File: coreutils.info,  Node: chmod invocation,  Next: touch invocation,  Prev: chgrp invocation,  Up: Changing file attributes

13.3 ‘chmod’: Change access permissions
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‘chmod’ changes the access permissions of the named files.  Synopsis:

     chmod [OPTION]... {MODE | --reference=REF_FILE} FILE...

   ‘chmod’ never changes the permissions of symbolic links, since 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.

   Only a process whose effective user ID matches the user ID of the
file, or a process with appropriate privileges, is permitted to change
the file mode bits of a file.

   A successful use of ‘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 REF_FILE 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.

   If used, MODE specifies the new file mode bits.  For details, see the
section on *note File permissions::.  If you really want MODE to have a
leading ‘-’, you should use ‘--’ first, e.g., ‘chmod -- -w file’.
Typically, though, ‘chmod a-w file’ is preferable, and ‘chmod -w file’
(without the ‘--’) complains if it behaves differently from what ‘chmod
a-w file’ would do.

   The program accepts the following options.  Also see *note Common
options::.

‘-c’
‘--changes’
     Verbosely describe the action for each FILE whose permissions
     actually change.

‘-f’
‘--silent’
‘--quiet’
     Do not print error messages about files whose permissions cannot be
     changed.

‘--preserve-root’
     Fail upon any attempt to recursively change the root directory,
     ‘/’.  Without ‘--recursive’, this option has no effect.  *Note
     Treating / specially::.

‘--no-preserve-root’
     Cancel the effect of any preceding ‘--preserve-root’ option.  *Note
     Treating / specially::.

‘-v’
‘--verbose’
     Verbosely describe the action or non-action taken for every FILE.

‘--reference=REF_FILE’
     Change the mode of each FILE to be the same as that of REF_FILE.
     *Note File permissions::.  If REF_FILE is a symbolic link, do not
     use the mode of the symbolic link, but rather that of the file it
     refers to.

‘-R’
‘--recursive’
     Recursively change permissions of directories and their contents.

   An exit status of zero indicates success, and a nonzero value
indicates failure.

   Examples:

     # Change file permissions of FOO to be world readable
     # and user writable, with no other permissions.
     chmod 644 foo
     chmod a=r,u+w foo

     # Add user and group execute permissions to FOO.
     chmod +110 file
     chmod ug+x file

     # Set file permissions of DIR and subsidiary files to
     # be the umask default, assuming execute permissions for
     # directories and for files already executable.
     chmod -R a=,+rwX dir