NAME

chmod — change file modes

SYNOPSIS

chmod

[-R [-H | -L | -P]] [-fh] mode file ...

chmod

[-R [-H | -L | -P]] [-fh] --reference=rfile file ...

DESCRIPTION

The chmod utility modifies the file mode bits of the listed files as specified by the mode operand, or copied from a reference rfile, as specified with the --reference argument.

The options are as follows:

-H

If the -R option is specified, symbolic links on the command line are followed. (Symbolic links encountered in the tree traversal are not followed.)

-L

If the -R option is specified, all symbolic links are followed.

-P

If the -R option is specified, no symbolic links are followed.

-R

Change the modes of the file hierarchies rooted in the files instead of just the files themselves.

-f

Do not display a diagnostic message or modify the exit status if chmod fails to change the mode of a file.

-h

If file is symbolic link, the mode of the link is changed.

The -H, -L and -P options are ignored unless the -R option is specified. In addition, these options override each other and the command's actions are determined by the last one specified. The default is as if the -P option had been specified.

Only the owner of a file or the super-user is permitted to change the mode of a file.

EXIT STATUS

The chmod utility exits 0 on success, and >0 if an error occurs.

MODES

Modes may be absolute or symbolic. An absolute mode is an octal number constructed by or'ing the following values:

4000

set-user-ID-on-execution

2000

set-group-ID-on-execution

1000

sticky bit, see chmod(2)

0400

read by owner

0200

write by owner

0100

execute (or search for directories) by owner

0070

read, write, execute/search by group

0007

read, write, execute/search by others

The read, write, and execute/search values for group and others are encoded as described for owner.

The symbolic mode is described by the following grammar:

mode         ::= clause [, clause ...] 
clause       ::= [who ...] [action ...] last_action 
action       ::= op [perm ...] 
last_action  ::= op [perm ...] 
who          ::= a | u | g | o 
op           ::= + | - | = 
perm         ::= r | s | t | w | x | X | u | g | o

The who symbols ``u'', ``g'', and ``o'' specify the user, group, and other parts of the mode bits, respectively. The who symbol ``a'' is equivalent to ``ugo''.

The perm symbols represent the portions of the mode bits as follows:

r

The read bits.

s

The set-user-ID-on-execution and set-group-ID-on-execution bits.

t

The sticky bit.

w

The write bits.

x

The execute/search bits.

X

The execute/search bits if the file is a directory or any of the execute/search bits are set in the original (unmodified) mode. Operations with the perm symbol ``X'' are only meaningful in conjunction with the op symbol ``+'', and are ignored in all other cases.

u

The user permission bits in the mode of the original file.

g

The group permission bits in the mode of the original file.

o

The other permission bits in the mode of the original file.

The op symbols represent the operation performed, as follows:

+

If no value is supplied for perm, the ``+'' operation has no effect. If no value is supplied for who, each permission bit specified in perm, for which the corresponding bit in the file mode creation mask is clear, is set. Otherwise, the mode bits represented by the specified who and perm values are set.

-

If no value is supplied for perm, the ``-'' operation has no effect. If no value is supplied for who, each permission bit specified in perm, for which the corresponding bit in the file mode creation mask is clear, is cleared. Otherwise, the mode bits represented by the specified who and perm values are cleared.

=

The mode bits specified by the who value are cleared, or, if no who value is specified, the owner, group and other mode bits are cleared. Then, if no value is supplied for who, each permission bit specified in perm, for which the corresponding bit in the file mode creation mask is clear, is set. Otherwise, the mode bits represented by the specified who and perm values are set.

Each clause specifies one or more operations to be performed on the mode bits, and each operation is applied to the mode bits in the order specified.

Operations upon the other permissions only (specified by the symbol ``o'' by itself), in combination with the perm symbols ``s'' or ``t'', are ignored.

EXAMPLES

644

make a file readable by anyone and writable by the owner only.

go-w

deny write permission to group and others.

=rw,+X

set the read and write permissions to the usual defaults, but retain any execute permissions that are currently set.

+X

make a directory or file searchable/executable by everyone if it is already searchable/executable by anyone.

755

u=rwx,go=rx

u=rwx,go=u-w

make a file readable/executable by everyone and writable by the owner only.

go=

clear all mode bits for group and others.

g=u-w

set the group bits equal to the user bits, but clear the group write bit.

SEE ALSO

chflags(1), install(1), chmod(2), stat(2), umask(2), fts(3), setmode(3), symlink(7), chown(8)

STANDARDS

The chmod utility is expected to be IEEE Std 1003.2-1992 (“POSIX.2”) compatible with the exception of the perm symbol “t” which is not included in that standard.

BUGS

There's no perm option for the naughty bits.