chmod - change file mode bits
]... OCTAL-MODE FILE
]... --reference=RFILE FILE
This manual page documents the GNU version of chmod
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
...]...], where perms
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
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
all users ( a
). If none of these are given, the effect is as if (
) 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
The letters rwxXst
select file mode bits for the affected users: read (
), 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 (
), restricted deletion flag or sticky bit ( t
). Instead of one
or more of these letters, you can specify exactly one of the letters
: 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 (
), 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.
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¶
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
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.
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 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
directory, and is commonly found on world-writable directories like
. 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. With --reference
, change the mode
of each FILE to that of RFILE.
- -c, --changes
- like verbose but report only when a change is made
- -f, --silent, --quiet
- suppress most error messages
- -v, --verbose
- output a diagnostic for every file processed
- do not treat '/' specially (the default)
- fail to operate recursively on '/'
- use RFILE's mode instead of MODE values
- -R, --recursive
- change files and directories recursively
- display this help and exit
- output version information and exit
Each MODE is of the form '[ugoa]*([-+=]([rwxXst]*|[ugo]))+|[-+=][0-7]+'.
Written by David MacKenzie and Jim Meyering.
GNU coreutils online help: <http://www.gnu.org/software/coreutils/
Report chmod translation bugs to <http://translationproject.org/team/
Copyright © 2016 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.
Full documentation at: <http://www.gnu.org/software/coreutils/chmod
or available locally via: info '(coreutils) chmod invocation'