chmod command

chmod - change file mode bits

The chmod command in Linux is used to change file permissions for owners, groups, and others. It controls who can read, write, or execute a file or directory, making it essential for security and access management.

Usage: chmod [OPTION]... MODE[,MODE]... FILE...

• MODE: The permissions to set (symbolic like u+x or numeric like 755).
• FILE: The target file(s) or directory(s).
• OPTION: Flags to modify behavior (e.g., -R for recursive).

Examples

• Understanding Permissions

  Linux permissions are divided into:

  • Users: Owner (u), Group (g), Others (o), All (a).
  • Types: Read (r, 4), Write (w, 2), Execute (x, 1`).

  Check permissions with ls -l:

  -rwxr-xr--  1 alice users  123 Apr 1 10:00 script.sh
  • -rwxr-xr--: Owner (rwx), Group (r-x), Others (r--).

• Symbolic Mode

  Use letters and operators to modify permissions.

  Operators:

  • +: Add permission.
  • -: Remove permission.
  • =: Set exact permission.

  Example (Add Execute):

  chmod u+x script.sh
  • Adds execute (x) for the owner (u).
  • Before: -rw-r--r--, After: -rwxr--r--.

  Example (Remove Write):

  chmod g-w file.txt
  • Removes write (w) for the group (g).

  Example (Set Exact):

  chmod o=r file.txt
  • Sets others (o) to read-only (r).

  Multiple Changes:

  chmod u+rwx,g+rx,o-rwx secret.txt
  • Owner: read/write/execute; Group: read/execute; Others: none.

• Numeric (Octal) Mode

  Use a 3-digit number to set permissions for owner, group, and others:

  • r = 4, w = 2, x = 1 (add them up).
  • Examples:
    • 7 = rwx (4+2+1).
    • 5 = r-x (4+1).
    • 0 = ---.

  chmod 755 script.sh
  • 7 (owner: rwx), 5 (group: r-x), 5 (others: r-x).
  • Result: -rwxr-xr-x.

  Example (Read-Only):

  chmod 644 file.txt
  • 6 (owner: rw-), 4 (group: r--), 4 (others: r--).
  • Result: -rw-r--r--.

• Recursive Changes

  Use -R to apply permissions to a directory and its contents.

  chmod -R 755 /var/www
  • Sets rwx for owner, r-x for group/others on /var/www and all files inside.

• Verbose Output

  Use -v to see what changes are made.

  chmod -v u+x script.sh
  • Output: mode of 'script.sh' changed from 0644 (rw-r--r--) to 0744 (rwxr--r--).

• Common Permissions Examples

Mode	Symbolic	Use Case
755	rwxr-xr-x	Executables (scripts, binaries)
644	rw-r--r--	Regular files
700	rwx------	Private scripts for owner only
777	rwxrwxrwx	Full access (use cautiously)

To get help related to the chmod command use --help option

$ chmod --help
Usage: chmod [OPTION]... MODE[,MODE]... FILE...
  or:  chmod [OPTION]... OCTAL-MODE FILE...
  or:  chmod [OPTION]... --reference=RFILE FILE...
Change the mode of each FILE to MODE.
With --reference, change the mode of each FILE to that of RFILE.

  -c, --changes          like verbose but report only when a change is made
  -f, --silent, --quiet  suppress most error messages
  -v, --verbose          output a diagnostic for every file processed
      --no-preserve-root  do not treat '/' specially (the default)
      --preserve-root    fail to operate recursively on '/'
      --reference=RFILE  use RFILE's mode instead of MODE values
  -R, --recursive        change files and directories recursively
      --help     display this help and exit
      --version  output version information and exit

Each MODE is of the form '[ugoa]*([-+=]([rwxXst]*|[ugo]))+|[-+=][0-7]+'.

For more details, check the manual with man chmod