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13.3 chmod: Change access permissions

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 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 Common options.


Verbosely describe the action for each file whose permissions actually changes.


Do not print error messages about files whose permissions cannot be changed.


Fail upon any attempt to recursively change the root directory, /. Without --recursive, this option has no effect. See Treating / specially.


Cancel the effect of any preceding --preserve-root option. See Treating / specially.


Verbosely describe the action or non-action taken for every file.


Change the mode of each file to be the same as that of ref_file. See 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.


Recursively change permissions of directories and their contents.

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

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