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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 representing 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 unmen-
tioned 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 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 com-
mand 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 underlying 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.
RESTRICTED DELETION FLAG OR STICKY BIT
The restricted deletion flag or sticky bit is a single bit, whose interpretation depends on the file type. For directo-
ries, it prevents unprivileged users from removing or renaming a file in the directory unless they own the file or the
directory; this is called the restricted deletion flag for the directory, and is commonly found on world-writable direc-
tories like /tmp. For regular files on some older systems, the bit saves the program's text image on the swap device so
it will load more quickly when run; this is called the sticky bit.
OPTIONS
Change the mode of each FILE to MODE.
-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 chmod bugs to bug-coreutilsATgnu.org
GNU coreutils home page: <http://www.gnu.org/software/coreutils/>;
General help using GNU software: <http://www.gnu.org/gethelp/>;
Report chmod translation bugs to <http://translationproject.org/team/>;
COPYRIGHT
Copyright (C) 2010 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 permitted by law.
SEE ALSO
chmod(2)
The full documentation for chmod is maintained as a Texinfo manual. If the info and chmod programs are properly
installed at your site, the command
info coreutils 'chmod invocation'
should give you access to the complete manual.
GNU coreutils 8.5 November 2010 CHMOD(1)
