Linux Commands

chmod command

Purpose:

chmod stands for "change mode" and is used to modify the permissions of files and directories.

It controls who can read, write, and execute (run) files or access and traverse directories.

Key Concepts:

Permissions: Each file or directory has three sets of permissions:

Owner: The user who owns the file.

Group: The group to which the file belongs.

Others: Everyone else on the system.

Read (r): Allows viewing the contents of a file or listing directory contents.

Write (w): Allows modifying a file or adding/removing files within a directory.

Execute (x): Allows running a file as a program or entering a directory.

Syntax:

Bash

chmod [options] mode file(s)

Common Options:

-R: Applies changes recursively to files and directories within a directory.

-v: Provides verbose output, listing changed files.

-c: Lists only files whose permissions would be changed.

Specifying Permissions:

• Symbolic mode: Uses letters to represent permissions:

  • u for user, g for group, o for others, a for all.

  • + to add permissions, - to remove permissions, = to set specific permissions.

  • Example: chmod u+x myscript.sh (adds execute permission for the owner)

• Octal mode: Uses a three-digit number to represent permissions:

  • Each digit represents read, write, and execute permissions for each class (user, group, others). 

  • Example: chmod 755 myfile.txt (gives read, write, and execute permissions to owner, read and execute to group and others)

Common Examples:

• Make a script executable: chmod +x myscript.sh

• Allow everyone to read a file: chmod ugo+r myfile.txt

• Make a directory accessible to group members: chmod g+rx mydirectory

Remember:

• Use ls -l to view current permissions before making changes.

• Use sudo with chmod if you need administrative privileges.

• Exercise caution, as incorrect permissions can cause security issues or prevent access to files.