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, sepa-
rated 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 ex-
isting 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 af-
fected.
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 dele-
tion 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 sec-
ond 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 doesn't change the permissions of symbolic links; the chmod system
call cannot change their permissions on most systems, and most systems
ignore permissions of symbolic links. 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. Options that modify this behavior
are described in the OPTIONS section.
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.
For directories chmod preserves 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. To clear these bits for directories
with a numeric mode requires an additional leading zero like 00755,
leading minus like -6000, or leading equals like =755.
RESTRICTED DELETION FLAG OR STICKY BIT
The restricted deletion flag or sticky bit is a single bit, whose inter-
pretation depends on the file type. For directories, it prevents un-
privileged users from removing or renaming a file in the directory un-
less 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
--dereference
affect the referent of each symbolic link, rather than the sym-
bolic link itself
-h, --no-dereference
affect each symbolic link, rather than the referent
--no-preserve-root
do not treat '/' specially (the default)
--preserve-root
fail to operate recursively on '/'
--reference=RFILE
use RFILE's mode instead of specifying MODE values. RFILE is al-
ways dereferenced if a symbolic link.
-R, --recursive
change files and directories recursively
The following options modify how a hierarchy is traversed when the -R
option is also specified. If more than one is specified, only the final
one takes effect. -H is the default.
-H if a command line argument is a symbolic link to a directory,
traverse it
-L traverse every symbolic link to a directory encountered
-P do not traverse any symbolic links
--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: <https://www.gnu.org/software/coreutils/>
Report any translation bugs to <https://translationproject.org/team/>
SEE ALSO
chmod(2)
Full documentation <https://www.gnu.org/software/coreutils/chmod>
or available locally via: info '(coreutils) chmod invocation'
Packaged by Debian (9.7-3)
Copyright © 2025 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <https://gnu.org/li-
censes/gpl.html>.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
GNU coreutils 9.7 June 2025 CHMOD(1)
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