$ Linux Command

Files

chmod

Change file and directory permissions.

permissionsexecutablefilessecuritymode

Additional Notes

chmod changes the permission mode of files and directories. Permissions decide who can read, write, or execute a file.

Linux permissions are usually shown as three groups: user, group, and others. For example, rwxr-xr-x means the owner can read/write/execute, the group can read/execute, and others can read/execute.

Syntax

chmod [options] MODE file...

Parameters

  • options: Flags that change how chmod behaves.
  • path: File or directory path to inspect or change.
  • arguments: Names, patterns, or values used by the command.

Permission Meaning

  • r: Read permission.
  • w: Write permission.
  • x: Execute permission. For directories, this means enter/traverse the directory.
  • u: User/owner.
  • g: Group.
  • o: Others.
  • a: All users.

Symbolic Modes

  • u+x: Add execute permission for the owner.
  • g-w: Remove write permission from the group.
  • o-rwx: Remove all permissions from others.
  • a+r: Add read permission for everyone.
  • u=rw,g=r,o=: Set exact permissions symbolically.
  • X: Add execute only to directories and files that already have execute permission.

Numeric Modes

Each permission has a number:

  • 4: Read.
  • 2: Write.
  • 1: Execute.

Common modes:

  • 600: Owner can read/write. Nobody else has access.
  • 644: Owner can read/write. Group and others can read.
  • 700: Owner has full access. Nobody else has access.
  • 755: Owner has full access. Others can read and execute.
  • 777: Everyone has full access. Avoid this unless you fully understand the risk.

Common Options

  • -R, --recursive: Change permissions recursively.
  • -v, --verbose: Print what changed.
  • -c, --changes: Print only files that changed.
  • --reference=FILE: Copy permissions from another file.

Examples

chmod +x script.sh

Make a script executable.

chmod 644 config.ini

Set a normal readable config-file mode.

chmod 600 ~/.ssh/id_rsa

Protect a private SSH key.

chmod 755 deploy.sh

Allow everyone to run the script while only the owner can edit it.

chmod -R u+rwX,g+rX,o-rwx private-data

Recursively give the owner access, allow group read/traverse, and remove access for others.

chmod --reference=known-good.conf new.conf

Copy permission mode from another file.

Practical Notes

  • Be careful with chmod -R; it can change thousands of files quickly.
  • Do not use chmod 777 as a quick fix. It often creates security problems.
  • Directories need execute permission to be entered.
  • Files need execute permission to run as programs or scripts.
  • For SSH keys, strict permissions matter. Private keys are commonly 600.