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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 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), exe‐
cute/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 let‐
ters 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 prob‐
lem since the permissions of symbolic links are never used. How‐
ever, 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 direc‐
tory 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 appropri‐
ate privileges. Additional restrictions may cause the set-user-ID
and set-group-ID bits of MODE or RFILE to be ignored. This behav‐
ior 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, or leading = like 00755 , or =755
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 pre‐
vents 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 com‐
monly 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: <https://www.gnu.org/software/core‐
utils/>
Report any translation bugs to <https://translationpro‐
ject.org/team/>
COPYRIGHT
Copyright © 2020 Free Software Foundation, Inc. License GPLv3+:
GNU GPL version 3 or later <https://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 <https://www.gnu.org/software/coreutils/chmod>
or available locally via: info '(coreutils) chmod invocation'
GNU coreutils 8.32 April 2020 CHMOD(1)