POiSON WORLD

What is chmod ?

chmod ( Change Mode ) is a command line utility in Unix , Linux and other Unix like systems to change the read, write, execute permissions of a file for owner , group and others.

How to use chmod?

Chmod command is used in two ways :
1. Using octal value & position : Sets the permission for owner, group and others with octal values , 4 for read , 2 for write , 1 for execute and any sum of these number to get cumulative permissions.

chmod syntax using octal mode
chmod [OPTION] MODE FILE

2. Using symbolic values to add, remove the file permission
u for user , g for group , o for others a for all ; r for read , w for write , x for execute , + , – & = for adding , removing and assigning r w x permissions.

chmod syntax for symbolic values
chmod [OPTION] MODE1,MODE2 FILE

3. chmod options

-R – Recursively change the permissions in the file under the directory.

chmod examples using octal mode :

• First column shows the chmod command ,
• second column shows how the value is calculated for the permission
• last columns of owner, group, others shows individual octal values and actual bit set on file as seen by ls -l.
• For setting any other permission combination for owner, group & other , pick corresponding value from each column and use with chmod command , for example chmod 264 file , chmod 400 file , chmod  755 file etc.

chmod examples using symbolic mode :

Symbol are used to assign the permissions :

• u – user , g – group,  o – others ,  a – all
• +  to add permission  ,  –  to remove permission ,  = to assign permission
• r w x   is used for read , write,execute ,  s  is used to set the sticky bit

Examples

• chmod  ug+x file   ;  assign execute permission to user and group in file
• chmod a+x  <file>  ;  assign execute permission to all in file
• chmod o-x  <file>  ;  remove execute permission to others in file
• chmod go+r  <file>  ;  assign read permission to group & others in file
• chmod u+rwx ,g+rw,o+r  file

Special permissions – sticky bit , setuid , setguid bit

sticky bit

when sticky bit is set the file or script is kept in swap space and loaded in to memory on next request. This makes memory loading faster.

sticky bit is represented by t in symbol mode , for example

$chmod +t file.sh

In octal mode sticky bit is represented by 1

$chmod 1755 file.sh

Set user id – setuid and set group id –  setguid bits

when these bits are set for user or groups , they are given effective permission of owner of the file during run time

for example , if file.txt is owned by root and you need to give just run time root permission to some user , you can set the setuid for file.

Following examples sets the setuid , setguid on file along with normal 755 permission.

$chmod 4755 file.txt

you need to give just run time root permission to some group of users , you can set the setgid for file.

$chmod 4755 file.txt