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112 lines
4.5 KiB
Plaintext
112 lines
4.5 KiB
Plaintext
'\" Copyright (C) 1998-2012 Free Software Foundation, Inc.
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'\"
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'\" This is free software. You may redistribute copies of it under the terms
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'\" of the GNU General Public License <http://www.gnu.org/licenses/gpl.html>.
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'\" There is NO WARRANTY, to the extent permitted by law.
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[NAME]
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chmod \- change file mode bits
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[DESCRIPTION]
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This manual page
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documents the GNU version of
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.BR chmod .
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.B chmod
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changes the file mode bits of each given file according to
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.IR mode ,
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which can be either a symbolic representation of changes to make, or
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an octal number representing the bit pattern for the new mode bits.
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.PP
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The format of a symbolic mode is [\c
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\fBugoa\fP.\|.\|.][[\fB+-=\fP][\fIperms\fP.\|.\|.].\|.\|.],
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where
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.I "perms"
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is either zero or more letters from the set
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\fBrwxXst\fP, or a single letter from the set \fBugo\fP.
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Multiple symbolic
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modes can be given, separated by commas.
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.PP
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A combination of the letters \fBugoa\fP controls which users' access
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to the file will be changed: the user who owns it (\fBu\fP), other
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users in the file's group (\fBg\fP), other users not in the file's
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group (\fBo\fP), or all users (\fBa\fP). If none of these are given,
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the effect is as if \fBa\fP were
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given, but bits that are set in the umask are not affected.
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.PP
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The operator \fB+\fP causes the selected file mode bits to be added to
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the existing file mode bits of each file; \fB-\fP causes them to be
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removed; and \fB=\fP causes them to be added and causes unmentioned
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bits to be removed except that a directory's unmentioned set user and
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group ID bits are not affected.
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.PP
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The letters \fBrwxXst\fP select file mode bits for the affected users:
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read (\fBr\fP), write (\fBw\fP), execute (or search for directories)
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(\fBx\fP), execute/search only if the file is a directory or already
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has execute permission for some user (\fBX\fP), set user or group ID
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on execution (\fBs\fP), restricted deletion flag or sticky bit
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(\fBt\fP). Instead of one or more of these letters, you can specify
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exactly one of the letters \fBugo\fP: the permissions granted to the
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user who owns the file (\fBu\fP), the permissions granted to other
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users who are members of the file's group (\fBg\fP),
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and the permissions granted to users that are in neither of the two preceding
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categories (\fBo\fP).
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.PP
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A numeric mode is from one to four octal digits (0\-7), derived by
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adding up the bits with values 4, 2, and 1. Omitted digits are
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assumed to be leading zeros.
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The first digit selects the set user ID (4) and set group ID (2) and
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restricted deletion or sticky (1) attributes. The second digit
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selects permissions for the user who owns the file: read (4), write (2),
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and execute (1); the third selects permissions for other users in the
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file's group, with the same values; and the fourth for other users not
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in the file's group, with the same values.
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.PP
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.B chmod
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never changes the permissions of symbolic links; the
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.B chmod
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system call cannot change their permissions. This is not a problem
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since the permissions of symbolic links are never used.
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However, for each symbolic link listed on the command line,
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.B chmod
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changes the permissions of the pointed-to file.
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In contrast,
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.B chmod
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ignores symbolic links encountered during recursive directory
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traversals.
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.SH "SETUID AND SETGID BITS"
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.B chmod
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clears the set-group-ID bit of a
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regular file if the file's group ID does not match the user's
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effective group ID or one of the user's supplementary group IDs,
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unless the user has appropriate privileges. Additional restrictions
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may cause the set-user-ID and set-group-ID bits of
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.I MODE
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or
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.I RFILE
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to be ignored. This behavior depends on the policy and
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functionality of the underlying
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.B chmod
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system call. When in
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doubt, check the underlying system behavior.
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.PP
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.B chmod
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preserves a directory's set-user-ID and set-group-ID bits unless you
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explicitly specify otherwise. You can set or clear the bits with
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symbolic modes like
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.B u+s
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and
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.BR g\-s ,
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and you can set (but not clear) the bits with a numeric mode.
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.SH "RESTRICTED DELETION FLAG OR STICKY BIT"
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The restricted deletion flag or sticky bit is a single bit, whose
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interpretation depends on the file type. For directories, it prevents
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unprivileged users from removing or renaming a file in the directory
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unless they own the file or the directory; this is called the
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.I "restricted deletion flag"
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for the directory, and is commonly found on world-writable directories
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like \fB/tmp\fP. For regular files on some older systems, the bit
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saves the program's text image on the swap device so it will load more
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quickly when run; this is called the
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.IR "sticky bit" .
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.SH OPTIONS
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[SEE ALSO]
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chmod(2)
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