What are Linux File Permissions?

In Linux, file permissions control who can read, write, or execute a file. Permissions are fundamental to system security and user privacy. Each file or directory has a set of permissions associated with it, determining which users or groups can access or modify the file.

Understanding File Permission Representation

Linux file permissions are represented in a symbolic or numeric format. The symbolic format uses letters to denote permissions, while the numeric format uses octal numbers. Here's how they work:

• Symbolic Format: Permissions are shown as a string of characters, such as rwxr-xr--. Each character represents a permission:
  • r (read): Allows viewing the contents of the file.
  • w (write): Allows modifying the file.
  • x (execute): Allows running the file as a program.
• Numeric Format: Permissions are represented by a three-digit octal number, such as 755. Each digit corresponds to a set of permissions:
  • The first digit represents the owner’s permissions.
  • The second digit represents the group’s permissions.
  • The third digit represents others’ permissions.
  Each digit is a sum of:
  • 4 for read permission
  • 2 for write permission
  • 1 for execute permission

Changing File Permissions

You can change file permissions using the chmod command. This command allows you to modify permissions using either symbolic or numeric notation. Here are some examples:

chmod 755 filename
chmod u+x filename
chmod g-w filename
chmod o=r filename

In these examples:

• chmod 755 filename sets the permissions to rwxr-xr-x for the file named filename.
• chmod u+x filename adds execute permission for the owner of the file.
• chmod g-w filename removes write permission for the group.
• chmod o=r filename sets read-only permission for others.

Viewing File Permissions

You can view file permissions using the ls -l command. This command lists files and directories with detailed information, including permissions. The output looks like this:

ls -l
-rwxr-xr-x 1 user group 12345 Aug 13 12:34 filename

In this output:

• -rwxr-xr-x represents the file's permissions.
• 1 is the number of hard links to the file.
• user is the file's owner.
• group is the file's group.
• 12345 is the file's size in bytes.
• Aug 13 12:34 is the file's last modification date and time.
• filename is the name of the file.

Changing File Ownership

The chown command is used to change the ownership of a file or directory. Ownership can be assigned to a user and/or a group. Here are some examples:

chown user filename
chown user:group filename
chown :group filename
chown user: filename

In these examples:

• chown user filename changes the owner of the file named filename to user.
• chown user:group filename changes the owner of the file to user and the group to group.
• chown :group filename changes the group ownership of the file to group, leaving the user unchanged.
• chown user: filename changes the owner of the file to user, leaving the group ownership unchanged.

Note that only the superuser (root) can change the ownership of files that are not owned by the current user.

Viewing File Permissions

You can view file permissions using the ls -l command. This command lists files and directories with detailed information, including permissions. The output looks like this:

ls -l
-rwxr-xr-x 1 user group 12345 Aug 13 12:34 filename

In this output:

• -rwxr-xr-x represents the file's permissions.
• 1 is the number of hard links to the file.
• user is the file's owner.
• group is the file's group.
• 12345 is the file's size in bytes.
• Aug 13 12:34 is the file's last modification date and time.
• filename is the name of the file.

Differences Between chown and chmod

The chown and chmod commands are both used for managing file permissions, but they serve different purposes:

Understanding Permission Representation

File permissions are displayed in a format like -rwxr-xr--, where:

• - indicates a regular file (or d for a directory).
• rwx shows the permissions for the file's owner.
• r-x shows the permissions for the group associated with the file.
• r-- shows the permissions for others.
• 1024 is the file's size in bytes.
• Aug 13 12:34 is the file's last modification date and time.
• filename is the name of the file.

Conclusion

Understanding and managing file permissions is crucial for maintaining system security and ensuring that users have the appropriate access to files and directories. Mastering file permissions allows for better control over who can view, modify, or execute files in a Linux environment.