In this guide, you will learn about the chmod command. Linux chmod command is one of the most commonly used commands especially by system administrators when assigning modifying file and folder permissions.
It’s usually used when installing and configuring various services and features in a Linux system. The command is usually used together with a set of octal notations or alphabetical characters to change file permissions.
Linux File Permissions
Each file on the Linux systems bears a set of permissions. There are 3 permission types that are associated with a file.
- Read permissions identified by ‘r‘
- Write permissions identified by ‘w‘
- Execute permissions identified by ‘x‘
To check the file permissions of any file, use the ls command and the
-l option as shown below.
$ ls -l
For instance, to list permissions of file1.txt run the command.
$ ls -l file1.txt
Let’s break down this output
The first hyphen (-) denotes that the permissions belong to a file, and not a folder. Permissions of a folder begins with a symbol d to denote a directory as shown.
ls -l /var/www
In the output, the string
rwxr-xr-x denotes the permissions of this file. This is divided into 3 sections as follows.
- user permissions
(rwx)– This represents the permissions of the owner or user who created the file. The
rwximplies that the owner can read, write and execute the file.
- Group permissions
(r-x)-This represents permissions belonging to users in the file’s group. The r-x permissions imply that the group users can read and execute the file, but cannot write onto the file.
- Other user permissions
(r-x)– These are permissions for other users who neither belong to the two categories as discussed above. In this case, other users can only read the file.
There are 2 permission modes that can be passed to
- Octal notation
- Alphabetical characters
1. Octal notation
Consider the permissions
rwx. This implies the following:
r=4 w=2 x=1
so for instance,
rwx is the equivalent of 4+2+1 which is equal to 7.
The corresponding numerical values to each of the alphabets are added to get the file permissions.
For example, a file with permissions
rwxr-xr-- will have an octal notation of
rwx = 4+2+1 = 7 r-x = 4+0+1 = 5 r-- = 4+0+0 = 4
As you may have noted, the hyphen takes the null value and is assigned 0.
Let’s take another example:
Say we have another file with permissions
The octal notation would be calculated as follows:
rwx = 4+2+1 = 7 r-x = 4+2+0 = 6 r-- = 4+2+0 = 6
Ultimately, this would give us
766 as the corresponding octal notation to
Changing file permissions with chmod command using octal notation
To change file permissions of a file use the syntax below.
chmod [octal value] file-name
For example, to change file permissions of a file file1.txt, to say
chmod 644 file1.txt
This is illustrated in the calculation below
(user) rw- = 4+2+0 = 6 (group) r-- = 4+0+0 = 4 (others)r-- = 4+0+0 = 4
2. Alphabetical Notation
In alphabetical notation, the write permissions are segmented into 3 sections with each section bearing the
From the left, we have the following notations:
u (user) g (group) o (others) a (all)
This is better illustrated below:
Changing file permissions with chmod command using alphabetical notation
To change file permissions using alphabetical notation, use the syntax below.
chmod [user type(u/g/o/a)] [add/revoke(+/-)] [permission type(r/w/x)]
For instance to change permissions of the owner of a file to read and write, execute:
chmod u+rw file1.txt
To give write permissions to everyone, execute:
chmod a+w file1.txt
To remove the write permission for all other users, we run:
chmod o-w file1.txt
To change the permissions of a directory, we run:
chmod [permission] [directory name]
To change the permissions of a directory with its files and sub-directories recursively, we run:
chmod -R [permission] [directory name]
For example, to set the permission to
755 recursively to
/var/www/ diirectory execute the command.
chmod -R 755 /var/www
We hope this article was insightful and helped you with the basics of the chmod command usage. As always, your feedback is most welcome.