chmod

chmod [options] mode file...

chmod (change mode) modifies file access permissions. It controls read, write, and execute permissions for three classes of users: the file owner (user), the file's group, and all other users.Permissions can be specified in two ways: numeric (octal) mode using digits 0-7, or symbolic mode using letters (u/g/o/a, +/-/=, r/w/x). Numeric mode sets all permissions at once, while symbolic mode allows adding or removing individual permissions. The command is fundamental to Unix file security and access control.

-R, --recursive

Change files and directories recursively

Verbose output

Report only changes

Use permissions from reference file

Octal digits (0-7):- 4 - Read (r)- 2 - Write (w)- 1 - Execute (x)Common modes:- 755 - rwxr-xr-x (owner full, others read/execute)- 644 - rw-r--r-- (owner read/write, others read)- 600 - rw------- (owner only)- 777 - rwxrwxrwx (all permissions, usually bad idea)

Format: [ugoa][+-=][rwxXst]Who:- u - User (owner)- g - Group- o - Others- a - AllOperation:- + - Add permission- - - Remove permission- = - Set exact permissionPermissions:- r - Read- w - Write- x - Execute- X - Execute only if directory- s - Setuid/setgid- t - Sticky bit

Setuid (4000):

Run as file owner

Run as file group or inherit directory group

Only owner can delete (common for /tmp)

Requires ownership or root privileges. 777 permissions are security risk. Recursive changes can break systems. Symbolic links not affected (use chmod -h on some systems). Execute on files vs directories different meaning.

chmod has been part of Unix since the early 1970s, implementing the Unix permission model designed by Dennis Ritchie and Ken Thompson.

chown(1), chgrp(1), umask(1), ls(1)

Source code · Documentation