1Week 4 - Jan 31, 2005 Week 4 Agenda UNIX Directory Structure Absolute pathname Relative pathname Permissions chmod (symbolic/absolute)

11 Week 4 - Jan 31, 2005Week 4 - Jan 31, 2005

Week 4 AgendaWeek 4 Agenda

UNIX Directory StructureUNIX Directory Structure

Absolute pathnameAbsolute pathname

Relative pathnameRelative pathname

PermissionsPermissions

chmod (symbolic/absolute)chmod (symbolic/absolute)

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The UNIX system is structured The UNIX system is structured hierarchically (upside down tree-like).hierarchically (upside down tree-like).

You can have any number of files and You can have any number of files and subdirectories under it organized in any subdirectories under it organized in any way. This structure leads to a parent-child way. This structure leads to a parent-child relationship between a directory and its relationship between a directory and its sub-directories.sub-directories.

http://cs.senecac.on.ca/%7Efac/int120/labs/unixnotes.html

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The file system structure starts with one The file system structure starts with one main directory, called the root directory.main directory, called the root directory.

From the root. At the end of each path is From the root. At the end of each path is an ordinary file or a directory file. an ordinary file or a directory file.


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Ordinary file VS Directory FileOrdinary file VS Directory File

Ordinary files (files) are at the end of paths Ordinary files (files) are at the end of paths and cannot support other paths whileand cannot support other paths while

Directory files (directories) are the points Directory files (directories) are the points that other paths can branch from. that other paths can branch from. Directories directly connected by a path Directories directly connected by a path called parents (closer to the root) and called parents (closer to the root) and children (farther from the root)children (farther from the root)


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//Root directoryRoot directory – – the ancestor of all directories and the start of all absolute pathnames.

~~ or or $HOME$HOME

Home directoryHome directory - - the working directory when you first login

Different DirectoriesDifferent Directories


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Different DirectoriesDifferent Directories

..Current working directoryCurrent working directory – t – the directory under which you are working

. .. .Parent directoryParent directory – t – the directory above the working directory


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Pathname – Absolute VS RelativePathname – Absolute VS Relative

Absolute pathnameAbsolute pathname – A pathname that – A pathname that starts with root directory. It locates a file starts with root directory. It locates a file without regard to the working directory.without regard to the working directory.

Relative pathnameRelative pathname – A pathname that – A pathname that starts from the working directory.starts from the working directory.

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s im p son s flin s ton es

cartoon s ac tion _ h eros

th e_ p en g u in

th e_ joker

b ad _ g u ys

/(roo t)

Your working directory is the_joker and home directory is action_herosYour working directory is the_joker and home directory is action_heros1. Copy flinstones to your working directory by using relative pathname1. Copy flinstones to your working directory by using relative pathname2. Copy flinstones to your working directory by using absolute pathname2. Copy flinstones to your working directory by using absolute pathname3. Delete simpsons by using relative pathname3. Delete simpsons by using relative pathname4. Rename cartoons to animation by using absolute pathname4. Rename cartoons to animation by using absolute pathname

Pathname – ExamplePathname – Example

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Pathname – ExamplePathname – Example/ (root)

JLoBTaylor

BenAffleck

MattDamon

VCerfPuffy

EMendes

GPaltrowBerners_LeeLTorvalds

KThompson

DRitchie

directory file

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PermissionsPermissions- objectives- objectives1. How do you find out what

permissions are on a file?

2. How do you change permissions

3. Directory Permissions

4. Default Permissions

5. What permissions do you need to do the following tasks...?

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Key Concepts – File Permissions

• UNIX security for users and for the system is based on who has access to files and directories • Setting “permissions” on a file controls who can read, write to or execute a file.• Use the ls -l command to see the permissions on a file

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Your grocery lists andyour Top Secret Buried Treasure maps

should probably have different permissions!

Warning!

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The output of the ls - l command will look

like this:

total 66

-r-xr-x- - - 1 ling.zhu users 30 Mar26 19:49 colours

drwxr-xr-x 4 ling.zhu sys 512 Apr 1 17:41 comfind

Key Concepts - File Permissions

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It is10 characters long.example:[ling.zhu@zenit ling.zhu]>$ ls -l -r-xr-x- - - 1 ling.zhu users 54 Sep 26 19:49 lab2

Permissions Field

The Permission Field

- the first field in the long listing entry

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file type

- r-x r-x - - -

The Permission Field- The first character position

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The first character position represents the file type:- a normal filed a directoryl a soft or symbolic link s a socketc a character-special deviceb a block-special device


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-r-xr-x- - - 1 ling.zhu staff 30 Mar26 19:49 colours


- r-x r-x - - -

The next 9 characters in the Permissions Field are read as groups of 3


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Each set of three characters is read the same way:

r

w

x

an “r” in the first position gives read permission for the file

a “w” in the second position gives write permission for the file

an “x” in the third position gives execute permission for the file

The next 9 characters in the Permissions Field are read as groups of 3


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Similarly for each set of three characters....

-

-

-

a “-” in the first position means no read permission for the file

a “-” in the second position means no write permission for the file

a “-” in the third position means no execute permission for the file

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Warning!

For normal files execute permission means it is executable!

In DOS a file would have to have the proper extension in the filename to be executable. (.com, .exe , .bat)

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- r-x r-x - - -

user

The user that is the owner of a file is found in field 3 of the ls -l output.

In this case ling.zhu has read and execute permission, no write permission


The first set of three characters - indicates the permissions for the user who owns the file.

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UNIX,

If you created it.... you own it!

Remember!

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- r-x r-x - - -group

The group that the file belongs to is found in field 4 of the ls -l output. In this case users have read & execute but no write permissions


The second set of three characters - indicates the permissions for the members of the file’s group.

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- r-x r-x - - -others


In this case others have no read, no write & no execute permissions...

The third set of three characters - indicates the permissions for all others on the system.

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- r-x r-x - - -file type

usergroup others


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chmod

Name: What it does

chmod changes permissions (or access mode) of one or more files

chmod -R changes permissions recursively for a directory, all of its files and subdirectories

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1. add / remove permissions implicitly - symbolic method

2. set permissions explicitly

- absolute or octal method

2 ways to specify permissions

with the chmod command

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You need to provide 3 pieces of information:

1. WHO are you changing permissions for?

2. what OPERATION do you want to do?

3. WHAT are the permissions you want to

add/remove?

To add/remove permission

implicitly (symbolic method)

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1. WHO are you changing permissions for?

u for the user

g for the group

o for all others

a for all three



you need to provide 3 pieces of information:you need to provide 3 pieces of information:

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2. what OPERATION do you want to do?

+ (plus sign) add the permission

- (minus sign) remove the permission

= (equal sign) set it to indicated mode




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3. WHAT are the permissions you want to add/remove?

r read

w write

x execute




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- r-x r-x - - -file type

usergroup others




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- r-x r-x - - -

file typeuser group others

To add write permission for user...

1. who? u (user)

2. what operation? + (add)

3. what permission? w (write)




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- r-x r-x - - -


To add write permission for user for the file called colours the command would be:

chmod u+w colours





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- rwx r-x - - -file type

user group others

The chmod u+w colours command results in the following permissions for the file colours:

-rwxr-x- - - 1 ling.zhu users 30 Mar26 19:49 colours




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- rwx r-x - - -


To remove write permission for user, and add write permission for the group for the file called colours the command would be:

chmod u-w,g+w colours





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- r-x rwx - - -file type

user group others

The chmod u-w,g+w colours command results in the following permissions for the file colours:

-r-xrwx- - - 1 ling.zhu users 30 Mar26 19:49 colours




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Using Implicit or Symbolic Permissions!

To get the permissions exactly as you want them you may have to use a series of addition and removal commands....

you can combine them all on one line, just make sure there are no spaces between them!

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There are 2 ways to specify permissions with the chmod command:

1. add / remove permissions implicitly

2. set permissions explicitly


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Assign individual permission letters “point values”

r = 4

w = 2

x = 1

deny all = 0

Setting permissionsexplicitly = octal = absolute

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2. Setting permissions explicitly: r = 4 w = 2 x = 1

rwx = 4 + 2 + 1 = 7 read,write,execute

rw- = 4 + 2 + 0 = 6 read, write

r-x = 4 + 0 + 1 = 5 read, execute

r-- = 4 + 0 + 0 = 4 read only

-wx = 0 + 2 + 1 = 3 write, execute

-w- = 0 + 2 + 0 = 2 write only

--x = 0 + 0 + 1 = 1 execute only

--- = 0 + 0 + 0 = 0 no permission

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chmod 777 colours

- rwx rwx rwx


rwx=4+2+1=7 rwx=4+2+1=7 rwx=4+2+1=7

-rwxrwxrwx 1 ling.zhu users 30 Mar26 19:49 colours


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chmod 640 colours

- rw- r-- ---


rw-= 4+2+0=6 r--=4+0+0=4 ---= 0+0+0=0

-rw-r----- 1 ling.zhu users 30 Mar26 19:49 colours


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Using Explicit or Octal Permissions !

allows you need to specify the exact permissions you want... regardless of the current permissions

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Directory PermissionsDirectory permissions are much the same as filesDirectory permissions are much the same as files

the owner of a directory decides who has the owner of a directory decides who has access to its filesaccess to its filesdirectory permissions are assigned using the directory permissions are assigned using the same letters as for filessame letters as for files

but the meaning of the letters r,w,x are but the meaning of the letters r,w,x are different.....different.....

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ReadRead permission for a directory means you are permission for a directory means you are allowed to list the files in the directory with allowed to list the files in the directory with lsls

WriteWrite permission for a directory means you can permission for a directory means you can create or remove files in itcreate or remove files in it

ExecuteExecute permission for a directory means you permission for a directory means you can make it your working directory or can make it your working directory or pass pass throughthrough it in a path it in a path

Read & executeRead & execute = “ = “searchsearch” permissions are ” permissions are required to get a required to get a long listinglong listing..

Directory Permissions

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Question:Question:What permission do you need for a What permission do you need for a

directory in order to remove a file directory in order to remove a file under it?under it?

Directory Permissions

rw-rw- ??

-wx-wx ??

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Warning!

For directories, you need “execute” permission to search them!

... without “search” permission in a directory you need to know the exact name of a file to read it

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Question

What is the minimum permission needed to view hiding.html under public_html/sub1/subsub1/week4 ?

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Two approaches to file security:

1. Permit wide access to your files and just turn off permissions for specific files and directories when more security is needed.

2. Permit limited access to your files and turn on permissions for specific files as

needed. It’s YOUR choice!

Setting Default Permissions for New Files

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Name: What it does

umask umask nnn

displays file creation mask set file creation mask to octal value nnn

umask

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The umask command allows you to control access to new files and directories

umask acts like chmod in reverse - each digit in umask tells what default permission to turn off

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default permissions for files are:

rw-rw-rw- read, write for everyone

no execute until you debug code

default permissions for directories are:

rwxrwxrwx read,write and execute so that the directory can be accessed


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default permissions for files are:

file default rw-rw-rw-

umask of 27 0 2 7

results in rw-r----- permission 640

turn off write for group turn off all for others


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default permissions for directories are:

directory default rwx rwx rwx

umask of 27 0 2 7

results in rwxr-x--- permission 750

turn off write for group turn off all for others


Documents

1Week 4 - Jan 31, 2005 Week 4 Agenda UNIX Directory Structure Absolute pathname Relative pathname Permissions chmod (symbolic/absolute)