"chmod" Command

Learn all about chmod command

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On Linux and other Unix-like operating systems, there is a set of rules for each file which defines who can access that file, and how they can access it. These rules are called file permissions or file modes. The command name chmod stands for "change mode", and it is used to define the way a file can be accessed.

In general, chmod commands take the form:

root:~# chmod options permissions FILENAME

permissions defines the permissions for the owner of the file (the "user"), members of the group who owns the file (the "group"), and anyone else ("others"). There are two ways to represent these permissions: with symbols (alpha characters), or with octal numbers (the digits 0 through 7).

Numerical Method

2^0 = 1 -- execute
2^1 = 2 -- write
2^2 = 4 -- read

Alpha Method

w -- write
r -- read
x -- execute
u -- user
g -- group
o -- others
a -- all

+ -- Add specified permissions to the mention user/group/others/all
- -- Remove specified permissions to the mention user/group/others/all
= -- Replicate the permissions to other class of the group/user/others

root:~# ls -l /
drwxr-xr-x    2 root root  4096 Mar 21 08:51 bin
drwxr-xr-x    4 root root  4096 Mar 21 08:53 boot
drwxr-xr-x   14 root root 13880 Mar 21 08:55 dev
drwxr-xr-x  109 root root  8192 May 29 05:14 etc
-rwxr-xr-x  109 root root  8192 May 29 05:14 my_file

d/- = First character represents Type of File. 
Here 'bin','boot','dev','etc' are directory and 'my_file' is file so it will be represented by -.
rwx = Next three characters represent User's Read,Write and Execute.
rwx = Next three characters represent Group's Read,Write and Execute.
rwx = Next three characters represent Other's Read,Write and Execute.

More information can be found:

root:~# chmod --h
root:~# man chmod

Example-1

1.1 Making read only file
To make a file read only means only owner is able to read it, You have to give read permissions to owner and no permissions to others and group, so '400' is the octal representation for this.

root:~# chmod 400 my_file
root:~# ls -l my_file
-r-------- 1 root root 12 Jul 21 14:24 my_file

- = Its File.
r-- = User is able to read only but doesnt have permission for write and others.
--- = Group doesnt have permission for read, write and others.
--- = Others dont have permission for read, write and others.

1.2 Providing write access to a user

root:~# ls -l my_file
-r-------- 1 root root 12 Jul 21 14:24 my_file

root:~# chmod u+w my_file

root:~# ls -l
total 4
-rw------- 1 root root 12 Jul 21 14:28 my_file

1.3 Adding write permissions to a group

root:~# chmod g+w my_file
root:~# ls -l my_file
-rw--w---- 1 root root 12 Jul 21 14:28 my_file

1.4 Remove all permissions for group and add only read access

root:~# chmod g=r my_file
root:~# ls -l my_file
-rw-r----- 1 root root 12 Jul 21 14:28 my_file

Example-2

2.1 Apply Permission Recursively to the Directory and their contents

root:~# ls -l TEMP_DIR
-rw-r--r-- 1 root root 0 Jul 21 14:43 x
-rw-r--r-- 1 root root 0 Jul 21 14:43 y
-rw-r--r-- 1 root root 0 Jul 21 14:43 z

root:~# chmod -R 777 TEMP_DIR

root:~# ls -l TEMP_DIR
-rwxrwxrwx 1 root root 0 Jul 21 14:43 x
-rwxrwxrwx 1 root root 0 Jul 21 14:43 y
-rwxrwxrwx 1 root root 0 Jul 21 14:43 z