Man Page Minute – chmod

NAME
chmod – change file access permissionsSYNOPSIS
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
permissions of each given file according to mode, which can be either a
symbolic  representation  of changes to make, or an octal number repre
senting the bit pattern for the new permissions.

The   format   of   a   symbolic   mode   is    [ugoa…][[+-=][rwxXs
tugo…]…][,…].   Multiple symbolic operations can be given, sepa
rated 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
files group (g), other users not in the files 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 permissions selected to  be  added  to  the
existing  permissions  of each file; – causes them to be removed; and
= causes them to be the only permissions that the file has.

The letters rwxXstugo select the new  permissions  for  the  affected
users:  read  (r),  write (w), execute (or access for directories) (x),
execute only if the file is a directory or already has execute  permis
sion  for  some user (X), set user or group ID on execution (s), sticky
(t), the permissions granted to the user who owns  the  file  (u),  the
permissions  granted to other users who are members of the files 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.   Any  omitted  digits  are
assumed  to  be leading zeros.  The first digit selects the set user ID
(4) and set group ID (2) and 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  files group, with the same values; and the fourth for other users
not in the files 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  sym
bolic link listed on the command line, chmod changes the permissions of
the pointed-to file.  In contrast, chmod ignores symbolic links encoun
tered during recursive directory traversals.

STICKY FILES
On  older  Unix  systems,  the sticky bit caused executable files to be
hoarded in swap space.  This feature is not useful on  modern  VM  sys
tems, and the Linux kernel ignores the sticky bit on files.  Other ker
nels may use the sticky bit on files for system-defined  purposes.   On
some systems, only the superuser can set the sticky bit on files.

STICKY DIRECTORIES
When  the sticky bit is set on a directory, files in that directory may
be unlinked or renamed only by the directory owner as well as  by  root
or the file owner.  Without the sticky bit, anyone able to write to the
directory can delete or rename files.  The sticky bit is commonly found
on directories, such as /tmp, that are world-writable.

OPTIONS
Change the mode of each FILE to MODE.

-c, –changes
like verbose but report only when a change is made

–no-preserve-root
do not treat / specially (the default)

–preserve-root
fail to operate recursively on /

-f, –silent, –quiet
suppress most error messages

-v, –verbose
output a diagnostic for every file processed

–reference=RFILE
use RFILEs 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]))+.

AUTHOR
Written by David MacKenzie and Jim Meyering.

REPORTING BUGS
Report bugs to <bug-coreutils@gnu.org>.

COPYRIGHT
Copyright  2005 Free Software Foundation, Inc.
This  is  free  software.   You may redistribute copies of it under the
terms      of      the      GNU      General       Public       License
<http://www.gnu.org/licenses/gpl.html>.   There  is NO WARRANTY, to the
extent permitted by law.

SEE ALSO
The full documentation for chmod is maintained as a Texinfo manual.  If
the  info  and  chmod programs are properly installed at your site, the
command

info chmod

should give you access to the complete manual.