Chmod Command in Linux (File Permissions)

File access in Linux is controlled by managing file permissions, attributes, and ownership. By implementing this security measure, only authorized users and processes are granted access to files and directories.

This tutorial provides a comprehensive guide on utilizing the chmod command to modify the access permissions of files and directories.

Understanding Linux File Permissions

Now, let’s delve into the fundamentals of the Linux permissions model. Linux assigns ownership and permission access rights to each file for different classes of users.

• Owner of the file.
• The members of the group.
• Others (everyone else).

You can easily change file ownership by utilizing the chown and chgrp commands.

There are three file permission types that apply to each class:

• The read permission.
• The write permission.
• The execute permission.

This concept enables you to define the user permissions for reading, writing, and executing the file.

You can view file permissions by using the ls command.

ls -l filename.txt

-rw-r--r-- 12 linuxize users 12.0K Apr  8 20:51 filename.txt
|[-][-][-]-   [------] [---]
| |  |  | |      |       |
| |  |  | |      |       +-----------> 7. Group
| |  |  | |      +-------------------> 6. Owner
| |  |  | +--------------------------> 5. Alternate Access Method
| |  |  +----------------------------> 4. Others Permissions
| |  +-------------------------------> 3. Group Permissions
| +----------------------------------> 2. Owner Permissions
+------------------------------------> 1. File Type

The first character indicates the file type. It can be a conventional file (-), directory (d), a symbolic link (l) is a specific sort of file.

The following nine characters are file permissions, three triplets of three characters each. The first triplet displays the owner’s permissions, the second shows group permissions, and the final triplet displays permissions for everyone else. Permissions might indicate different things based on the file type.

The example above (rw-r--r--) indicates that the file owner has both read and write permissions (rw-), but the group and others only have read permissions (r--).

Each of the three permission triplets can be built from the following characters and have distinct effects depending on whether they are set to a file or a directory:

Effect of Permissions on Files

Effect of Permissions on Directories (Folders)

In Linux, directories are unique file types that include other files and directories.

By using chmod

This is how the chmod command usually looks:

chmod [OPTIONS] MODE FILE...

You can use a symbolic or numeric mode or a reference file with the chmod command to change a file’s rights. We’ll talk about the modes in more depth later in this piece. It’s possible for the command to be given more than one file or directory path split by a space.

It is only possible for root, the file owner, or a user with sudo rights to change a file’s access. When you use chmod, be extra careful, especially when changing rights over and over again.

The Symbolic (Text) Method

It looks like this when you use the symbolic mode with the chmod command:

Chmod THE OPTIONS [ugoa][-+=]Perms... [,] FILE...

This first group of flags, called “users flags,” tells the file’s owner which types of users can change its permissions.

• u – The file owner.
• g – The users who are members of the group.
• o – All other users.
• a – All users, identical to ugo.

If the users flag is omitted, the default one is a and the permissions that are set by umask are not affected.

The second set of flags ([-+=]), the operation flags, defines whether the permissions are to be removed, added, or set:

• - Removes the specified permissions.
• + Adds specified permissions.
• = Changes the current permissions to the specified permissions. If no permissions are specified after the =symbol, all permissions from the specified user class are removed.

The permissions (perms...) can be explicitly set using either zero or one or more of the following letters: r, w, x, X, s, and t. Use a single letter from the set u, g, and o when copying permissions from one to another users class.

When setting permissions for more than one user classes ([,…]), use commas (without spaces) to separate the symbolic modes.

Below are some examples of how to use the chmod command in symbolic mode:

• Give the members of the group permission to read the file, but not to write and execute it:

  chmod g=r filename

• Remove the execute permission for all users:

  chmod a-x filename

• Recursively remove the write permission for other users:

  chmod -R o-w dirname

• Remove the read, write, and execute permission for all users except the file’s owner:

  chmod og-rwx filename

  The same thing can be also accomplished by using the following form:

  chmod og= filename

• Give read, write and execute permission to the file’s owner, read permissions to the file’s group and no permissions to all other users:

  chmod u=rwx,g=r,o= filename

• Add the file’s owner permissions to the permissions that the members of the file’s group have:

  chmod g+u filename

• Add a sticky bit to a given directory:

  chmod o+t dirname

Numeric Method

The syntax of the chmod command when using numeric method has the following format:

chmod [OPTIONS] NUMBER FILE...

stat -c "%a" filename

644

Using a Reference File

The --reference=ref_file option allows you to set the file’s permissions to be same as those of the specified reference file (ref_file).

chmod --reference=file1 file2

chmod -R 755 /var/www

chmod 755 symlink

find /var/www/my_website -type d -exec chmod 755 {} \;
find /var/www/my_website -type f -exec chmod 644 {} \;

find /var/www/my_website -type d -exec chmod u=rwx,go=rx {} \;
find /var/www/my_website -type f -exec chmod u=rw,go=r {} \;