Changing file permissions

To change the permissions on a file, use the chmod(C) (change mode) command, which has two formats, ``symbolic'' and ``absolute'', as follows:

chmod who operator permission filename
chmod mode filename

Using the first, symbolic, format, the who field is one or more of the following characters:

a

all users; change all three sets of permissions at once

u

user; change the user, or owner, permissions

g

group; change the group members' permissions

o

others; change the other users' permissions

+

add a new permission

-

remove a new permission

=

set permissions while clearing (removing) all other permissions

$ chmod g+w memo

adds write permission for group members on the file memo.

$ chmod o-wx memo

removes write and execute permission for others (users other than the owner or those in the file's group).

$ chmod o= memo

clears (removes) all permissions for other (setting a NULL permission clears any existing value).

$ chmod u=rx memo

sets read and execute for user, clearing (removing) write permission (which is not specified in the ``='' command.)

$ chmod a+w memo

adds write permission to the existing permissions for all categories of user.

Using octal numbers to set permissions

Permissions	Octal number
---	0
--x	1
-w-	2
-wx	3
r--	4
r-x	5
rw-	6
rwx	7

Permission to execute a file is represented by a value of 1. Permission to write a file is represented by a value of 2. Permission to read a file is represented by a value of 4. These values are added together to produce the combinations in the table above.

Three octal numbers (numbers in the range 0 to 7) are used to represent the owner, group and other permissions respectively. Thus, by adding the permissions for a given category of user, you produce a digit; and by specifying three digits (one for each set of users) you can specify all the permissions on a file, as follows:

   $ l myfile
   -rw-r--r--   1 johnd techpubs    5061 Feb 10 15:01 myfile
   $ chmod 640 myfile
   $ l myfile
   -rw-r-----   1 johnd techpubs    5061 Feb 10 15:01 myfile