Mac OS X  
linux chmod

chmod -- change file modes or Access Control Lists (ACL)

chmod [-fv ] [-R [-H | -L | -P] mode file
chmod [-fv ] [-R [-H | -L | -P] [-a | +a | =a] ACE file
chmod [-fhv] [-R [-H | -L | -P] [-E] file
chmod [-fhv] [-R [-H | -L | -P] [-C] file
chmod [-fhv] [-R [-H | -L | -P] [-N] file

Modifies the file mode bits and Access Control Lists (ACLs)

-f force (?)Do not display a message if modify fails for file.
-R Change the modes of the file hierarchies rooted in the files instead of just the files the
-H, -L and -P are ignored without -R , override each other and the results are the last specified.
•Symbolic link behavor ( with -R)
-H symbolic links on the command line are followed.
Symbolic links encountered in the tree traversal are not followed by default.
-L followed.
-P NOT followed.Default.
-h change the mode of the link itself rather than the target
-v verbose, showing filenames as the mode is modified.
Specified more than once, the old and new modes of the file will also be displayed, in both octal and symbolic notation.

Only the owner of a file or the super-user is permitted to change the mode of a file.

DIAGNOSTICS

The chmod utility exits 0 on success, and >0 if an error occurs.

MODES

Modes may be absolute or symbolic. An absolute mode is an octal number constructed from the sum of one or more of the following values:
4000 set-user-ID-on-execution.
Executable with this bit set will run with effective uid set to the uid of the file owner.
Directories with the set-user-id bit set will force all files and sub-directories created in them to be owned by the directory owner and not by the uid of the creating process, if the underlying file system supports this feature: see chmod(2) and the suiddir option to mount(8).
2000 set-group-ID-on-execution.
Executable with this bit set will run with effective gid set to the gid of the file owner.
1000 sticky bit See chmod(2) and sticky(8).
0400 read by owner.
0200 write by owner.
0100 execution by owner, for files.
allow search by owner , for directories.
0040 read by group members.
0020 write by group members.
0010 execution by group members.
Search group members in the directory.
0004 read by others.
0002 write by others.
0001 execution by others.
others search in the directory.

For example, the absolute mode that permits read, write and execute by the owner, read and execute by group members, read and execute by others, and no set-uid or set-gid behaviour is 755 (400+200+100+040+010+004+001).

The symbolic mode is described by the following grammar:

           mode         ::= clause [, clause ...]
           clause       ::= [who ...] [action ...] action
           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 mode bits, a (all) is equivalent to ugo.

The perm symbols represent the portions of the mode (permission) bits:
r read bits.
w write bits.
x execute/search bits.
s set-user-ID-on-execution and set-group-ID-on-execution bits.
t sticky bit.
X 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 user bits
g group bits
o other bits not owner!

ls -l
-rw------- 1 5595 Feb 22 12:31 .bash_history
-rwxr----- 1 27 Dec 15 16:31 .profile
The op symbols represent the operation performed:
     +     If no value is supplied for perm, + 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 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.  
           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.

EXAMPLES OF VALID MODES

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.

Access Control List

A file may have an ACL, containing an ordered list of entries.
Each entry refers to a user or group, and grants or denies a set of permissions for operations.
ls -l  displays + when ACL entries are present
ls -le displays the ACL entries

drwx------+ 108 dgerman staff 3672 Feb 9 20:09 Documents
0: group:everyone deny delete

The Access Control List is manipultaed with extensions to chmod symbolic mode grammar.
Operations applicable to all filesystem objects:
delete Deletion may be granted by either this permission on an object or the delete_child right on the containing directory.
readattr implicitly granted if the object can be looked up and not explicitly denied.
readextattr
writeattr
writeextattr
readsecurity
writesecurity
chown Change ownership
Operations applicable to directories:
list
search Look up files by name.
add_file
add_subdirectory
delete_child Delete a contained object. See the file delete permission above.
Operations Applicable to non-directory filesystem objects:
read
write
append Open for writing, but only allow writes into areas of the file not previously written
execute
Inheritance is controlled on directories with
file_inherit Inherit to files.
directory_inherit Inherit to directories.
limit_inherit Clears directory_inherit in the entry that is inherited, preventing further nested subdirectories from also inheriting the entry. only relevant to entries inherited by subdirectories
only_inherit The entry is inherited by created items but not considered when processing the ACL.

If the user and group are the same (example mail mail) , prefix the user/group with "user:" or "group:"

+a inserts into the canonical location.
local deny, local allow, inherited deny, inherited allow
By default, chmod adds entries to the top of the local deny and local allow .

If the entry refers to an user/group already listed, the entries are combined.

Examples

   # ls -dle Documents     #list the directory, long format, including ACL
  drwx------+ 108   3672 Feb  9 20:09 Documents
    0: group:everyone deny delete


  # ls -le     # file1 initially has no ACL
  -rw-r--r--+ 1 juser  wheel  0 Apr 28 14:06 file1

  # chmod +a "admin allow write" file1
  # ls -le
  -rw-r--r--+ 1 juser  wheel  0 Apr 28 14:06 file1
    owner: juser
    1: admin allow write

  # chmod +a "guest deny read" file1   #add in the proper order
  # ls -le
  -rw-r--r--+ 1 juser  wheel  0 Apr 28 14:06 file1
    owner: juser
    1: guest deny read
    2: admin allow write

  # chmod +a "admin allow delete" file1       # add delete to existing  write entry
  # ls -le
  -rw-r--r--+ 1 juser  wheel  0 Apr 28 14:06 file1
    owner: juser
    1: guest deny read
    2: admin allow write,delete
+ai user permission access adds Inherited entries

Examples

             # ls -le
              -rw-r--r--+ 1 juser  wheel  0 Apr 28 14:06 file1
                owner: juser
                1: guest deny read
                2: admin allow write,delete
                3: juser inherited deny delete
                4: admin inherited allow delete
                5: backup inherited deny read
                6: admin inherited allow write-security

              # chmod +ai "others allow read" file1

              # ls -le
              -rw-r--r--+ 1 juser  wheel  0 Apr 28 14:06 file1
                owner: juser
                1: guest deny read
                2: admin allow write,delete
                3: juser inherited deny delete
                4: others inherited allow read
                5: admin inherited allow delete
                6: backup inherited deny read
                7: admin inherited allow write-security

+a# insert entry at a specific location

Examples

              # ls -le
              -rw-r--r--+ 1 juser  wheel  0 Apr 28 14:06 file1
                owner: juser
                1: guest deny read
                2: admin allow write
              # chmod +a# 2 "others deny read" file1
              # ls -le
              -rw-r--r--+ 1 juser  wheel  0 Apr 28 14:06 file1
                owner: juser
                1: guest deny read
                2: others deny read
                3: admin allow write

+ai# inserts inherited entries at a specific location.
These modes allow non-canonical ACL ordering to be constructed.!
-a user premission action deletes matching ACL entries.
If the entry lists a subset of rights granted by an entry, only the rights listed are removed.
Entries may also be deleted by index using -a# .
Inheritance is not considered .

Examples

              # ls -le
              -rw-r--r--+ 1 juser  wheel  0 Apr 28 14:06 file1
                owner: juser
                1: guest deny read
                2: admin allow write,delete
              # chmod -a# 1 file1
              # ls -le
              -rw-r--r--+ 1 juser  wheel  0 Apr 28 14:06 file1
                owner: juser
                1: admin allow write,delete
              # chmod -a "admin allow write" file1
              # ls -le
              -rw-r--r--+ 1 juser  wheel  0 Apr 28 14:06 file1
                owner: juser
                1: admin allow delete

=a# rewritten Individual entries , but may not add new entries. Examples
              # ls -le
              -rw-r--r--+ 1 juser  wheel  0 Apr 28 14:06 file1
                owner: juser
                1: admin allow delete
              # chmod =a# 1 "admin allow write,chown"
              # ls -le
              -rw-r--r--+ 1 juser  wheel  0 Apr 28 14:06 file1
                owner: juser
                1: admin allow write,chown

-E Reads the ACL information from stdin, as a sequential list of ACEs, separated by newlines. If the information parses correctly, the existing information is replaced.
-C Returns false if any of the named files have ACLs in non-canonical order.
-i Removes the 'inherited' bit from all entries in the named file(s) ACLs.
-I Removes all inherited entries from the named file(s) ACL(s).
-N Removes the ACL from the named file(s).

COMPATIBILITY

The -v option is non-standard and its use in scripts is not recommended.

SEE ALSO

chflags(1), fsaclctl(1), install(1), chmod(2), stat(2), umask(2), fts(3), setmode(3), symlink(7), chown(8), mount(8), sticky(8)

STANDARDS

The chmod utility is expected to be IEEE Std 1003.2 (POSIX.2) compatible with the exception of the perm symbol t which is not included in that standard.