# chmod

## 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**),
           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** 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
           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 _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
                  symbolic 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 always 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]+'.
    


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