CHMOD(1) User Commands CHMOD(1)
NAME
chmod - change file mode bits
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 file mode bits of each given file
according to mode, which can be either a symbolic representation of changes to make, or an octal number repre‐
senting the bit pattern for the new mode bits.
The format of a symbolic mode is [ugoa...][[+-=][perms...]...], where perms is either zero or more letters
from the set rwxXst, or a single letter from the set ugo. Multiple symbolic modes 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 selected file mode bits to be added to the existing file mode bits of each file; -
causes them to be removed; and = causes them to be added and causes unmentioned bits to be removed except that
a directory's unmentioned set user and group ID bits are not affected.
The letters rwxXst select file mode bits for the affected users: read (r), write (w), execute (or search for
directories) (x), execute/search only if the file is a directory or already has execute permission for some
user (X), set user or group ID on execution (s), restricted deletion flag or sticky bit (t). Instead of one
or more of these letters, you can specify exactly one of the letters ugo: 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.
Omitted digits are assumed to be leading zeros. The first digit selects the set user ID (4) and set group ID
(2) and restricted deletion or 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 val‐
ues.
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.
SETUID AND SETGID BITS
chmod clears the set-group-ID bit of a regular file if the file's group ID does not match the user's effective
group ID or one of the user's supplementary group IDs, unless the user has appropriate privileges. Additional
restrictions may cause the set-user-ID and set-group-ID bits of MODE or RFILE to be ignored. This behavior
depends on the policy and functionality of the underlying chmod system call. When in doubt, check the under‐
lying system behavior.
chmod preserves a directory's set-user-ID and set-group-ID bits unless you explicitly specify otherwise. You
can set or clear the bits with symbolic modes like u+s and g-s, and you can set (but not clear) the bits with
a numeric mode.
-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]+'.
GNU coreutils online help: <http://www.gnu.org/software/coreutils/> Report chmod translation bugs to
<http://translationproject.org/team/>
AUTHOR
Written by David MacKenzie and Jim Meyering.
COPYRIGHT
Copyright © 2013 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later
<http://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent per‐
mitted by law.
SEE ALSO
chmod(2)
The full documentation for chmod is maintained as a Texinfo manual. If the info and chmod programs are prop‐
erly installed at your site, the command
info coreutils 'chmod invocation'
should give you access to the complete manual.
GNU coreutils 8.22 November 2016 CHMOD(1)
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