Linux : chmod: Changes the access mode (permissions) of a file

chmod [options] who operator permission file-list (symbolic)

chmod [options] mode file-list (absolute)

chmod [options] ––reference=rfile file-list (referential)

The chmod utility changes the ways in which a file can be accessed by the owner of the file, the group to which the file belongs, and/or all other users. Only the owner of a file or Superuser can change the access mode, or permissions, of a file. You can specify the new access mode absolutely, symbolically, or referentially.

Arguments

Arguments specify which files are to have their modes changed in what ways.

Symbolic

You can specify multiple sets of symbolic modes (who operator permission) by separating each set from the next with a comma.

The chmod utility changes the access permission for the class of users specified by who. The class of users is designated by one or more of the letters specified in the who column of Table V-4.

Table V-4. Symbolic mode user class specification

who	User class	Meaning
u	User	Owner of the file
g	Group	Group to which the owner belongs
o	Other	All other users
a	All	Can be used in place of ugo

Table V-5 lists the symbolic mode operators.

Table V-5. Symbolic mode operators

operator	Meaning
+	Adds permission for the specified user class
–	Removes permission for the specified user class
=	Sets permission for the specified user class—resets all other permissions for that user class

The access permission is specified by one or more of the letters listed in Table V-6.

Table V-6. Symbolic mode permissions

permission	Meaning
r	Sets read permission
w	Sets write permission
x	Sets execute permission
s	Sets user ID or group ID (depending on the who argument) to that of the owner of the file while the file is being executed (For more information see page 94.)
t	Sets the sticky bit (Only Superuser can set the sticky bit, and it can be used only with u; see page 903.)
X	Makes the file executable only if it is a directory or if another user class has execute permission
u	Sets specified permissions to those of the owner
g	Sets specified permissions to those of the group
o	Sets specified permissions to those of others

Absolute

You can use an octal number to specify the access mode. Construct the number by ORing the appropriate values from Table V-7. To OR two octal numbers from this table, just add them. (Refer to Table V-8 for examples.)

Table V-7. Absolute mode specifications

mode	Meaning
4000	Sets user ID when the program is executed (page 94)
2000	Sets group ID when the program is executed (page 94)
1000	Sticky bit (page 903)
0400	Owner can read the file
0200	Owner can write to the file
0100	Owner can execute the file
0040	Group can read the file
0020	Group can write to the file
0010	Group can execute the file
0004	Others can read the file
0002	Others can write to the file
0001	Others can execute the file

Table V-8. Examples of absolute mode specifications

Mode	Meaning
0777	Owner, group, and others can read, write, and execute file
0755	Owner can read, write, and execute file; group and others can read and execute file
0711	Owner can read, write, and execute file; group and others can execute file
0644	Owner can read and write file; group and others can read file
0640	Owner can read and write file, group can read file, and others cannot access file

Table V-8 lists some typical modes.

Options

––changes	–c	Displays a message giving the new permissions for each file whose mode is changed.
––quiet or ––silent	–f	Prevents the display of warning messages about files whose permissions prevent chmod from changing the permissions of the file.
––recursive	–R	Recursively descends a directory specified in file-list and changes the permissions on all files in the directory hierarchy.

––reference=rfile

		Changes the permissions of the files in file-list to that of rfile.
––verbose	–v	Displays for each file a message saying that its permissions were changed (even if they were not changed) and specifying the permissions. Use ––changes to display messages only when permissions are actually changed.

Notes

When you are using symbolic arguments, you can omit the permission from the command line only when the operator is =. This omission takes away all permissions. See the second example in the next section.

Examples

The following examples show how to use the chmod utility to change the permissions of the file named temp. The initial access mode of temp is shown by ls (see "Discussion" on page 710 for information about the ls display):

$ ls -l temp

-rw-rw-r-- 1 alex pubs 57 Jul 12 16:47 temp

When you do not follow an equal sign with a permission, chmod removes all permissions for the specified user class. The following command removes all access permissions for the group and all other users so that only the owner has access to the file:

$ chmod go= temp

$ ls -l temp

-rw------- 1 alex pubs 57 Jul 12 16:47 temp

The next command changes the access modes for all users (owner, group, and others) to read and write. Now anyone can read from or write to the file.

$ chmod a=rw temp

$ ls -l temp

-rw-rw-rw- 1 alex pubs 57 Jul 12 16:47 temp

Using an absolute argument, a=rw becomes 666. The next command performs the same function as the previous one:

$ chmod 666 temp

The next command removes write access permission for other users. As a result members of the pubs group can still read from and write to the file, but other users can only read from the file:

$ chmod o-w temp

$ ls -l temp

-rw-rw-r-- 1 alex pubs 57 Jul 12 16:47 temp

The following command yields the same result, using an absolute argument:

$ chmod 664 temp

The next command adds execute access permission for all users:

$ chmod a+x temp

$ ls -l temp

-rwxrwxr-x 1 alex pubs 57 Jul 12 16:47 temp

If temp is a shell script or other executable file, all users can now execute it. (You need read and execute access to execute a shell script but only execute access to execute a binary file.) The absolute command that yields the same result is

$ chmod 775 temp

The final command uses symbolic arguments to achieve the same result as the preceding one. It sets permissions to read, write, and execute for the owner, and to read and write for the group and other users. A comma separates the sets of symbolic modes.

$ chmod u=rwx,go=rw temp