chmod —
change file modes
  
    | chmod | [ -R[-H|-L|-P]]
      [-fh] mode
      file ... | 
  
    | chmod | [ -R[-H|-L|-P]]
      [-fh]--reference=rfilefile ... | 
The chmod utility modifies the file mode bits of the
  listed files as specified by the mode operand, or copied
  from a reference rfile, as specified with the
  --reference argument.
The options are as follows:
  - -H
- If the -Roption is specified, symbolic links on
      the command line are followed. (Symbolic links encountered in the tree
      traversal are not followed.)
- -L
- If the -Roption is specified, all symbolic links
      are followed.
- -P
- If the -Roption is specified, no symbolic links
      are followed.
- -R
- Change the modes of the file hierarchies rooted in the files instead of
      just the files themselves.
- -f
- Do not display a diagnostic message or modify the exit status if
      chmodfails to change the mode of a file.
- -h
- If file is symbolic link, the mode of the link is
      changed.
The -H, -L and
    -P options are ignored unless the
    -R option is specified. In addition, these options
    override each other and the command's actions are determined by the last one
    specified. The default is as if the -P option had
    been specified.
Only the owner of a file or the super-user is permitted to change
    the mode of a file.
The chmod utility exits 0 on success,
  and >0 if an error occurs.
Modes may be absolute or symbolic. An absolute mode is an octal number
  constructed by or'ing the following values:
  - 4000
- set-user-ID-on-execution
- 2000
- set-group-ID-on-execution
- 1000
- sticky bit, see chmod(2)
- 0400
- read by owner
- 0200
- write by owner
- 0100
- execute (or search for directories) by owner
- 0070
- read, write, execute/search by group
- 0007
- read, write, execute/search by others
 
The read, write, and execute/search values for group and others
    are encoded as described for owner.
The symbolic mode is described by the following grammar:
mode         ::= clause [, clause ...]
clause       ::= [who ...] [action ...] last_action
action       ::= op [perm ...]
last_action  ::= op [perm ...]
who          ::= a | u | g | o
op           ::= + | - | =
perm         ::= r | s | t | w | x | X | u | g | o
 
The who symbols ``u'', ``g'', and ``o''
    specify the user, group, and other parts of the mode bits, respectively. The
    who symbol ``a'' is equivalent to ``ugo''.
The perm symbols represent the portions of
    the mode bits as follows:
  - r
- The read bits.
- s
- The set-user-ID-on-execution and set-group-ID-on-execution bits.
- t
- The sticky bit.
- w
- The write bits.
- x
- The execute/search bits.
- X
- The execute/search bits if the file is a directory or any of the
      execute/search bits are set in the original (unmodified) mode. Operations
      with the perm symbol ``X'' are only meaningful in
      conjunction with the op symbol ``+'', and are
      ignored in all other cases.
- u
- The user permission bits in the mode of the original file.
- g
- The group permission bits in the mode of the original file.
- o
- The other permission bits in the mode of the original file.
 
The op symbols represent the operation
    performed, as follows:
  - +
- If no value is supplied for perm, the ``+''
      operation has no effect. If no value is supplied for
      who, each permission bit specified in
      perm, for which the corresponding bit in the file
      mode creation mask is clear, is set. Otherwise, the mode bits represented
      by the specified who and perm
      values are set.
- -
- If no value is supplied for perm, the ``-''
      operation has no effect. If no value is supplied for
      who, each permission bit specified in
      perm, for which the corresponding bit in the file
      mode creation mask is clear, is cleared. Otherwise, the mode bits
      represented by the specified who and
      perm values are cleared.
- =
- The mode bits specified by the who value are
      cleared, or, if no who value is specified, the owner, group and other mode
      bits are cleared. Then, if no value is supplied for
      who, each permission bit specified in
      perm, for which the corresponding bit in the file
      mode creation mask is clear, is set. Otherwise, the mode bits represented
      by the specified who and perm
      values are set.
Each clause specifies one or more operations
    to be performed on the mode bits, and each operation is applied to the mode
    bits in the order specified.
Operations upon the other permissions only (specified by the
    symbol ``o'' by itself), in combination with the perm
    symbols ``s'' or ``t'', are ignored.
  - 644
- make a file readable by anyone and writable by the owner only.
    
  
- go-w
- deny write permission to group and others.
    
  
- =rw,+X
- set the read and write permissions to the usual defaults, but retain any
      execute permissions that are currently set.
    
  
- +X
- make a directory or file searchable/executable by everyone if it is
      already searchable/executable by anyone.
    
  
- 755
-  
- u=rwx,go=rx
-  
- u=rwx,go=u-w
- make a file readable/executable by everyone and writable by the owner
      only.
    
  
- go=
- clear all mode bits for group and others.
    
  
- g=u-w
- set the group bits equal to the user bits, but clear the group write
    bit.
Thechmod utility is expected to be
  IEEE Std 1003.2-1992 (“POSIX.2”)
  compatible with the exception of the perm symbol
  “t” which is not included in that standard.
A chmod utility appeared in
  Version 1 AT&T UNIX.
There's no perm option for the naughty bits.