chmod - change file modes
chmod [-R [-H | -L | -P]] mode [file ...]
The chmod utility modifies the file mode bits of the listed
files as
specified by the mode operand. The mode of a file dictates
its permissions,
among other attributes.
The options are as follows:
-R Recursively descend through any specified directory
arguments.
Change the modes of the file hierarchies rooted in
the files instead
of just the files themselves.
-H If the -R option is also 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 also specified, all symbolic
links are followed.
-P If the -R option is also specified, no symbolic
links are followed.
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 file's owner or the superuser is permitted to
change the mode of
a file.
The chmod utility exits 0 on success or >0 if an error occurred.
Absolute modes [Toc] [Back]
Absolute modes are specified according to the following format:
chmod nnnn file [...]
An absolute mode is an octal number (specified as nnnn,
where n is a number
from 0 to 7) constructed by ORing any of the following
values:
0400 Allow read by owner.
0200 Allow write by owner.
0100 Allow execution (or search in directories) by
owner.
0700 Allow read, write, and execute/search by owner.
0040 Allow read by group.
0020 Allow write by group.
0010 Allow execution (or search in directories) by
group.
0070 Allow read, write, and execute/search by
group.
0004 Allow read by others.
0002 Allow write by others.
0001 Allow execution (or search in directories) by
others.
0007 Allow read, write, and execute/search by others.
In addition to the file permission modes, the following mode
bits are
available:
4000 Set-user-ID on execution.
2000 Set-group-ID on execution.
1000 Enable sticky bit; see sticky(8) and chmod(2).
The execute bit for a directory is often referred to as the
``search''
bit. In order to access a file, a user must have execute
permission in
each directory leading up to it in the filesystem hierarchy.
For example,
to access the file /bin/ls, execute permission is needed on /, /bin,
and, of course, the ls binary itself.
Symbolic modes [Toc] [Back]
Symbolic modes are specified according to the following format:
chmod <[who]op[perm]>,[[who]op[perm]],[...] file [...]
The who symbols indicate whose permissions are to be changed
or assigned:
u User (owner) permissions.
g Group permissions.
o Others permissions.
a All of the above.
Do not confuse the `o' symbol with ``owner''. It is the user bit, `u',
that refers to the owner of the 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 user, 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.
The perm (permission symbols) represent the portions of the
mode bits as
follows:
r Read bits.
s Set-user-ID and set-group-ID on execution
bits.
t Sticky bit.
w Write bits.
x 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 User permission bits in the mode of the original file.
g Group permission bits in the mode of the original file.
o Other permission bits in the mode of the original file.
Each clause (given in a comma-delimited list on the command
line) specifies
one or more operations to be performed on the mode
bits, and each
operation is applied in the order specified.
Operations upon the ``other'' permissions (specified by the
symbol `o' by
itself), in combination with the perm symbols `s' or `t',
are ignored.
$ chmod 644 file
Set file readable by anyone and writable by the owner only.
$ chmod go-w file
Deny write permission to group and others.
$ chmod =rw,+X file
Set the read and write permissions to the usual defaults,
but retain any
execute permissions that are currently set.
$ chmod +X file
Make a directory or file searchable/executable by everyone
if it is already
searchable/executable by anyone.
$ chmod 755 file
$ chmod u=rwx,go=rx file
$ chmod u=rwx,go=u-w file
Any of these commands will make a file readable/executable
by everyone
and writable by the owner only.
$ chmod go= file
Clear all mode bits for group and others.
$ chmod g=u-w file
Set the group bits equal to the user bits, but clear the
group write bit.
chflags(1), chgrp(1), find(1), install(1), chmod(2),
stat(2), umask(2),
fts(3), setmode(3), symlink(7), chown(8), sticky(8)
The chmod utility is expected to be IEEE Std 1003.2
(``POSIX.2'') compatible
with the exception of the perm symbols `t' and `X'
which are not included
in that standard.
A chmod command appeared in Version 1 AT&T UNIX.
There's no perm option for the naughty bits.
OpenBSD 3.6 March 31, 1994
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