CHMOD(1)

NAME
chmod - change file modes

SYNOPSIS
chmod [ -R [-H | -L | -P]] mode file ...

DESCRIPTION
The chmod utility modifies the file mode bits of the listed files as specified by the mode operand.

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.

Symbolic links do not have modes, so unless the -H or -L option is set, chmod on a symbolic link always succeeds and has no effect. 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.

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

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 from the sum of one or more of the following values:

4000
    (the set-user-ID-on-execution bit) Executable files with this bit set will run with effective uid set to the uid of the file owner. Directories with the set-user-id bit set will force all files and sub-directories created in them to be owned by the directory owner and not by the uid of the creating process, if the underlying file system supports this feature: see chmod(2) and the suiddir option to mount(1).
2000
    (the set-group-ID-on-execution bit) Executable files with this bit set will run with effective gid set to the gid of the file owner.
1000
    (the sticky bit) When set on a directory, unprivileged users can delete and rename only those files in the directory that are owned by them, regardless of the permissions on the directory. Under FreeBSD, the sticky bit is ignored for executable files and may only be set for directories (see sticky(8) ).
0400
    Allow read by owner.
0200
    Allow write by owner.
0100
    For files, allow execution by owner. For directories, allow the owner to search in the directory.
0040
    Allow read by group members.
0020
    Allow write by group members.
0010
    For files, allow execution by group members. For directories, allow group members to search in the directory.
0004
    Allow read by others.
0002
    Allow write by others.
0001
    For files, allow execution by others. For directories allow others to search in the directory.

For example, the absolute mode that permits read, write and execute by the owner, read and execute by group members, read and execute by others, and no set-uid or set-gid behaviour is 755 (400+200+100+040+010+004+001).

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.

BUGS
There's no perm option for the naughty bits.

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

STANDARDS
The chmod utility is expected to be p1003.2 compatible with the exception of the perm symbols “t” and “X” which are not included in that standard.

HISTORY
A chmod command appeared in AT&T UNIX.