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.
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.
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
Change the mode of each FILE to MODE. With --reference, change the
mode of each FILE to that of RFILE.
like verbose but report only when a change is made
-f, --silent, --quiet
suppress most error messages
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
change files and directories recursively
--help display this help and exit
output version information and exit
Each MODE is of the form
This page is part of the coreutils (basic file, shell and text
manipulation utilities) project. Information about the project can
be found at ⟨http://www.gnu.org/software/coreutils/⟩. If you have a
bug report for this manual page, see
⟨http://www.gnu.org/software/coreutils/⟩. This page was obtained from
the tarball coreutils-8.25.tar.xz fetched from
⟨http://www.gnutls.org/download.html⟩ on 2016-08-07. If you discover
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you have corrections or improvements to the information in this
COLOPHON (which is not part of the original manual page), send a mail
GNU coreutils 8.25 January 2016 CHMOD(1)