Provided by: coreutils_7.4-2ubuntu2_i386
chmod - change file mode bits
chmod [OPTION]... MODE[,MODE]... FILE...
chmod [OPTION]... OCTAL-MODE FILE...
chmod [OPTION]... --reference=RFILE FILE...
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 unmentioned set user and group ID bits are
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
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 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 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 directories, 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 directories 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.
Change the mode of each FILE to MODE.
like verbose but report only when a change is made
do not treat ‘/’ specially (the default)
fail to operate recursively on ‘/’
-f, --silent, --quiet
suppress most error messages
output a diagnostic for every file processed
use RFILE’s mode instead of MODE values
change files and directories recursively
--help display this help and exit
output version information and exit
Each MODE is of the form ‘[ugoa]*([-+=]([rwxXst]*|[ugo]))+’.
Written by David MacKenzie and Jim Meyering.
Report chmod bugs to firstname.lastname@example.org
GNU coreutils home page: <http://www.gnu.org/software/coreutils/>
General help using GNU software: <http://www.gnu.org/gethelp/>
Copyright © 2009 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.
The full documentation for chmod is maintained as a Texinfo manual. If
the info and chmod programs are properly installed at your site, the
info coreutils 'chmod invocation'
should give you access to the complete manual.