man-pages/man1/chmod.1

.\" Copyright Andries Brouwer, Ragnar Hojland Espinosa and A. Wik, 1998.
.\"
.\" This file may be copied under the conditions described
.\" in the LDP GENERAL PUBLIC LICENSE, Version 1, September 1998
.\" that should have been distributed together with this file.
.\" 
.\" Modified 2004-06-17, Michael Kerrisk
.\"
.TH CHMOD 1 2004-06-17 "GNU fileutils 4.0"
.SH NAME
chmod \- change access permissions of files
.SH SYNOPSIS
.BI "chmod [" options "] " "mode file..."
.sp
POSIX options:
.B "[\-R] [\-\-]"
.sp
GNU mode denotation:
.BI [\-\-reference= rfile ]
.sp
GNU options (shortest form):
.B [\-cfvR]
.B "[\-\-help] [\-\-version] [\-\-]"
.SH DESCRIPTION
.B chmod
changes the permissions of each given
.I file
according to
.IR mode ,
which can be either a symbolic representation of changes to make, or
an octal number representing the bit pattern for the new permissions.
.PP
The format of a symbolic mode change argument is
.br
\&`[ugoa...][[+\-=][rwxXstugo...]...][,...]'.
.PP
Such an argument is a list of symbolic mode change commands,
separated by commas.
Each symbolic mode change command starts with zero or more
of the letters `ugoa'; these control 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).  Thus, `a' is here equivalent to `ugo'.
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.
.PP
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.
.PP
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
permission for some user (X), set user or group ID on execution (s),
sticky bit (t), the permissions that the user
who owns the file currently has for it (u), the permissions that other
users in the file's group have for it (g), and the permissions that
other users not in the file's group have for it (o).
(Thus, `chmod g\-s file' removes the set-group-ID bit,
\&`chmod ug+s file' sets both the set-user-ID and set-group-ID bits, while
\&`chmod o+s file' does nothing.)
.PP
The name of the `sticky bit' derives from the original meaning:
keep program text on swap device.
These days, when set for a directory, it means that
only the owner of the file and the owner of that directory
may remove the file from that directory.
(This is commonly used on directories like /tmp that have
general write permission.)
.PP
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 save text image [`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.
.PP
.B chmod
never changes the permissions of symbolic links, since the
.B chmod
system call cannot change their permissions.  This is not a problem
since the permissions of symbolic links are never used. However, for 
each symbolic link listed on the command line,
.B chmod
changes the permissions of the pointed-to file.  In contrast,
.B chmod
ignores symbolic links encountered during recursive directory traversals.
.SH "POSIX OPTIONS"
.TP
.B "\-R"
Recursively change permissions of directories and their contents.
.TP
.B "\-\-"
Terminate option list.
.SH "ADDITIONAL GNU DESCRIPTION"
A GNU extension (new in fileutils 4.0) allows one to use
.BI "\-\-reference=" "rfile"
as a mode description: the same mode as that of
.IR rfile .
.SH "GNU OPTIONS"
.TP
.B "\-c, \-\-changes"
Verbosely describe the action for each
.I file
whose permissions actually changes.
.TP
.B "\-f, \-\-silent, \-\-quiet"
Do not print error messages about files whose permissions cannot be changed.
.TP
.B "\-v, \-\-verbose"
Verbosely describe the action or non-action taken for every
.IR file .
.TP
.B "\-R, \-\-recursive"
Recursively change permissions of directories and their contents.
.SH "GNU STANDARD OPTIONS"
.TP
.B "\-\-help"
Print a usage message on standard output and exit successfully.
.TP
.B "\-\-version"
Print version information on standard output, then exit successfully.
.TP
.B "\-\-"
Terminate option list.
.SH ENVIRONMENT
The variables LANG, LC_ALL, LC_CTYPE and LC_MESSAGES have the
usual meaning. For an XSI-conforming system: NLSPATH has the
usual meaning.
.SH "CONFORMING TO"
POSIX 1003.2 only requires the \-R option. Use of other options
may not be portable. This standard does not describe the 't' permission
bit. This standard does not specify whether \fBchmod\fP must preserve
consistency by clearing or refusing to set the set-user-ID and set-group-ID
bits, e.g., when all execute bits are cleared, or whether \fBchmod\fP
honors the `s' bit at all.
.SH "NONSTANDARD MODES"
Above we described the use of the `t' bit on directories.
Various systems attach special meanings to otherwise
meaningless combinations of mode bits.
In particular, Linux, following System V (see
System V Interface Definition (SVID) Version 3),
uses the combination of having the set-group-ID bit enabled
while group execute bit is disabled to mean that
mandatory locking is enabled for the file.
For more details, see
the file
.IR /usr/src/linux/Documentation/mandatory.txt .
.SH NOTES
This page describes
.B chmod
as found in the fileutils-4.0 package;
other versions may differ slightly.
.SH "SEE ALSO"
.BR chattr (1),
.BR chown (1),
.BR install (1),
.BR chmod (2),
.BR stat (2),
.BR umask (2)