CHMOD
Section: User Commands (1)
Updated: September 2017
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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 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 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.
OPTIONS
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
- --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]+'.
AUTHOR
Written by David MacKenzie and Jim Meyering.
REPORTING BUGS
GNU coreutils online help: <
http://www.gnu.org/software/coreutils/>
Report chmod translation bugs to <
http://translationproject.org/team/>
COPYRIGHT
Copyright © 2017 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)
Full documentation at: <http://www.gnu.org/software/coreutils/chmod>
or available locally via: info aq(coreutils) chmod invocationaq
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- SETUID AND SETGID BITS
-
- RESTRICTED DELETION FLAG OR STICKY BIT
-
- OPTIONS
-
- AUTHOR
-
- REPORTING BUGS
-
- COPYRIGHT
-
- SEE ALSO
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