chmod - Unix, Linux Command

NAME

chmod - change file access permissions

SYNOPSIS

chmod [OPTION]... MODE[,MODE]... FILE...
chmod [OPTION]... OCTAL-MODE FILE...
chmod [OPTION]... --reference=RFILE FILE...

DESCRIPTION

This manual page documents the GNU version of chmod. chmod changes the permissions of each given file according to mode, which can be either a symbolic representation of changes to make, or an octal number representing the bit pattern for the new permissions.

The format of a symbolic mode is ‘[ugoa...][[+-=][rwxXstugo...]...][,...]’. Multiple symbolic operations can be given, separated by commas.

A combination of the letters ‘ugoa’ controls which users’ access to the file will be changed: the user who owns it (u), other users in the file’s group (g), other users not in the file’s group (o), or all users (a). If none of these are given, the effect is as if ‘a’ were given, but bits that are set in the umask are not affected.

The operator ‘+’ causes the permissions selected to be added to the existing permissions of each file; ‘-’ causes them to be removed; and ‘=’ causes them to be the only permissions that the file has.

The letters ‘rwxXstugo’ select the new permissions for the affected users: read (r), write (w), execute (or access for directories) (x), execute only if the file is a directory or already has execute permission for some user (X), set user or group ID on execution (s), sticky (t), the permissions granted to the user who owns the file (u), the permissions granted to other users who are members of the file’s group (g), and the permissions granted to users that are in neither of the two preceding categories (o).

A numeric mode is from one to four octal digits (0-7), derived by adding up the bits with values 4, 2, and 1. Any omitted digits are assumed to be leading zeros. The first digit selects the set user ID (4) and set group ID (2) and sticky (1) attributes. The second digit selects permissions for the user who owns the file: read (4), write (2), and execute (1); the third selects permissions for other users in the file’s group, with the same values; and the fourth for other users not in the file’s group, with the same values.

chmod never changes the permissions of symbolic links; the chmod system call cannot change their permissions. This is not a problem since the permissions of symbolic links are never used. However, for each symbolic link listed on the command line, chmod changes the permissions of the pointed-to file. In contrast, chmod ignores symbolic links encountered during recursive directory traversals.

STICKY FILES

On older Unix systems, the sticky bit caused executable files to be hoarded in swap space. This feature is not useful on modern VM systems, and the Linux kernel ignores the sticky bit on files. Other kernels may use the sticky bit on files for system-defined purposes. On some systems, only the superuser can set the sticky bit on files.

STICKY DIRECTORIES

When the sticky bit is set on a directory, files in that directory may be unlinked or renamed only by root or their owner. Without the sticky bit, anyone able to write to the directory can delete or rename files. The sticky bit is commonly found on directories, such as /tmp, that are world-writable.

OPTIONS

Change the mode of each FILE to MODE.

Tag	Description
-c, --changes
	like verbose but report only when a change is made
--no-preserve-root
	do not treat ‘/’ specially (the default)
--preserve-root
	fail to operate recursively on ‘/’
-f, --silent, --quiet
	suppress most error messages
-v, --verbose
	output a diagnostic for every file processed
--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]))+’.

AUTHOR

Written by David MacKenzie and Jim Meyering.

REPORTING BUGS

Report bugs to <bug-coreutils@gnu.org>.

COPYRIGHT

Copyright © 2006 Free Software Foundation, Inc.
This is free software. You may redistribute copies of it under the terms of the GNU General Public License <http://www.gnu.org/licenses/gpl.html>. There is NO WARRANTY, to the extent permitted by law.