CHMOD(1) User Commands CHMOD(1)
NAME
chmod - change file mode bits
SYNOPSIS
chmod [OPTION]... MODE[,MODE]... FILE...
chmod [OPTION]... OCTAL-MODE FILE...
chmod [OPTION]... --reference=RFILE FILE...
DESCRIPTION
This manual page documents the GNU version of chmod. chmod changes the file mode bits of each given file according to mode, which can be
either a symbolic representation of changes to make, or an octal number representing the bit pattern for the new mode bits.
The format of a symbolic mode is [ugoa...][[+-=][perms...]...], where perms is either zero or more letters from the set rwxXst, or a single
letter from the set ugo. Multiple symbolic modes can be given, separated by commas.
A combination of the letters ugoa controls 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). 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.
The operator + causes the selected file mode bits to be added to the existing file mode bits of each file; - causes them to be removed; and
= causes them to be added and causes unmentioned bits to be removed except that a directory's unmentioned set user and group ID bits are not
affected.
The letters rwxXst select file mode bits for the affected users: read (r), write (w), execute (or search for directories) (x), exe‐
cute/search only if the file is a directory or already has execute permission for some user (X), set user or group ID on execution (s),
restricted deletion flag or sticky bit (t). Instead of one or more of these letters, you can specify exactly one of the letters ugo: the
permissions granted to the user who owns the file (u), the permissions granted to other users who are members of the file's group (g), and
the permissions granted to users that are in neither of the two preceding categories (o).
A numeric mode is from one to four octal digits (0-7), derived by adding up the bits with values 4, 2, and 1. Omitted digits are assumed to
be leading zeros. The first digit selects the set user ID (4) and set group ID (2) and restricted deletion or 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.
chmod never changes the permissions of symbolic links; the 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, chmod changes the permissions
of the pointed-to file. In contrast, chmod ignores symbolic links encountered during recursive directory traversals.
SETUID AND SETGID BITS
chmod clears the set-group-ID bit of a regular file if the file's group ID does not match the user's effective group ID or one of the user's
supplementary group IDs, unless the user has appropriate privileges. Additional restrictions may cause the set-user-ID and set-group-ID
bits of MODE or RFILE to be ignored. This behavior depends on the policy and functionality of the underlying chmod system call. When in
doubt, check the underlying system behavior.
chmod preserves a directory's set-user-ID and set-group-ID bits unless you explicitly specify otherwise. You can set or clear the bits with
symbolic modes like u+s and g-s, and you can set (but not clear) the bits with a numeric mode.
RESTRICTED DELETION FLAG OR STICKY BIT
The restricted deletion flag or sticky bit is a single bit, whose interpretation depends on the file type. For directories, it prevents
unprivileged users from removing or renaming a file in the directory unless they own the file or the directory; this is called the
restricted deletion flag for the directory, and is commonly found on world-writable directories like /tmp. For regular files on some older
systems, the bit saves the program's text image on the swap device so it will load more quickly when run; this is called the sticky bit.
OPTIONS
Change the mode of each FILE to MODE. With --reference, change the mode of each FILE to that of RFILE.
-c, --changes
like verbose but report only when a change is made
-f, --silent, --quiet
suppress most error messages
-v, --verbose
output a diagnostic for every file processed
--no-preserve-root
do not treat '/' specially (the default)
--preserve-root
fail to operate recursively on '/'
--reference=RFILE
use RFILE's mode instead of MODE values
-R, --recursive
change files and directories recursively
--help display this help and exit
--version
output version information and exit
Each MODE is of the form '[ugoa]* ([-+=]([rwxXst]*|[ugo]))+ | [-+=][0-7]+'.
1 #chmod u=rwx,g=rw,o=r FILE 2 3 #chmod a=r FILE 所有用户加上可读属性
1 $ chmod u+x file 给file的属主增加执行权限 2 $ chmod 751 file 给file的属主分配读、写、执行(7)的权限，给file的所在组分配读、执行(5)的权限，给其他用户分配执行(1)的权限 3 $ chmod u=rwx,g=rx,o=x file 上例的另一种形式 4 $ chmod =r file 为所有用户分配读权限 5 $ chmod 444 file 同上例 6 $ chmod a-wx,a+r file 同上例 7 $ chmod -R u+r directory 递归地给directory目录下所有文件和子目录的属主分配读的权限 8 $ chmod 4755 设置用ID，给属主分配读、写和执行权限，给组和其他用户分配读、执行的权限。
来自：
http://www.cnblogs.com/younes/archive/2009/11/20/1607174.html
AUTHOR
Written by David MacKenzie and Jim Meyering.
REPORTING BUGS
Report chmod bugs to bug-coreutils@gnu.org
GNU coreutils home page: <http://www.gnu.org/software/coreutils/>
General help using GNU software: <http://www.gnu.org/gethelp/>
Report chmod translation bugs to <http://translationproject.org/team/>
COPYRIGHT
Copyright © 2013 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law.
SEE ALSO
chmod(2)
The full documentation for chmod is maintained as a Texinfo manual. If the info and chmod programs are properly installed at your site, the
command
info coreutils 'chmod invocation'
should give you access to the complete manual.
GNU coreutils 8.21 March 2014 CHMOD(1)