Linux -- chmod

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 [email protected]
       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)