Chmod Command Examples in Unix / Linux

Chmod (change mode) is one of the most frequently used commands in unix or linux operating system. The chmod command is used to change the file or directory access permissions. To know about the access permissions of a file or directory, use the ls -l command as shown below:

$ ls -l sample.sh
-rwx-rw-r-- 1 matt deploy 94 Oct  4 03:12 sample.sh

Here in the above example: Use matt has the read, write and execute permissions on the file. Group deploy has read and write permissions. Others have only the read permission.
File and Directory permissions:

There are three different permissions. They are: 

◈ Read (4): Permitted to read the contents of the file. In case of directory, you can view all the files and sub-directories in that directory.
◈ Write (2): Permitted to write to the file. In case of directory, you can create files and sub-directories.
◈ Execute (1): Execute the file as a program/shell script. In case of directory, You can enter into that directory.

Here in the above, the numbers in the brackets represents the numeric values for the corresponding permissions. If you want to have a combination of permissions add the required numbers. For example, for read and execute, it is 4+1=5.

The syntax of chmod command is 

chmod [options] mode filename

THe important options are:

-R : recursively change the permissions of a directory.
-v : Verbose

Chmod Examples in Linux / Unix: 

1. Give read, write and execute permissions to everyone. 

Read, write and execute: 4+2+1=7

$ chmod 777 sample.sh

In the above example, you can see that the permissions are specified with a three digit number. The first digit is for user permissions, second is for group and third is for others permission. This type of representation is called octal representation. Alternatively, you can use the symbolic representation to give the permissions.

chmod ugo+rwx sample.sh

We will see more details about the symbolic representation later.

2. Give read permission to user, write permission to group and execute permission to others. 

$ chmod 421 sample.sh

3. Recursive permissions to directory 

To give read and write permissions to all the users to a directory (including files and subdirectories) use the recursive option -R.

chmod -R 666 /dir

Symbolic Representation of Permissions: 

The following symbols are used to represent the users, groups and others:

u : User
g : Group
o : Others a : All (user, group and others)

The following symbols represent the permissions:

r : read
w : write
x : execute

The following symbols represent the permissions grant or revoke:

+ : Additional permissions. Selected permissions are added.
- : Revoke the permissions. Selected permissions are revoked.
= : Specific permissions. Only selected permissions are assigned.

Examples: 

1. Remove write permission from group 

$ chmod g-w sample.sh

This will only removes the write permission for the group.

2. Add new permission execute to others 

$ chmod o+x sample.sh

In addition to the existing permissions, this will add execute permission to others.

3. Give only read permissions to the user 

$ chmod u=w sample.sh

This will remove the existing permissions to the user and gives only write permission to the user.