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Linux File Permissions Commands

Overview

File permissions in Linux control the access to files and directories. Permissions determine who can read, write, or execute files. This document covers basic commands for viewing and modifying file permissions.

Viewing Permissions

ls -l

Lists files and directories with detailed information, including permissions.

ls -l

Example

ls -l
# Output: Lists files and directories with permissions, owner, group, size, and modification date
# Example output:
# -rwxr-xr-- 1 user group 1234 Aug  3 10:00 example.txt

Understanding File Permissions

File permissions are represented by a string of 10 characters:

-rwxr-xr--
  • File Type: - (regular file), d (directory), l (symlink)
  • Owner Permissions: rwx (read, write, execute)
  • Group Permissions: r-x (read, execute)
  • Others Permissions: r-- (read)

Changing Permissions

chmod

Changes the permissions of a file or directory.

chmod [options] <permissions> <file>

Example

chmod 755 example.txt
# Sets the permissions of 'example.txt' to rwxr-xr-x (owner: read, write, execute; group and others: read, execute)

Numeric Mode

  • 7: rwx (read, write, execute)
  • 6: rw- (read, write)
  • 5: r-x (read, execute)
  • 4: r-- (read)
  • 3: wx- (write, execute)
  • 2: w-- (write)
  • 1: --x (execute)
  • 0: --- (no permissions)

Example

chmod 644 example.txt
# Sets the permissions of 'example.txt' to rw-r--r-- (owner: read, write; group and others: read)

Symbolic Mode

  • u: user (owner)
  • g: group
  • o: others
  • a: all

Use + to add permissions, - to remove, and = to set exact permissions.

Example

chmod u+x example.txt
# Adds execute permission for the owner

chmod go-w example.txt
# Removes write permission for group and others

chown

Changes the owner and/or group of a file or directory.

sudo chown [owner][:group] <file>

Example

sudo chown john:admins example.txt
# Changes the owner of 'example.txt' to 'john' and the group to 'admins'

chgrp

Changes the group ownership of a file or directory.

sudo chgrp <group> <file>

Example

sudo chgrp admins example.txt
# Changes the group ownership of 'example.txt' to 'admins'

Special Permissions

Setuid (Set User ID)

When set on an executable file, the process runs with the permissions of the file's owner.

chmod u+s <file>

Example

chmod u+s /usr/bin/someprogram
# Sets the setuid bit on 'someprogram'

Setgid (Set Group ID)

When set on a directory, new files and directories inherit the group of the directory.

chmod g+s <directory>

Example

chmod g+s /var/shared
# Sets the setgid bit on the '/var/shared' directory

Sticky Bit

When set on a directory, only the file owner can delete files within that directory.

chmod +t <directory>

Example

chmod +t /tmp
# Sets the sticky bit on the '/tmp' directory

Summary

Understanding and managing file permissions is crucial for maintaining security and proper access control in Linux. These commands allow you to view and modify permissions, ownership, and special attributes of files and directories. For more detailed information on each command, refer to the Linux manual pages.