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2'\" Foundation, Inc.
3'\"
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7[NAME]
8chmod \- change file mode bits
9[DESCRIPTION]
10This manual page
11documents the GNU version of
12.BR chmod .
13.B chmod
14changes the file mode bits of each given file according to
15.IR mode ,
16which can be either a symbolic representation of changes to make, or
17an octal number representing the bit pattern for the new mode bits.
18.PP
19The format of a symbolic mode is [\c
20\fBugoa\fP.\|.\|.][[\fB+-=\fP][\fIperms\fP.\|.\|.].\|.\|.],
21where
22.I "perms"
23is either zero or more letters from the set
24\fBrwxXst\fP, or a single letter from the set \fBugo\fP.
25Multiple symbolic
26modes can be given, separated by commas.
27.PP
28A combination of the letters \fBugoa\fP controls which users' access
29to the file will be changed: the user who owns it (\fBu\fP), other
30users in the file's group (\fBg\fP), other users not in the file's
31group (\fBo\fP), or all users (\fBa\fP).  If none of these are given,
32the effect is as if \fBa\fP were
33given, but bits that are set in the umask are not affected.
34.PP
35The operator \fB+\fP causes the selected file mode bits to be added to
36the existing file mode bits of each file; \fB-\fP causes them to be
37removed; and \fB=\fP causes them to be added and causes unmentioned
38bits to be removed except that a directory's unmentioned set user and
39group ID bits are not affected.
40.PP
41The letters \fBrwxXst\fP select file mode bits for the affected users:
42read (\fBr\fP), write (\fBw\fP), execute (or search for directories)
43(\fBx\fP), execute/search only if the file is a directory or already
44has execute permission for some user (\fBX\fP), set user or group ID
45on execution (\fBs\fP), restricted deletion flag or sticky bit
46(\fBt\fP).  Instead of one or more of these letters, you can specify
47exactly one of the letters \fBugo\fP: the permissions granted to the
48user who owns the file (\fBu\fP), the permissions granted to other
49users who are members of the file's group (\fBg\fP),
50and the permissions granted to users that are in neither of the two preceding
51categories (\fBo\fP).
52.PP
53A numeric mode is from one to four octal digits (0\-7), derived by
54adding up the bits with values 4, 2, and 1.  Omitted digits are
55assumed to be leading zeros.
56The first digit selects the set user ID (4) and set group ID (2) and
57restricted deletion or sticky (1) attributes.  The second digit
58selects permissions for the user who owns the file: read (4), write (2),
59and execute (1); the third selects permissions for other users in the
60file's group, with the same values; and the fourth for other users not
61in the file's group, with the same values.
62.PP
63.B chmod
64never changes the permissions of symbolic links; the
65.B chmod
66system call cannot change their permissions.  This is not a problem
67since the permissions of symbolic links are never used.
68However, for each symbolic link listed on the command line,
69.B chmod
70changes the permissions of the pointed-to file.
71In contrast,
72.B chmod
73ignores symbolic links encountered during recursive directory
74traversals.
75.SH "SETUID AND SETGID BITS"
76.B chmod
77clears the set-group-ID bit of a
78regular file if the file's group ID does not match the user's
79effective group ID or one of the user's supplementary group IDs,
80unless the user has appropriate privileges.  Additional restrictions
81may cause the set-user-ID and set-group-ID bits of
82.I MODE
83or
84.I RFILE
85to be ignored.  This behavior depends on the policy and
86functionality of the underlying
87.B chmod
88system call.  When in
89doubt, check the underlying system behavior.
90.PP
91.B chmod
92preserves a directory's set-user-ID and set-group-ID bits unless you
93explicitly specify otherwise.  You can set or clear the bits with
94symbolic modes like
95.B u+s
96and
97.BR g\-s ,
98and you can set (but not clear) the bits with a numeric mode.
99.SH "RESTRICTED DELETION FLAG OR STICKY BIT"
100The restricted deletion flag or sticky bit is a single bit, whose
101interpretation depends on the file type.  For directories, it prevents
102unprivileged users from removing or renaming a file in the directory
103unless they own the file or the directory; this is called the
104.I "restricted deletion flag"
105for the directory, and is commonly found on world-writable directories
106like \fB/tmp\fP.  For regular files on some older systems, the bit
107saves the program's text image on the swap device so it will load more
108quickly when run; this is called the
109.IR "sticky bit" .
110.SH OPTIONS
111[SEE ALSO]
112chmod(2)
113