIZEIfyouwanttodisablehistoryalltogetheranddon’twantbashshellto

rememberthecommandsyou’vetyped,settheHISTSIZEto0asshownbelow.

# exportHISTSIZE=0

#history

#[Note:

Historydidnotdisplayanything]

Hack64.IgnorespecificcommandsfromthehistoryusingHISTIGNORESometimesyoumaynotwanttoclutteryourhistorywithbasiccommands

suchaspwdandls.UseHISTIGNOREtospecifyallthecommandsthatyou

wanttoignorefromthehistory.

78


PleasenotethataddinglstotheHISTIGNOREignoresonlylsandnotls-l.So,

youhavetoprovidetheexactcommandthatyouwouldliketoignorefrom

thehistory.

# exportHISTIGNORE=”pwd:ls:ls–ltr:”

#pwd

#ls

#ls-ltr

#servicehttpdstop

#history|tail-3

79exportHISTIGNORE=”pwd:ls:ls-ltr:”

80servicehttpdstop

81history

[Note:Historydidnotdisplaypwdandls]

79


Chapter9:SystemAdministrationTasksHack65.PartitionusingfdiskAfteryou’veinstalledbrandnewdisksonyourserver,youhavetousetools

likefdisktopartitionitaccordingly.

Followingarethe5typicalactions(commands)thatyoucanexecuteinside

fdisk.

o

o

o

o

o

n–NewPartitioncreation

d–Deleteanexistingpartition

p-PrintPartitionTable

w–Writethechangestothepartitiontable.i.esave.

q–Quitthefdiskutility

CreateapartitionInthefollowingexample,Icreateda/dev/sda1primarypartition.

# fdisk/dev/sda

DevicecontainsneitheravalidDOSpartitiontable,

norSun,SGIorOSFdisklabelBuildinganewDOS

disklabel.Changeswillremaininmemoryonly,

untilyoudecidetowritethem.Afterthat,ofcourse,

thepreviouscontentwon’tberecoverable.

Thenumberofcylindersforthisdiskissetto34893.

Thereisnothingwrongwiththat,butthisislarger

than1024,andcouldincertainsetupscauseproblems

with:

80


1)softwarethatrunsatboottime(e.g.,oldversions

ofLILO)

2)bootingandpartitioningsoftwarefromotherOSs

(e.g.,DOSFDISK,OS/2FDISK)

Warning:invalidflag0x0000ofpartitiontable4will

becorrectedbyw(rite)

Command(mforhelp): p

Disk/dev/sda:287.0GB,287005343744bytes

255heads,63sectors/track,34893cylinders

Units=cylindersof16065*512=8225280bytes

DeviceBoot

Start

End

Blocks

Id

System

Command(mforhelp): n

Commandaction

eextended

pprimarypartition(1-4)

p

Partitionnumber(1-4): 1

Firstcylinder(1-34893,default1):

Usingdefaultvalue1

Lastcylinderor+sizeor+sizeMor+sizeK(1-34893,

default34893):

Usingdefaultvalue34893

Command(mforhelp): w

Thepartitiontablehasbeenaltered!

Callingioctl()tore-readpartitiontable.

Syncingdisks.

Verifythatthepartitiongotcreatedsuccessfully# fdisk/dev/sda

Thenumberofcylindersforthisdiskissetto34893.

Thereisnothingwrongwiththat,butthisislarger

than1024,andcouldincertainsetupscauseproblems

with:

81


1)softwarethatrunsatboottime(e.g.,oldversions

ofLILO)

2)bootingandpartitioningsoftwarefromotherOSs

(e.g.,DOSFDISK,OS/2FDISK)

Command(mforhelp):p

Disk/dev/sda:287.0GB,287005343744bytes

255heads,63sectors/track,34893cylinders

Units=cylindersof16065*512=8225280bytes

DeviceBoot

/dev/sda1

Start

1

End

34893

Blocks

280277991

Id

83

System

Linux

Command(mforhelp): q

Hack66.Formatapartitionusingmke2fskAfterpartitioningthedisks,itisstillnotreadyforusage,asweneedto

formatthedisk.Atthisstage,ifyoutrytoviewthediskinformation,itwill

givethefollowingerrormessageindicatingthatnovalidsuperblockis

present.

# tune2fs-l/dev/sda1

tune2fs1.35(28-Feb-2004)

tune2fs:Badmagicnumberinsuper-blockwhiletrying

toopen/dev/sda1

Couldn’tfindvalidfilesystemsuperblock.

Toformatthedisk,usemke2fsasshownbelow.

# mke2fs/dev/sda1

Youcanalsopassthefollowingoptionalparametertothemke2fs.

82

LinuxHacko

AjayKumarTiwari

-m0:reserved-blocks-percentage–Thisindicatesthe

percentageofthefilesystemblocksreservedfortherootuser.

Defaultis5%.Inthefollowingexample,itissetto0.

-b4096:block-sizespecifiedinbytes.Validvaluesare1024,

2048and4096bytesperblock.

o

# mke2fs-m0-b4096/dev/sda1

mke2fs1.35(28-Feb-2004)

Filesystemlabel=

OStype:Linux

Blocksize=4096(log=2)

Fragmentsize=4096(log=2)

205344inodes,70069497blocks

0blocks(0.00%)reservedforthesuperuser

Firstdatablock=0

Maximumfilesystemblocks=71303168

2139blockgroups

32768blockspergroup,32768fragmentspergroup

96inodespergroup

Superblockbackupsstoredonblocks:

32768,98304,163840,229376,294912,819200,884736,

1605632,2654208,4096000,7962624,11239424,20480000,

23887872

Writinginodetables:done

Writingsuperblocksandfilesystemaccounting

information:done

Thisfilesystemwillbeautomaticallycheckedevery32

mountsor180days,whichevercomesfirst.Usetune2fs

-cor-itooverride.

Theabovecommandwillcreateanext2filesystem.Tocreateanext3file

systemdothefollowing:

# mkfs.ext3/dev/sda1

# mke2fs–j/dev/sda1

83


Hack67.MountthepartitionAftercreatingapartitionandformatting,youcanmountittoamountpoint.

Firstcreateadirectorywherethepartitionshouldbemounted.

#mkdir/home/database

Mountthefilesystem.

# mount/dev/sda1/home/database

Toautomaticallymountthefilesystemafterthereboot,addthefollowing

entrytothe/etc/fstab

/dev/sdaa/home/databaseext3defaults02

Hack68.Finetunethepartitionusingtune2fsUsethetune2fs–l/dev/sda1toviewthefilesysteminformationasshown

below.

# tune2fs-l/dev/sda1

tune2fs1.35(28-Feb-2004)

Filesystemvolumename:/home/database

Lastmountedon:<notavailable>

FilesystemUUID:f1234556-e123-1234-abcd-

bbbbaaaaae11

Filesystemmagicnumber:0xEF44

Filesystemrevision#:1(dynamic)

Filesystemfeatures:resize_inodefiletype

sparse_super

Defaultmountoptions:(none)

Filesystemstate:notclean

84

LinuxHackErrorsbehavior:

FilesystemOStype:

Inodecount:

Blockcount:

Reservedblockcount:

Freeblocks:

Freeinodes:

Firstblock:

Blocksize:

Fragmentsize:

ReservedGDTblocks:

Blockspergroup:

Fragmentspergroup:

Inodespergroup:

Inodeblockspergroup:

Filesystemcreated:

Lastmounttime:

Lastwritetime:

Mountcount:

Maximummountcount:

Lastchecked:

Checkinterval:

Nextcheckafter:

Reservedblocksuid:

Reservedblocksgid:

Firstinode:

Inodesize:

Defaultdirectoryhash:

DirectoryHashSeed:

ccccc123292b

AjayKumarTiwari

Continue

Linux

1094912

140138994

0

16848481

1014969

0

2048

2048

512

16384

16384

128

8

TueJul100:06:032008

ThuAug2105:58:252008

FriJan215:40:362009

2

20

TueJul100:06:032008

15552000(6months)

SatDec2723:06:032008

0(userroot)

0(grouproot)

11

128

tea

12345829-1236-4123-9aaa-

Youcanalsousethetune2fstotunetheex2/ext3filesystemparameter.For

example,ifyouwanttochangetheFilesystemvolumename,youcandoitas

shownbelow.

# tune2fs-l/dev/sda1|grepvolume

Filesystemvolumename:/home/database

# tune2fs-Ldatabase-home/dev/emcpowera1

tune2fs1.35(28-Feb-2004)

85


# tune2fs-l/dev/sda1|grepvolume

Filesystemvolumename:database-home

Hack69.Createaswapfilesystem.Createafileforswapusageasshownbelow.

# ddif=/dev/zeroof=/home/swap-fsbs=1Mcount=512

512+0recordsin

512+0recordsout

#ls-l/home/swap-fs

-rw-r—r—1rootroot536870912Jan

/home/swap-fs

223:13

UsemkswaptosetupaLinuxswapareainthe/home/swap-fsfilethatwas

createdabove.

# mkswap/home/swap-fs

Settingupswapspaceversion1,size=536866kB

OncethefileiscreatedandhasbeensetupforLinuxswaparea,itistimeto

enabletheswapusingswaponasshownbelow.

# swapon/home/swap-fs

Addthefollowinglineto/etc/fstabandrebootthesystemfortheswapto

takeintoeffect.

/home/swap-fsswapswapdefaults00

86


Hack70.CreateanewuserAddanewuser–BasicmethodSpecifyonlytheusername.

#useraddjsmith

AddanewuserwithadditionalParameterYoucanalsospecifythefollowingparametertotheuseradd

o

o

-c:Descriptionabouttheuser.

-e:expirydateoftheuserinmm/dd/yyformat

# adduser-c“JohnSmith-OracleDeveloper”-e

12/31/09jsmith

Verifythattheusergotaddedsuccessfully.

#grepjsmith/etc/passwd

jsmith:x:510:510:JohnSmith-Oracle

Developer:/home/jsmith:/bin/bash

Changetheuserpassword.# passwdjsmith

Changingpasswordforuserjsmith.

NewUNIXpassword:

BADPASSWORD:itisbasedonadictionaryword

RetypenewUNIXpassword:

passwd:allauthenticationtokensupdatedsuccessfully.

87


Note: Makesuretofollowthesebestpracticestocreateastrongpassword

fortheuser.

Howtoidentifythedefaultvaluesusedbyuseradd?Followingarethedefaultvaluesthatwillbeusedwhenanuseriscreated.

# useradd–D

GROUP=100

HOME=/home

INACTIVE=-1

EXPIRE=

SHELL=/bin/bash

SKEL=/etc/skel

Hack71.CreateanewgroupandassigntoanuserCreateanewdevelopergroup.#groupadddevelopers

Validatethatthegroupwascreatedsuccessfully.#grepdeveloper/etc/group

developers:x:511:

Addanusertoanexistinggroup.Youcannotuseuseraddtomodifyanexistinguser,asyou’llgetthefollowing

errormessage.

88

LinuxHack#useradd-Gdevelopersjsmith

useradd:userjsmithexists

#usermod-gdevelopersjsmith

AjayKumarTiwari

Validatetheusersgroupwasmodifiedsuccessfully.#grepjsmith/etc/passwd

jsmith:x:510:511:Oracle

Developer:/home/jsmith:/bin/bash

# idjsmith

uid=510(jsmith)gid=511(developers)

groups=511(developers)

#grepjsmith/etc/group

jsmith:x:510:

developers:x:511:jsmith

Hack72.SetupSSHpasswordlesslogininOpenSSHYoucanlogintoaremoteLinuxserverwithoutenteringpasswordin3simple

stepsusingssky-keygenandssh-copy-idasexplainedinthisexample.

ssh-keygencreatesthepublicandprivatekeys.ssh-copy-idcopiesthelocal-

host’spublickeytotheremote-host’sauthorized_keysfile.ssh-copy-idalso

assignsproperpermissiontotheremote-host’shome,~/.ssh,and

~/.ssh/authorized_keys.

Step1:Createpublicandprivatekeysusingssh-key-genonlocal-

hostjsmith@local-host$ ssh-keygen

Generatingpublic/privatersakeypair.

89


Enterfileinwhichtosavethekey

(/home/jsmith/.ssh/id_rsa):[Enterkey]

Enterpassphrase(emptyfornopassphrase):[Press

enterkey]

Entersamepassphraseagain:[Pessenterkey]

Youridentificationhasbeensavedin

/home/jsmith/.ssh/id_rsa.

Yourpublickeyhasbeensavedin

/home/jsmith/.ssh/id_rsa.pub.

Thekeyfingerprintis:

33:b3:fe:af:95:95:18:11:31:d5:de:96:2f:f2:35:f9

jsmith@local-host

Step2:Copythepublickeytoremote-hostusingssh-copy-idjsmith@local-host$ ssh-copy-id-i~/.ssh/id_rsa.pub

remote-host

jsmith@remote-host’spassword:

Nowtryloggingintothemachine,with“ssh‘remote-

host’”,andcheckin:

.ssh/authorized_keystomakesurewehaven’tadded

extrakeysthatyouweren’texpecting.

Note: ssh-copy-idappendsthekeystotheremote-host’s

.ssh/authorized_key.

Step3:Logintoremote-hostwithoutenteringthepasswordjsmith@local-host$ sshremote-host

Lastlogin:SunNov1617:22:332008from192.168.1.2

[Note:SSHdidnotaskforpassword.]

jsmith@remote-host$[Note:Youareonremote-hosthere]

90


Hack73.Usessh-copy-idalongwithssh-agentUsingssh-copy-idalongwiththessh-add/ssh-agentWhennovalueispassedfortheoption-iandIf~/.ssh/identity.pubisnot

available,ssh-copy-idwilldisplaythefollowingerrormessage.

jsmith@local-host$ssh-copy-id-iremote-host

/usr/bin/ssh-copy-id:ERROR:Noidentitiesfound

Ifyouhaveloadedkeystothessh-agentusingthessh-add,thenssh-copy-id

willgetthekeysfromthessh-agenttocopytotheremote-host.i.e,itcopies

thekeysprovidedbyssh-add-Lcommandtotheremote-host,whenyoudon’t

passoption-itothessh-copy-id.

jsmith@local-host$ssh-agent$SHELL

jsmith@local-host$ssh-add-L

Theagenthasnoidentities.

jsmith@local-host$ssh-add

Identityadded:/home/jsmith/.ssh/id_rsa

(/home/jsmith/.ssh/id_rsa)

jsmith@local-host$ssh-add-L

ssh-rsa

AAAAB3NzaC1yc2EAAAABIwAAAQEAsJIEILxftj8aSxMa3d8t6JvM79D

aHrtPhTYpq7kIEMUNzApnyxsHpH1tQ/Ow==

/home/jsmith/.ssh/id_rsa

jsmith@local-host$ssh-copy-id-iremote-host

jsmith@remote-host’spassword:

Nowtryloggingintothemachine,with“ssh‘remote-

host’”,andcheckin:.ssh/authorized_keystomakesure

wehaven’taddedextrakeysthatyouweren’texpecting.

[Note:Thishasaddedthekeydisplayedbyssh-add-L]

91

LinuxHack

ThreeMinorAnnoyancesofssh-copy-idAjayKumarTiwari

Followingarefewminorannoyancesofthessh-copy-id.

1.Defaultpublickey:ssh-copy-iduses~/.ssh/identity.pubasthe

defaultpublickeyfile(i.ewhennovalueispassedtooption-i).

Instead,Iwishitusesid_dsa.pub,orid_rsa.pub,oridentity.pub

asdefaultkeys.i.eIfanyoneofthemexist,itshouldcopythat

totheremote-host.Iftwoorthreeofthemexist,itshouldcopy

identity.pubasdefault.

2.Theagenthasnoidentities:Whenthessh-agentisrunningand

thessh-add-Lreturns“Theagenthasnoidentities”(i.enokeys

areaddedtothessh-agent),thessh-copy-idwillstillcopythe

message“Theagenthasnoidentities”totheremote-host’s

authorized_keysentry.

3.Duplicateentryinauthorized_keys:Iwishssh-copy-idvalidates

duplicateentryontheremote-host’sauthorized_keys.Ifyou

executessh-copy-idmultipletimesonthelocal-host,itwillkeep

appendingthesamekeyontheremote-host’sauthorized_keys

filewithoutcheckingforduplicates.Evenwithduplicateentries

everythingworksasexpected.But,Iwouldliketohavemy

authorized_keysfileclutterfree.

Hack74.CrontabUsingcronyoucanexecuteashell-scriptorLinuxcommandsataspecific

timeanddate.Forexampleasysadmincanscheduleabackupjobthatcan

runeveryday.

Howtoaddajobtothecron?#crontab–e

05***/root/bin/backup.sh

92


Thiswillexecute/root/bin/backup.shat5a.meveryday.

DescriptionofCronfields.Followingistheformatofthecrontabfile.{minute}{hour}{day-of-month}{month}{day-of-week}{full-path-to-shell-script}

o

o

o

o

o

minute:Allowedrange0–59

hour:Allowedrange0–23

day-of-month:Allowedrange0–31

month:Allowedrange1–12.1=January.12=December.

Day-of-week:Allowedrange0–7.Sundayiseither0or7.

Crontabexamples1.Runat12:01a.m.1minuteaftermidnighteveryday.Thisisagoodtimeto

runbackupwhenthesystemisnotunderload.

10***/root/bin/backup.sh

2.Runbackupeveryweekday(Mon–Fri)at11:59p.m.

5911**1,2,3,4,5/root/bin/backup.sh

Followingwillalsodothesame.

5911**1-5/root/bin/backup.sh

93


3.Executethecommandevery5minutes.

*/5****/root/bin/check-status.sh

4.Executeat1:10p.mon1stofeverymonth

10131**/root/bin/full-backup.sh

5.Execute11p.monweekdays.

023**1-5/root/bin/incremental-backup.sh

CrontabOptionsFollowingaretheavailableoptionswithcrontab:

o

crontab–e:Editthecrontabfile.Thiswillcreateacrontab,ifit

doesn’texist

crontab–l:Displaythecrontabfile.

crontab-r:Removethecrontabfile.

crontab-ir:Thiswillpromptuserbeforedeletingacrontab.

o

o

o

Hack75.SafeRebootOfLinuxUsingMagicSysRqKeyThemagicSysRqkeyisakeycombinationintheLinuxkernelwhichallows

theusertoperformvariouslowlevelcommandsregardlessofthesystem’s

state.

Itisoftenusedtorecoverfromfreezes,ortorebootacomputerwithout

corruptingthefilesystem.Thekeycombinationconsistsof

94


Alt+SysRq+commandkey.InmanysystemstheSysRqkeyistheprintscreen

key.

First,youneedtoenabletheSysRqkey,asshownbelow.

echo“1”>/proc/sys/kernel/sysrq

ListofSysRqCommandKeysFollowingarethecommandkeysavailableforAlt+SysRq+commandkey.

o

o

o

‘k’–Killsalltheprocessrunningonthecurrentvirtualconsole.

’s’–Thiswillattempttosyncallthemountedfilesystem.

‘b’-Immediatelyrebootthesystem,withoutunmountingpartitions

orsyncing.

‘e’–SendsSIGTERMtoallprocessexceptinit.

‘m’–Outputcurrentmemoryinformationtotheconsole.

‘i’-SendtheSIGKILLsignaltoallprocessesexceptinit

‘r’-Switchthekeyboardfromrawmode(themodeusedbyprograms

suchasX11),toXLATEmode.

’s’–syncallmountedfilesystem.

‘t’-Outputalistofcurrenttasksandtheirinformationtothe

console.

‘u’-Remountallmountedfilesystemsinreadonlymode.

‘o’–Shutdownthesystemimmediately.

‘p’–Printthecurrentregistersandflagstotheconsole.

‘0-9′-Setstheconsoleloglevel,controllingwhichkernelmessages

willbeprintedtoyourconsole.

o

o

o

o

o

o

o

o

o

o

95

LinuxHacko

o

AjayKumarTiwari

‘f’-Willcalloom_killtokillprocesswhichtakesmorememory.

‘h’–Usedtodisplaythehelp.Butanyotherkeysthantheabove

listedwillprinthelp.

Wecanalsodothisbyechoingthekeystothe/proc/sysrq-triggerfile.For

example,tore-bootasystemyoucanperformthefollowing.

echo“b”>/proc/sysrq-trigger

PerformaSaferebootofLinuxusingMagicSysRqKeyToperformasaferebootofaLinuxcomputerwhichhangsup,dothe

following.Thiswillavoidthefsckduringthenextre-booting.i.ePress

Alt+SysRq+letterhighlightedbelow.

o

o

unRaw(takecontrolofkeyboardbackfromX11,

tErminate(sendSIGTERMtoallprocesses,allowingthemtoterminate

gracefully),

kIll (sendSIGILLtoallprocesses,forcingthemtoterminate

immediately),

Sync (flushdatatodisk),

Unmount (remountallfilesystemsread-only),

reBoot.

o

o

o

o

96


Chapter10:ApachectlandHttpdExamplesAfteryouhaveinstalledApache2,ifyouwanttouseapachectlandhttpdto

it’smaximumpotential,youshouldgobeyondusingstart,stopandrestart.

The9practicalexamplesprovidedinthischapterwillhelpyoutouse

apachectlandhttpdveryeffectively.

ApachectlactsasSysVinitscript,takingargumentslikestart,stop,restart

andstatus.Italsoactsasfront-endtohttpdcommand,bysimplypassingthe

commandlineargumentstohttpd.So,allthecommandsyouexecuteusing

apachectl,canalsobeexecuteddirectlybycallinghttpd.

Ifyoudon’thaveApache,refertothetutorials:installapachefromsourceor

installLAMPstackusingyum.

Hack76.Passdifferenthttpd.conffilenametoapachectlTypicallyyou’llmodifytheoriginalhttpd.conftotryoutdifferentApache

directives.Ifsomethingdoesn’tworkout,you’llrevertbackthechanges.

Insteadofplayingaroundwiththeoriginalhttpd.conf,copyittoanew

httpd.conf.debugandusethisnewhttpd.conf.debugfilewithApachefor

testingpurposeasshownbelowusingoption-f.

# apachectl-fconf/httpd.conf.debug

# httpd-kstart-fconf/httpd.conf.debug

[Note:youcanuseeitherapachectlorhttpdasshown

above]

#ps-ef|grephttp

root250801023:2600:00:00/usr/sbin/httpd-f

97


conf/httpd.conf.debug

apache2509925080023:2800:00:00/usr/sbin/httpd-f

conf/httpd.conf.debug

[Note:psshowsthehttpdrunningwithhttpd.conf.debug

file]

OnceyouaresatisfiedwiththechangesandApacherunswithoutanyproblem

withhttpd.conf.debug,youcancopythechangestohttpd.confandstartthe

Apachenormallyasshownbelow.

#cphttpd.conf.debughttpd.conf

#apachectlstop

#apachectlstart

#ps-ef|grephttpd

root251141023:2800:00:00/usr/sbin/httpd

-kstart

daemon2511525114023:2800:00:00/usr/sbin/httpd

-kstart

[Note:psindicatesthatthehttpdisrunningusingthe

defaultconfigfile]

Hack77.UseatemporaryDocumentRootwithoutmodifyinghttpd.confThisisveryhelpful,whenyouaretryingoutdifferentlayoutforyourwebsite

anddon’twanttomodifytheoriginalfilesunderthedefaultDocumentRoot.

TakeacopyofyouroriginalDocumentRootdirectory(/var/www/html)toa

newtemporaryDocumentRootdirectory(/var/www/html_debug).Makeall

yourchangesunderthistemporaryDocumentRootdirectory

(/var/www/html_debug)andstarttheApachewiththistemporarydirectory

asshownbelowusingoption-c.

98


# httpd-kstart-c“DocumentRoot/var/www/html_debug/”

Ifyouwanttogobacktooriginalconfigurationusingthedefault

DocumentRoot(/var/www/html),simplyrestarttheApacheasshownbelow.

#httpd-kstop

#apachectlstart

Hack78.IncreasetheLogLeveltemporarilyWhileyouaredebugginganissue,youcanchangetheLogLeveloftheApache

temporarily,withoutmodifyingtheLogLeveldirectiveinthehttpd.confas

shownbelowusingoption-e.Inthisexample,theLogLevelissettodebug.

# httpd-kstart-edebug

[SunAug1713:53:062008][debug]mod_so.c(246):

loadedmoduleauth_basic_module


loadedmoduleauth_digest_module

Possiblevaluesyoucanpasstooption-eare:

o

o

o

o

o

o

o

o

debug

info

notice

warn

error

crit

alert

emerg

99


Hack79.DisplaythemodulesinsideApacheDisplaythemodulescompiledinsideApache# httpd-l

Compiledinmodules:

core.c

prefork.c

http_core.c

mod_so.c

DisplaybothstaticanddynamicmoduleloadedbyApacheWhenyoupassoption-l,tohttpd,itwilldisplayonlythestaticmodules.

Passingoption-M,willdisplaybothstaticandsharedmodulesasshown

below.

# httpd–M

LoadedModules:

core_module(static)

mpm_prefork_module(static)

http_module(static)

so_module(static)

auth_basic_module(shared)

auth_digest_module(shared)

authn_file_module(shared)

authn_alias_module(shared)

SyntaxOK

100


Hack80.Showallaccepteddirectivesinsidehttpd.confThisislikeanextendedhelpforhttpd,whichwilldisplayallthehttpd.conf

directivesandtheplaceswheretheyarevalid.Foraspecificdirective,it

tellsallthepossiblevaluesandwhereitcanbeusedinsidethehttpd.conf.

Thiscanbeveryhelpful,whenyouwanttoquicklyknowaboutaparticular

Apachedirective.

# httpd–L

HostnameLookups(core.c)

“on”toenable,“off”todisablereverseDNSlookups,

or“double”toenabledouble-reverseDNSlookups

Allowedin*.confanywhere

ServerLimit(prefork.c)

MaximumvalueofMaxClientsforthisrunofApache

Allowedin*.confonlyoutside<Directory>,<Files>or

<Location>

KeepAlive(http_core.c)

WhetherpersistentconnectionsshouldbeOnorOff


<Location>

LoadModule(mod_so.c)

amodulenameandthenameofasharedobjectfileto

loaditfrom


<Location>

Hack81.Validatethehttpd.confaftermakingchangesUseoption-ttovalidatewhetherthereareanyissueswithaspecificApache

configurationfile.Intheexampleshownbelow,itdisplaysthatthereisa

101


problematline148inthehttpd.conf.debug.mod_auth_basicsoismissinga.

(period)beforetheso.

# httpd-t-fconf/httpd.conf.debug

httpd:Syntaxerroronline148of

/etc/httpd/conf/httpd.conf.debug:

Cannotload/etc/httpd/modules/mod_auth_basicsointo

server:

/etc/httpd/modules/mod_auth_basicso:cannotopenshared

objectfile:Nosuchfileordirectory

Onceyoufixtheissue,itwilldisplaySyntaxOK.

# httpd-t-fconf/httpd.conf.debug

SyntaxOK

Hack82.DisplaythehttpdbuildparametersUseoption-V(upper-caseV),todisplayApacheversionnumberandallthe

parametersthatareusedwhilebuildingtheApache.

# httpd–V

Serverversion:Apache/2.2.9(Unix)

Serverbuilt:Jul14200815:36:56

Server’sModuleMagicNumber:20051115:15

Serverloaded:APR1.2.12,APR-Util1.2.12

Compiledusing:APR1.2.12,APR-Util1.2.12

Architecture:32-bit

ServerMPM:Prefork

threaded:no

forked:yes(variableprocesscount)

Servercompiledwith….

-DAPACHE_MPM_DIR=”server/mpm/prefork”

-DAPR_HAS_SENDFILE

-DAPR_HAS_MMAP

102

LinuxHack-D

-D

-D

-D

-D

-D

-D

-D

-D

-D

-D

-D

-D

-D

-D

AjayKumarTiwari

APR_HAVE_IPV6(IPv4-mappedaddressesenabled)

APR_USE_SYSVSEM_SERIALIZE

APR_USE_PTHREAD_SERIALIZE

SINGLE_LISTEN_UNSERIALIZED_ACCEPT

APR_HAS_OTHER_CHILD

AP_HAVE_RELIABLE_PIPED_LOGS

DYNAMIC_MODULE_LIMIT=128

HTTPD_ROOT=”/etc/httpd”

SUEXEC_BIN=”/usr/sbin/suexec”

DEFAULT_PIDLOG=”logs/httpd.pid”

DEFAULT_SCOREBOARD=”logs/apache_runtime_status”

DEFAULT_LOCKFILE=”logs/accept.lock”

DEFAULT_ERRORLOG=”logs/error_log”

AP_TYPES_CONFIG_FILE=”conf/mime.types”

SERVER_CONFIG_FILE=”conf/httpd.conf”

IfyouwantdisplayonlytheApacheversionnumber,usetheoption-v(lower-

casev)asshownbelow.

# httpd-v

Serverversion:Apache/2.2.9(Unix)

Serverbuilt:Jul14200815:36:56

Hack83.LoadaspecificmoduleonlyondemandSometimesyoumaynotwanttoloadallthemodulesintheApache.Fore.g.

YoumaywanttoloadldaprelatedmodulestoApache,onlywhenyouare

testingLDAP.Thiscanbeachievedasshownbelow.

Modifythehttpd.confandaddIfDefinedirectivecalledload-ldap(youcan

namethisanythingyouwant).

<IfDefineload-ldap>

LoadModuleldap_modulemodules/mod_ldap.so

LoadModuleauthnz_ldap_module

103

LinuxHackmodules/mod_authnz_ldap.so

</IfDefine>

AjayKumarTiwari

WhenyouaretestingldapandwouldliketoLoadtheldaprelatedmodules,

passtheload-ldaptoOption-D,asshownbelow:

#httpd-kstart-edebug-Dload-ldap-f

/etc/httpd/conf/httpd.conf.debug


loadedmoduleldap_module


loadedmoduleauthnz_ldap_module

[Note:Pass-Dload-ldap,toloadtheldapmodulesinto

Apache]

#apachectlstart

[Note:StarttheApachenormally,ifyoudon’twantto

loadtheldapmodules.

104


Chapter11:BashScriptingHack84.ExecutionSequenceof.bash_*filesWhatisthesequenceinwhichthefollowingfilesareexecuted?

o

o

o

o

o

o

/etc/profile

~/.bash_profile

~/.bashrc

~/.bash_login

~/.profile

~/.bash_logout

ExecutionsequenceforinteractiveloginshellFollowingpseudocodeexplainsthesequenceofexecutionofthesefiles.

execute/etc/profile

IF~/.bash_profileexistsTHEN

execute~/.bash_profile

ELSE

IF~/.bash_loginexistTHEN

execute~/.bash_login

ELSE

IF~/.profileexistTHEN

execute~/.profile

ENDIF

ENDIF

ENDIF

105


Whenyoulogoutoftheinteractiveshell,followingisthesequenceof

execution:

IF~/.bash_logoutexistsTHEN

execute~/.bash_logout

ENDIF

Pleasenotethat/etc/bashrcisexecutedby~/.bashrcasshownbelow:

#cat~/.bashrc

if[-f/etc/bashrc];then

./etc/bashrc

Fi

Executionsequenceforinteractivenon-loginshellWhilelaunchinganon-logininteractiveshell,followingisthesequenceof

execution:

IF~/.bashrcexistsTHEN

execute~/.bashrc

ENDIF

Note: Whenanon-interactiveshellstartsup,itlooksforENVenvironment

variable,andexecutesthefile-namevaluementionedintheENVvariable.

Hack85.HowtogeneraterandomnumberinbashshellUsethe$RANDOMbashbuilt-infunctiontogeneraterandomnumberbetween

0–32767asshownbelow.

$echo$RANDOM

106

LinuxHack22543

$echo$RANDOM

25387

$echo$RANDOM

647

AjayKumarTiwari

Hack86.DebugashellscriptTodebugashellscriptuseset–xvinsidetheshellscriptatthetop.

Shellscriptwithnodebugcommand:$catfilesize.sh

#!/bin/bash


$5}’)

do


done

echo“Totalfilesizeincurrentdirectory:$totalsize”

OutputofShellscriptwithnodebugcommand:$./filesize.sh

Totalfilesizeincurrentdirectory:652

ShellscriptwithDebugcommandinside:Addset–xvinsidetheshellscriptnowtodebugtheoutputasshownbelow.

$catfilesize.sh

#!/bin/bash

set-xv


107


$5}’)

do


done

echo“Totalfilesizeincurrentdirectory:$totalsize”

OutputofShellscriptwithDebugcommandinside:$./fs.sh

++ls-l.

++grep‘^-‘

++awk‘{print$5}’

+forfilesizein‘$(ls-l

‘'’{print$5}’'’)’

+lettotalsize=+178


‘'’{print$5}’'’)’

+lettotalsize=178+285


‘'’{print$5}’'’)’

+lettotalsize=463+189

+echo‘Totalfilesizein

Totalfilesizeincurrent

.|grep“^-”|awk

.|grep“^-”|awk

.|grep“^-”|awk

currentdirectory:652’

directory:652

ExecuteShellscriptwithdebugoption:Insteadofgivingtheset–xvinsidetheshellscript,youcanalsoprovidethat

whileexecutingtheshellscriptasshownbelow.

$bash-xvfilesize.sh

Hack87.Quotingechostatementwithoutanyspecialcharacter.

$echoTheGeekStuff

108

LinuxHackTheGeekStuff

AjayKumarTiwari

Echostatementwithaspecialcharacter;.semi-colonisacommand

terminatorinbash.Inthefollowingexample,“TheGeek”worksfortheecho

and“Stuff”istreatedasaseparateLinuxcommandandgivescommandnot

found.

$echoTheGeek;Stuff

TheGeek

-bash:Stuff:commandnotfound

Toavoidthisyoucanadda\infrontofsemi-colon,whichwillremovethe

specialmeaningofsemi-colonandjustprintitasshownbelow.

$echoTheGeek\;Stuff

TheGeek;Stuff

SingleQuoteUsesinglequotewhenyouwanttoliterallyprinteverythinginsidethesingle

quote.Eventhespecialvariablessuchas$HOSTNAMEwillbeprintas

$HOSTNAMEinsteadofprintingthenameoftheLinuxhost.

$echo‘Hostname=$HOSTNAME;

Message=\$isUSD’

Hostname=$HOSTNAME;

Message=\$isUSD

CurrentUser=`whoami`;


DoubleQuoteUsedoublequoteswhenyouwanttodisplaytherealmeaningofspecial

variables.

$echo“Hostname=$HOSTNAME;

Message=\$isUSD”


109


Hostname=dev-db;CurrentUser=ramesh;Message=$is

USD

Doublequoteswillremovethespecialmeaningofallcharactersexceptthe

following:

o

o

o

o

o

o

$ParameterSubstitution.

`Backquotes

\$LiteralDollarSign.

\´LiteralBackquote.

\”EmbeddedDoublequote.

\EmbeddedBackslashes.

Hack88.ReaddatafilefieldsinsideashellscriptThisexampleshowshowtoreadaparticularfieldfromadata-fileand

manipulateitinsideashell-script.Forexample,letusassumethe

employees.txtfileisintheformatof{employee-name}:{employee-

id}:{department-name},withcolondelimitedfileasshownbelow.

$catemployees.txt


AlexJason:200:Sales




Thefollowingshellscriptexplainshowtoreadspecificfieldsfromthis

employee.txtfile.

$viread-employees.sh

#!/bin/bash

IFS=:

echo“EmployeeNames:”

echo“–––––”

whilereadnameempiddept

110


do

echo“$nameispartof$deptdepartment”

done<~/employees.txt

Assignexecuteprivilegetotheshellscriptandexecuteit.

$chmodu+xread-employees.sh

$./read-employees.sh

EmployeeNames:

–––––

EmmaThomasispartofMarketingdepartment

AlexJasonispartofSalesdepartment

MadisonRandyispartofProductDevelopmentdepartment

SanjayGuptaispartofSupportdepartment

NishaSinghispartofSalesdepartment

111


Chapter12:SystemMonitoringandPerformanceHack89.Freecommandfreecommanddisplaysallthenecessaryinformationaboutsystemphysical

(RAM)andswapmemory.

Syntax:free[options]

WhatisthetotalRAMonmysystem?Intheexamplebelow,thetotalphysicalmemoryonthissystemis1GB.The

valuesdisplayedbelowareinKB.

#free

totalused

Mem:10346241006696

-/+buffers/cache:

Swap:2031608

freesharedbuffers

279280174136

216668817956

02031608

cached

615892

WhatisthetotalmemoryonmysystemincludingRAMandSwap?Inthefollowingcommand:

o

o

optionmdisplaysthevaluesinMB

optiontdisplaysthe“Total”line,whichissumofphysicalandswap

memoryvalues

optionoistohidethebuffers/cachelinefromtheaboveexample.

o

# free–mto

112

LinuxHacktotal

1010

1983

2994

used

983

0

983

free

27

1983

2011

shared

0

AjayKumarTiwari

buffers

170

cached

601

Mem:

Swap:

Total:

Hack90.TopCommandtopcommanddisplaysrealtimeinformationaboutvariousperformance

metricsofthesystemsuchasCPULoad,MemoryUsage,Processeslistetc.

Syntax:top[options]

HowtoviewmycurrentsystemstatusincludingCPUusage?Executetopwithoutanyoptionfromthecommandline,whichwilldisplay

theoutputshownbelow.Thetopcommandoutputwillkeepdisplayingthe

real-timevalues,untilyoupress“Control+c”orqtoexitfromthecommand

output.

# top

top-13:10:13up171days,20:21,3users,loadaverage:0.01,0.05,0.00

Tasks:194total,1running,193sleeping,0stopped,0zombie

Cpu(s):0.6%us,0.7%sy,0.0%ni,98.7%id,0.0%wa,0.0%hi,0.0%si

Mem:1034624ktotal,1007420kused,27204kfree,174540kbuffers

Swap:2031608ktotal,0kused,2031608kfree,615904kcached

PIDUSERPRNIVIRTRES

11912apache15031828

19299oracle190279m

11398jsmith160107m

SHRS%CPU%MEMTIME+COMMAND

13m3916S10.20:46.35httpd

18m17mS10.20:00.03oracle

28m6404S00.40:03.07perl

113


Howtoreadtheoutputofthetopcommandshownabove?o

Line1“top”,indicatesthatthesystemhasbeenupandrunning

for171days.

Line2“Tasks”,displaysthetotalnumberofprocessesalongwith

abreakdownofrunning,sleeping,stoppedandzombieprocesses

count.

Line3“Cpu(s)”displaysthecurrentCPUutilizationofthe

system.Inthisexample,CPUis98.7%idle

Line4“Mem”andline5“Swap”providesthememory

information.Thisisthesameinformationfromthefree

command.

Therestofthelinesdisplayalltheactiveprocessesonthe

system,sorteddefaultbyCPUusage(%CPUcolumn).i.ethemost

CPUintensiveprocesseswillbedisplayedonthetopbydefault.

o

o

o

o

Thereareseveralcommandlineoptionsandinteractiveoptionsavailablefor

topcommands.Letusreviewcoupleofessentialoptionsfortopcommand.

Howtoidentifythemostmemoryintensiveprocesses?Whiletheoutputofthetopcommanddisplayed,pressF,whichwilldisplay

thefollowingmessageandshowallfieldsavailableforsorting,pressn(which

isforsortingtheprocessesbyMemory)andpressenter.Thiswilldisplaythe

processesinthetopoutputsortedbymemoryusage.

CurrentSortField:Kforwindow1:Def

Selectsortfieldviafieldletter,typeanyotherkeytoreturn

Howtoaddadditionalfields(fore.g.CPUTime)tothetopoutput?

114


Whilethetopcommandisrunning,pressf,whichwilldisplaythefollowing

messageandshowallfieldsavailablefordisplay,pressl,whichwilladdthe

CPUTimetothedisplaycolumnsinthetopoutput.

CurrentFields:AEHIOQTWKNMbcdfgjplrsuvyzXforwindow1:Def

Togglefieldsviafieldletter,typeanyotherkeytoreturn

Howtogetthefullpathnameandparametersoftherunning

processes?Whilethetopcommandisrunning,pressc,whichwilldisplayfullpathname

ofrunningprocessesasshownbelowinthecommandcolumn.i.eInsteadof

httpd,itdisplays/usr/local/apache2/bin/httpd.

PIDUSER

PRNIVIRTRESSHRS%CPU%MEM

1

TIME+COMMAND

0.20:46.35

11912apache1503182813m3916S

/usr/local/apache2/bin/httpd

HowtoviewtheindividualCPUsinthetopcommand?Whilethetopcommandisrunning,press1(numberone),whichwilldisplay

theperformancedataoftheindividualCPUsonthatmachineasshown

below.

top-13:10:13up171days,20:21,3users,loadaverage:0.01,0.05,0.00

Tasks:194total,1running,193sleeping,0stopped,0zombie

Cpu0:10.2%us,2.6%sy,0.0%ni,86.8%id,0.3%wa,0.0%hi,0.0%si




115


Mem:1034624ktotal,1007420kused,27204kfree,174540kbuffers

Swap:2031608ktotal,0kused,2031608kfree,615904kcached

Hack91.PsCommandpscommand(processstatus)willdisplaysnapshotinformationofallactive

processes.

Syntax:ps[options]

Howtodisplayalltheprocessesrunninginthesystem?Use“psaux”,asshownbelow.

# psaux|more

USERPID%CPU%MEMVSZRSSTTYSTATSTARTTIMECOMMAND

root10.00.02044588?SsJun27

0:00init[5]

apache311860.01.62373617556?SJul26

0:40/usr/local/apache2/bin/httpd

apache311870.01.32064014444?SJul26


Youcanalsouse“ps-ef|more”,togetasimilaroutput

PrinttheProcessTreeYoucanuseeither psaxuf or ps–ejH todisplayprocessesinatreeformat.

Thetreestructurewillhelptovisualizetheprocessandit’sparentprocess

immediately.Forclaritypurpose,fewcolumnshavebeencut-offinthe

outputbelow.# psaxuf

116

LinuxHackAjayKumarTiwariroot

root

root

root

root

root

root

root

root

511

511

Oct14

Oct14

Oct14

Oct14

Oct14

Oct14

Dec03

Dec22

23:35

23:35

0:00/opt/VRTSralus/bin/beremote

0:00\_/opt/VRTSralus/bin/beremote





0:01/usr/local/sbin/sshd

1:08/usr/local/sbin/sshd

0:00\_/usr/local/sbin/sshd

0:00\_-bash

\_psaxuf

Note:Youcanalsousepstreecommandtodisplayprocessintreestructure.

ViewProcessesOwnedbyaParticularUser

ThefollowingcommanddisplaysalltheprocessownedbyLinuxuser-name:

oracle.

$ psUoracle

PIDTTY

5014?

7124?

8206?

8852?

8854?

8911?

STAT

Ss

Ss

Ss

Ss

Ss

Ss

TIMECOMMAND

0:01/oracle/bin/tnslsnr

0:00ora_q002_med

0:00ora_cjq0_med

0:01ora_pmon_med

0:00ora_psp0_med

0:02oraclemed(LOCAL=NO)

ViewProcessesOwnedbyCurrentUserFollowingcommanddisplaysalltheprocessownedbythecurrentuser.

$ psU$USER

PIDTTY

10329?

10330pts/1

STAT

S

Ss

TIMECOMMAND

0:00sshd:ramesh@pts/1,pts/2

0:00-bash

117

LinuxHack10354pts/2

10530pts/1

Ss+

R+

0:00-bash

0:00psUramesh

AjayKumarTiwari

Hack92.DfCommanddfcommand(diskfree)displaystheamountoftotalandfreediskspace

availableonthemountedfilesystems.

Syntax:df[options][name]

HowmuchGBofdiskspaceisfreeonmysystem?Usedf-hasshownbelow.Option-hdisplaysthevaluesinhumanreadable

format(forexample:KforKb,MforMbandGforGb).Inthesampleoutput

below,/filesystemhas17GBofdiskspaceavailableand/home/user

filesystemhas70GBavailable.

# df–h

Filesystem

/dev/sda1

/dev/sdb1

Size

64G

137G

UsedAvailUse%Mountedon

44G17G73%/

67G70G49%/home/user

WhattypeoffilesystemdoIhaveonmysystem?Option-TwilldisplaytheinformationaboutthefilesystemType.Inthis

example/and/home/userfilesystemsareext2.Option-awilldisplayallthe

filesystems,includingthe0sizespecialfilesystemusedbythesystem.

# df-Tha

Filesystem

Type

SizeUsedAvailUse%Mountedon

118


/dev/sda1ext264G44G17G73%/

/dev/sdb1ext2137G67G70G49%/home/user

noneproc000-/proc

nonesysfs000-/sys

nonedevpts000-/dev/pts

nonetmpfs2.0G02.0G0%/dev/shm

Hack93.KillCommandkillcommandcanbeusedtoterminatearunningprocess.Typicallythis

commandisusedtokillprocessesthatarehangingandnotresponding.

Syntax:kill[options][pids|commands]

Howtokillahangingprocess?First,identifytheprocessidoftheparticularprocessthatyouwouldliketo

killusingthepscommand.Onceyouknowtheprocessid,passitasa

parametertothekillcommand.Theexamplebelowshowshowtokillthe

hangingapachehttpdprocess.Pleasenotethattypicallyyoushoulduse

“apachectlstop”tostopapache.

# psaux|grephttpd

USERPID%CPU%MEMVSZRSSTTYSTATSTARTTIMECOMMAND

apache311860.01.62373617556?SJul26


apache311870.01.32064014444?SJul26


# kill3118631187

119


Pleasenotethattheabovecommandtriestoterminatetheprocessgraciously

bysendingasignalcalledSIGTERM.Iftheprocessdoesnotgetterminated,

youcanforcefullyterminatetheprocessbypassingasignalcalledSIGKILL,

usingtheoption-9asshownbelow.Youshouldeitherbetheownerofthe

processoraprivilegedusertokillaprocess.

# kill-93118631187

Anotherwaytokillmultipleprocesseseasilyisbyaddingthefollowingtwo

functionstothe.bash_profile.functionpsgrep()

{

psaux|grep“$1”|grep-v‘grep’

}

functionpsterm()

{

[${#}-eq0]&&echo“usage:$FUNCNAMESTRING”&&return0

localpid

pid=$(psax|grep“$1”|grep-vgrep|awk‘{print$1}’)

echo-e“terminating‘$1’/process(es):\n$pid”

kill-SIGTERM$pid

}

Nowdothefollowing,toidentifyandkillallhttpdprocesses.# psgrephttp

USER

COMMAND

apache

Jul26

apache

Jul26

PID%CPU%MEM

31186

0:40

31187

0:37

VSZ

RSSTTY

STATSTART

TIME

S

S

0.01.62373617556?


0.01.32064014444?


# pstermhttpd

terminating‘httpd’/process(es):

31186

120

LinuxHack31187

AjayKumarTiwari

Hack94.DuCommandducommand(diskusage)willprintthefilespaceusageforaparticular

directoryanditssubdirectories.

Howmuchspaceistakenbymyhomedirectoryandallits

subdirectories?Inthefollowingexample,option-sstandsforsummaryonly.i.eitdisplays

onlythetotalsizeof/home/jsmithandnottheindividualsizesofallthe

subdirectoriesinsidethe/home/jsmith.Option-hdisplaystheinformationin

ahumanreadableformat.i.eKforKB,MforMBandGforGB.The~

indicatestheuserhomedirectory.Thiscommandissameas“du-sh

/home/jsmith”

# du-sh~

320M/home/jsmith

Togetthesubdirectoriesunder/home/jsmithlisted,executetheabove

commandwithoutthesoption.

Hack95.lsofcommands.Lsofstandsforlsopenfiles,whichwilllistalltheopenfilesinthesystem.

Theopenfilesincludenetworkconnection,devicesanddirectories.The

outputofthelsofcommandwillhavethefollowingcolumns:

o

o

o

COMMANDprocessname.

PIDprocessID

USERUsername

121

LinuxHacko

o

o

o

o

o

FDfiledescriptor

TYPEnodetypeofthefile

DEVICEdevicenumber

SIZEfilesize

NODEnodenumber

NAMEfullpathofthefilename.

AjayKumarTiwari

ViewallopenfilesofthesystemExecutethelsofcommandwithoutanyparameterasshownbelow.

# lsof|more

COMMANDPIDUSERFDTYPEDEVICESIZENODE

NAME

init1rootcwdDIR8,140962/

init1rootrtdDIR8,140962/

init1roottxtREG8,132684983101/sbin/init

init1rootmemREG8,1106397166798/lib/ld-

2.3.4.so

init1rootmemREG8,11454802166799

/lib/tls/libc-2.3.4.so


/lib/libsepol.so.1


/lib/libselinux.so.1

init1root10uFIFO0,13972/dev/initctl

migration2rootcwdDIR8,140962/

skipped…

Thelsofcommandbyitselfwithoutmayreturnlotofrecordsasoutput,

whichmaynotbeverymeaningfulexcepttogiveyouaroughideaabouthow

122


manyfilesareopeninthesystematanygivenpointofviewasshownbelow.

# lsof|wc-l

3093

ViewopenfilesbyaspecificuserUselsof–uoptiontodisplayallthefilesopenedbyaspecificuser.

# lsof–uramesh

vi7190ramesh

475196/bin/vi

txt

REG

8,1

474608

sshd7163ramesh3uIPv615088263

TCPdev-db:ssh->abc-12-12-12-12.socal.res.rr.com:2631

(ESTABLISHED)

Asystemadministratorcanusethiscommandtogetsomeideaonwhatusers

areexecutingonthesystem.

ListUsersofaparticularfileIfyouliketoviewalltheuserswhoareusingaparticularfile,uselsofas

shownbelow.Inthisexample,itdisplaysalluserswhoarecurrentlyusingvi.#lsof/bin/vi

COMMANDPID

vi7258

vi7300

USERFD

roottxt

rameshtxt

TYPEDEVICESIZENODENAME

REG8,1474608475196/bin/vi

REG8,1474608475196/bin/vi

123


Hack96.SarCommandSarcommandscomeswiththesysstatpackage.Makesuresysstatisinstalled.

Ifyoudon’thavesarinstalledonyoursystem,getitfromSysstatproject.

Sarisanexcellentmonitoringtoolthatdisplaysperformancedataofpretty

mucheveryresourceofthesystemincludingCPU,memory,IO,paging,

networking,interruptsetc.,

SarCollects,Reports(displays)andSavestheperformancedata.Letuslook

atallthethreeaspectsseparately

Sadc-Systemactivitydatacollector/usr/lib/sadc(Systemactivitydatacollector)commandcollectsthesystem

dataataspecifiedtimeinterval.Thisusesthedailyactivitydatafilethatis

locatedunder/va/log/sa/sa[dd],whereddisthecurrentday.

Sa1shell-script/usr/lib/sa1in-turncallsthe/usr/lib/sadcs.sa1isinvokedfromthecrontab

asshownbelow.Runthisevery5minutesor15minutesdependingonyour

need.Iprefertoscheduleitforevery5minutesinthecrontabasshown

below.

*/5****root/usr/lib/sa/sa111

Sa2shell-script/usr/lib/sa2isashellscriptthatwillwriteadailyreportinthe

/var/log/sa/sa[dd]file,whereddisthecurrentday.Invokethesa2fromthe

crontabonceadayatmidnight.

#5923***root/usr/lib/sa/sa2–A

124


Note: /etc/cron.d/sysstatfilescomeswiththesysstatpackagethatincludes

somedefaultvalueforthesa1andsa2,whichyoucanchangeaccordingly.

DisplayCPUStatisticsusingSarCommand# sar–u

Linux2.6.9-42.ELsmp(dev-db)

12:00:01AMCPU%user%nice

12:05:01AMall3.700.00

12:10:01AMall4.590.00

12:15:01AMall3.900.00

12:20:01AMall4.060.00

12:25:01AMall3.890.00

12:30:01AMall3.890.00

Skipped..

Average:all

4.56

0.00

1.00

0.15

94.29

01/01/2009

%system%iowait

0.850.00

1.190.06

0.950.04

1.000.01

0.870.00

0.870.00

%idle

95.45

94.16

95.11

94.93

95.23

95.23

Note: Ifyouneedabreakdownoftheperformancedatafortheindividual

CPU’s,executethefollowingcommand.

#sar-u-PALL

DisplayDiskIOStatisticsusingsarcommand# sar–d


12:00:01AMDEV

12:05:01AMdev2-0

12:10:01AMdev8-1

Skipped..

Average:

Average:

dev2-0

dev8-1

4.66

1.89

120.77

3.17

69.45

8.02

01/01/2009

tpsrd_sec/s

1.651.28

4.088.11

wr_sec/s

45.43

21.81

125


DisplaynetworkingStatisticsusingsarcommand# sar-nDEV|more


12:00:01AMIFACErxpck/s

rxcmp/stxcmp/

srxmcst/s

12:05:01AMlo0.17

0.000.0

00.00

12:10:01AMeth052.92

0.000.0

00.00

01/01/2009

txpck/srxbyt/s

txbyt/s

0.16

25.31

23.33

53.64

10169.74

12178.57

# sar-nSOCK|more


12:00:01AMtotscktcpsck

12:05:01AM5013

12:10:01AM5013

12:15:01AM5313

01/01/2009

udpsckrawsck

30

40

50

ip-frag

0

0

0

Hack97.vmstatCommandForatypicalperformancemonitoringallyouneedisonlyvmstatcommand.

Thisdisplaymemory,swap,IO,systemandcpuperformanceinformation.

Thefollowingcommandexecutesvmstatevery1secondfor100times.

# vmstat1100

procs–––—memory–––-–swap—–—io–-—system—–-cpu–-

rbswpdfreebuffcachesiso

00

00

02821201341085797012

02821201341085797012

126

0

0

bi

0

0

boin

0

0

2

0

csussyidwa

0001000

01007359001000

LinuxHack00

00

02821201341085797012

02821201341085797012

0

0

0

0

0

0

AjayKumarTiwari

01117577001000

01007366001000

VmstatprocsSectiono

o

rfield:Totalnumberofrunnableprocess

bfield:Totalnumberofblockedprocess

Memorysectiono

o

o

o

Swpdfield:Usedswapspace

Freefield:AvailablefreeRAM

Bufffield:RAMusedforbuffers

Cachefield:RAMusedforfilesystemcache

SwapSectiono

o

Sifield:Amountofmemoryswappedfromdiskpersecond

Sofield:Amountofmemoryswappedtodiskpersecond

IOSectiono

o

Bifield:Blocksreceivedfromdisk

Bofield:Blockssenttodisk.

SystemSectiono

o

Infield:Numberofinterruptspersecond.

Csfield:Numberofcontextswitchespersecond.

127


CPUSectiono

o

o

o

Usfield:Timespendrunningusercode.(non-kernelcode)

Syfield:Timespentrunningkernelcode.

Idfield:Idletime.

Wafield:TimespentwaitingfortheIO

Hack98.NetstatCommandNetstatcommanddisplaysthenetworkrelatedinformationsuchasnetwork

connections,routingtables,interfacestatistics.Followingarefewexamples

onhowtousenetstatcommand.

DisplayActiveInternetConnectionsanddomainsocketsusing

netstat# netstat–an

ActiveInternetconnections(serversandestablished)

ProtoRecv-QSend-QLocalAddressForeign

AddressState

tcp000.0.0.0:56660.0.0.0:*

LISTEN

tcp000.0.0.0:1110.0.0.0:*

LISTEN

tcp000.0.0.0:40860.0.0.0:*

LISTEN

skipped..

ActiveUNIXdomainsockets(serversandestablished)

ProtoRefCntFlagsTypeStateI-NodePath

unix2[ACC]STREAMLISTENING7894

/tmp/.font-unix/fs7100


/tmp/.gdm_socket


@/tmp/fam-root-

128


DisplayActiveConnectionswithProcessIDandProgramNameThiscouldbeveryhelpfultoidentifywhichprogramhasinitiatedaspecific

networkconnection.

# netstat-tap

ActiveInternetconnections(serversandestablished)

ProtoRecv-QSend-QLocalAddressForeign

AddressStatePID/Programname

tcp00*:nrpe*:*

LISTEN16277/xinetd

tcp00localhost.localdomain:smtp*:*

LISTEN7263/sendmail:acce

tcp340localhost.localdomain:54221

localhost.localdomain:4089CLOSE_WAIT29881/httpd

tcp03216dev-db:sshcpe-76-

94-215-154.soca:4682ESTABLISHED11717/sshd:ramesh

DisplayRoutingTable# netstat–-route

KernelIProutingtable

DestinationGateway

WindowirttIface

192.168.1.0*

0eth0

162.244.0.0*

0eth0

default192.168.1.1

0eth0

Genmask

255.255.255.0

255.255.0.0

0.0.0.0

U

Flags

MSS

00

U

UG

00

00

DisplayRAWnetworkstatistics# netstat—statistics—raw

Ip:

129


11080343totalpacketsreceived

0forwarded

1withunknownprotocol

0incomingpacketsdiscarded

11037744incomingpacketsdelivered

11199763requestssentout

Icmp:

577135ICMPmessagesreceived

64inputICMPmessagefailed.

ICMPinputhistogram:

destinationunreachable:537

timeoutintransit:65

sourcequenches:2

echorequests:576476

echoreplies:12

timestamprequest:3

addressmaskrequest:3

581558ICMPmessagessent

0ICMPmessagesfailed

ICMPoutputhistogram:

destinationunreachable:5079

echoreplies:576476

timestampreplies:3

MiscNetstatCommandso

#netstat–-tcp–-numeric ListofTCPconnectiontoandfromthe

machine.

#netstat—tcp—listening–-programs DisplayTCPportthatthe

serverislisteningonalongwiththeprogramthatislisteningon

thatparticularport.

#netstat–rnC Displaytheroutingcache

o

o

Hack99.SysctlCommandLinuxkernelparametercanbechangedontheflyusingsysctlcommand.

130


SysctlhelpstoconfiguretheLinuxkernelparametersduringruntime.

# sysctl–a

dev.cdrom.autoclose=1

fs.quota.writes=0

kernel.ctrl-alt-del=0

kernel.domainname=(none)

kernel.exec-shield=1

net.core.somaxconn=128

net.ipv4.tcp_window_scaling=1

net.ipv4.tcp_wmem=409616384

net.ipv6.route.mtu_expires=600

sunrpc.udp_slot_table_entries=16

vm.block_dump=0

131072

ModifyKernelparameterin/etc/sysctl.confforpermanentchangeAftermodifyingthekernelparameterinthe/etc/sysctl.conf,executesysctl–

ptocommitthechanges.Thechangeswillstillbethereafterthereboot.

#vi/etc/sysctl.conf

# sysctl–p

ModifykernelparametertemporarilyTotemporarilymodifyakernelparameter,executethefollowingcommand.

Pleasenotethatafterrebootthesechangeswillbelost.

#sysctl–w{variable-name=value}

131


Hack100.NiceCommandKerneldecideshowmuchprocessortimeisrequiredforaprocessbasedon

thenicevalue.Possiblenicevaluerangeis:-20to20.Aprocessthathasa

nicevalueof-20isveryhighpriority.Theprocessthathasanicevalueof20

isverylowpriority.

Usepsaxltodisplaythenicevalueofallrunningprocessasshownbelow.# psaxl

FUIDPIDPPIDPRINIVSZRSSWCHANSTATTTY

TIMECOMMAND

40101602172552-S?

0:17init[5]

1031341900ksoftiSN?

3:18[ksoftirqd/0]

101015-1000workerS<?

0:01[events/0]

405145125103212418592-SNs?

0:08/usr/bin/python/usr/bin/rhn-applet-gui—sm-client-id

default4

40514751421603528604-S?

0:00/sbin/pam_timestamp_check-droot

1503175524180160142083920-S?

0:01/home/www/apache2/bin/httpd-f

/home/www/apache2/conf/httpd.conf-kstart

Howtoassignalowprioritytoashell-script?(highernicevalue)Intheexamplebelow,whenIstartedthenice-test.shscriptinthe

background,ittookthenicevalueof0.

$./nice-test.sh&

[3]13009

$psaxl|grepnice-test

05091300912863170

4652

972wait

S

132

LinuxHackpts/1

AjayKumarTiwari

0:00/bin/bash./nice-test.sh

[Note:6thcolumnwithvalue0isthenice.]

Now,letusexecutethesameshellscriptwithadifferentnicevalueasshown

below.

$ nice-10./nice-test.sh&

[1]13016


0509130161286330104236968wait

pts/10:00/bin/bash./nice-test.sh

SN

[Note:6thcolumnwithvalue10isthenicevaluefor

theshell-script.]

Howtoassignahighprioritytoashell-script?(Lowernicevalue)Inthefollowingexample,letusassignanicevalueof-10(minus10)tothe

nice-test.shshellscript.

$ nice—10./nice-test.sh&

[1]13021

$nice:cannotsetpriority:Permissiondenied

Note: Onlyrootusercansetanegativenicevalue.Loginasrootandtrythe

same.Pleasenotethatthereisadoubledashbeforethe10inthenice

commandbelow.

# nice—10./nice-test.sh&

[1]13060

#psaxl|grepnice-test

40130601302410-105388964wait


133

S<


[Note:6thcolumnwithvalue-10isthenicevalueof

theshell-script.]

Hack101.ReniceCommandRenicealterstheschedulingpriorityofarunningprocess.

Howtodecreasethepriorityofarunningprocess?(Increasenice)Intheexamplebelow,anexistingshell-scriptisrunningatnicevalueof10.

(6thcolumninthepsoutput)


0509132451321630105244968wait


SN

Toincreasethenicevalue(thusreducingthepriority),executetherenice

commandasshownbelow.

$ renice16-p13245

13245:oldpriority10,newpriority16


0509132451321636165244968wait


SN

[Note:Now,the6thcolumnofthenice-test.sh(PID

13245)showsthenewnicevalueof16.]

Howtoincreasethepriorityofarunningprocess?(Decreasenice)Intheexamplebelow,anexistingshell-scriptisrunningatanicevalueof10.

134

LinuxHack(6thcolumninthepsoutput)

AjayKumarTiwari


0509132541321630104412968wait


SN

Inincreasethepriority,givealowernicevalueasshownbelow.However,

onlyrootcanincreasethepriorityofarunningprocess,elseyou’llgetthe

followingerrormessage.

$ renice5-p13254

renice:13254:setpriority:Permissiondenied

Loginasroottoincreasethepriorityofarunning

process

$su-

# renice5-p13254

13254:oldpriority10,newpriority5

#psaxl|grepnice-test

050913254132162554412968wait


SN

[Note:The6thcolumnnowshowsalowernicevalueof5

(increasedpriority)]

135

LinuxHackAjayKumarTiwa

LinuxHack

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Linux Hack - index-of.co.ukindex-of.co.uk › Hacking › Linux Hacker by Mr Ajay Kumar Tiwari.pdf · Linux Hack Ajay Kumar Tiwari Foreword Another collection of hacks? Yes! If you