Lecture on Linux 1

7/21/2019 Lecture on Linux 1

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Going with tradition

Background and History Unix

GNU : Richard Stallman

Linux : Linus Torvalds Unix philosophy

Small tools can do big tasks

Write programs that do one thing and do it well. Write programs to work together.

Everything is a file


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Organization of File System I

/ The root

/bin

Binaries

/dev

Even your devices are mapped to files

/proc

Your processes are files too :)


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Organization of File System II

/etc Configuration files

/lib

Essential System Libraries /home

Your home directories

/root

Root's home directory


5/19

Organization of File System III

/sbin System Binaries

/tmp

Temporary files /usr

User tools and binaries

/var

Variable data files including logs

/opt


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Users I

/etc/passwd [DEMO]

Every user on a Linux system has

a username

a user ID, or UID for short an integer between 0 and 65534

a group ID, or GID for short an integer between 0 and 65534

a home directory

a default shell (what s a shell? comes later!)

memberships to one (or more groups)

a password!!

root

The God of a Linux system called the super-user

Has UID=0

Has GID=0

Has access to everything


7/19

Users II

What are groups? A collection of users which would be sharing ownership to some

common service, directory or files.

/etc/groups [DEMO]

One user can belong to many groups and one group cancontain many users


8/19

Permissions I

Every file and directory has permissions Permissions with respect to

User (u)

Group (g) Others (o)

Permissions

read (r)

write (w)

execute (x)

rwxrwxrwx

Owner Group Other


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Meaning of rwx Read, write, execute mean different things wrtdirectories and files

Command Permissions on file Permissions on directory

cd /home/abhask N/A xls /home/abhask/*.c none r

ls -l /home/abhask/*.c none rx

cat myfile r x

cat >>myfile w x

./runme (executable) x x

./cleanup.sh (script) rx xrm filename none x

touch newfile N/A x

Permissions II


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Permissions III

Notations r = 4

w = 2

x = 1 Example:

rwxr-x--x

rwx => 4+2+1=7 r-x => 4+0+1=5

--x => 0+0+1=1

Permissions are 751


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Special Permissions

Sticky Bit t = 1000

Set UID

s = 4000 Running a program with privileges of the owner

Set GID

s = 2000

t-wxr-xr-x

1355

but shown as: -wxr-xr-t


12/19

Checking Permissions

Checking permissions of files [DEMO] ls -l

More about the ls command later


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Lets change permissions

chmod [options] Mode[,Mode] file

some option

-R

--reference=file

Modes [ugoa] [+-=][rwxXstugo]

Octal notation

Examples chmod 755 file.c

chmod u=rwx,g=rx,o=rx file.c

DEMO


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Changing Owner

chown [options] owner[:group] file Options

-R = recursive

--reference=rfile

owner

can be either string or UID

group

optional

can be either string or GID

DEMO

h


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Change group

chgrp [option] group file

option

-R = recursive

--reference=rfile

group can be either string or GID

DEMO


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Processes ps to view processes

ps ax ps aux

DEMO

top to view processes kill killing processes by pid

kill PID1 PID2 PID3 ...

killall killing processes by name killall NAME1 NAME2 NAME3 ...

options

-e, -g, -v

O h B i C d


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Other Basic Commands

rm [options] FILE .. options

-R = recursive

-f = force

-i = interactive

mv [options] SRC DEST

options

-f = force

-i = interactive

-b = backup

-u = update

G i h l !


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Getting help!

Manual pages getting help in Linux

man [-k] command

info [menu-item]

Dr. Google! Read this!

man man

info info

A i t


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Assignment

Run mozilla, and try to figure out why killall -e

mozilla doesnt work?

Why is the set UID bit set for su command?

How do you check the man page for /etc/passwd ?

How do you list all the files in a particular directory(including subdirectories)? (no use of cd)

What commands can be used for compressingfiles in linux? (Hint, use man -k).

Documents

Lecture on Linux 1