chmod(1) - Useful Manual Pages

NAME

chmod — change file modes or Access Control Lists

SYNOPSIS

chmod [-fhv] [-R [-H | -L | -P]] mode file ...

chmod [-fhv] [-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 ...

DESCRIPTION

The chmod utility modifies the file mode bits of the listed files as specified by the mode operand. It may also be used to modify the Access Control Lists (ACLs) associated with the listed files.

The generic options are as follows:

-f: Do not display a diagnostic message if chmod could not modify the mode for file, nor modify the exit status to reflect such failures.
-H: If the -R option is specified, symbolic links on the command line are followed and hence unaffected by the command. (Symbolic links encountered during tree traversal are not followed.)
-h: If the file is a symbolic link, change the mode of the link itself rather than the file that the link points to.
-L: If the -R option is specified, all symbolic links are followed.
-P: If the -R option is specified, no symbolic links are followed. This is the default.
-R: Change the modes of the file hierarchies rooted in the files, instead of just the files themselves. Beware of unintentionally matching the “..” hard link to the parent directory when using wildcards like “.*”.
-v: Cause chmod to be verbose, showing filenames as the mode is modified. If the -v flag is specified more than once, the old and new modes of the file will also be printed, in both octal and symbolic notation.

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.

If chmod receives a SIGINFO signal (see the status argument for stty(1)), then the current filename as well as the old and new modes are displayed.

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

EXIT STATUS

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: (the setuid bit). Executable files with this bit set will run with effective uid set to the uid of the file owner. Directories with this 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: (the setgid bit). Executable files with this bit set will run with effective gid set to the gid of the file owner.
1000: (the sticky bit). See chmod(2) and sticky(7).
0400: Allow read by owner.
0200: Allow write by owner.
0100: For files, allow execution by owner. For directories, allow the owner to search in the directory.
0040: Allow read by group members.
0020: Allow write by group members.
0010: For files, allow execution by group members. For directories, allow group members to search in the directory.
0004: Allow read by others.
0002: Allow write by others.
0001: For files, allow execution by others. For directories allow others to 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 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 original mode of the file.
g: The group permission bits in the original mode of the file.
o: The other permission bits in the original mode of the 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 (see umask(2)) 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 set, 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 (see umask(2)) 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.

The ``w'' permission on directories will permit file creation, relocation, and copy into that directory. Files created within the directory itself will inherit its group ID.

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.

ACL MANIPULATION OPTIONS

ACLs are manipulated using extensions to the symbolic mode grammar. Each file has one ACL, containing an ordered list of entries. Each entry refers to a user or group, and grants or denies a set of permissions. In cases where a user and a group exist with the same name, the user/group name can be prefixed with "user:" or "group:" in order to specify the type of name.

If the user or group name contains spaces you can use ':' as the delimiter between name and permission.

The following permissions are applicable to all filesystem objects:

delete: Delete the item. Deletion may be granted by either this permission on an object or the delete_child right on the containing directory.
readattr: Read an object's basic attributes. This is implicitly granted if the object can be looked up and not explicitly denied.
writeattr: Write an object's basic attributes.
readextattr: Read extended attributes.
writeextattr: Write extended attributes.
readsecurity: Read an object's extended security information (ACL).
writesecurity: Write an object's security information (ownership, mode, ACL).
chown: Change an object's ownership.

The following permissions are applicable to directories:

list: List entries.
search: Look up files by name.
add_file: Add a file.
add_subdirectory: Add a subdirectory.
delete_child: Delete a contained object. See the file delete permission above.

The following permissions are applicable to non-directory filesystem objects:

read: Open for reading.
write: Open for writing.
append: Open for writing, but in a fashion that only allows writes into areas of the file not previously written.
execute: Execute the file as a script or program.

ACL inheritance is controlled with the following permissions words, which may only be applied to directories:

file_inherit: Inherit to files.
directory_inherit: Inherit to directories.
limit_inherit: This flag is only relevant to entries inherited by subdirectories; it causes the directory_inherit flag to be cleared in the entry that is inherited, preventing further nested subdirectories from also inheriting the entry.
only_inherit: The entry is inherited by created items but not considered when processing the ACL.

The ACL manipulation options are as follows:

+a

The +a mode parses a new ACL entry from the next argument on the commandline and inserts it into the canonical location in the ACL. If the supplied entry refers to an identity already listed, the two entries are combined.

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

The +a mode strives to maintain correct canonical form for the ACL.
local deny
local allow
inherited deny
inherited allow

By default, chmod adds entries to the top of the local deny and local allow lists. Inherited entries are added by using the +ai mode.

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#

When a specific ordering is required, the exact location at which an entry will be inserted is specified with the +a# mode.

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

The +ai# mode may be used to insert inherited entries at a specific location. Note that these modes allow non-canonical ACL ordering to be constructed.

-a

The -a mode is used to delete ACL entries. All entries exactly matching the supplied entry will be deleted. 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 the -a# mode.

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

Inheritance is not considered when processing the -a mode; rights and entries will be removed regardless of their inherited state.

If the user or group name contains spaces you can use ':' as the delimiter

Example
# chmod +a "User 1:allow:read" file1

=a#

Individual entries are rewritten using the =a# mode.

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

This mode may not be used to add new entries.

-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