Octal to Binary Conversion Chart
Octal Binary
R W X
4 2 1
0 0 0 0
1 0 0 1
2 0 1 0
3 0 1 1
4 1 0 0
5 1 0 1
6 1 1 0
7 1 1 1

Command Examples

chmod

The chmod command can be used with either a text-based argument or 3 octal digits (see note 1) to change the permissions on a file.  An example of the text-based command to add "read" permission for group members and others to a file named foo is: 

    /home/user> ls -l foo
    -rwx--x--- 1 user user 78 Aug 14 13:08 foo
    /home/user> chmod go+r foo
    /home/user> ls -l foo
    -rwxr-xr-- 1 user user 78 Aug 14 13:08 foo
    /home/user>

Each permission can be thought of as a single binary digit that is "1"if the permission is granted and "0" if not.  The nine permissions are naturally grouped into three sets of 3 permissions each.  The first set is the permissions for the owner, the second set is the permissions for group members, and the last set is the permissions for others.

Each set of three binary digits can be represented with a single octal digit.  For example, to grant read ("R") and execute ("X") but not write ("W") permissions, the three binary digits should be 1 0 1.  Looking this up in the chart show that the corresponding octal digit is 5.

Another way to see the same result is to note the weights shown in the chart for each permission.  By adding up the weights for the permissions you wish to grant, the octal digit is easily obtained.  For example, to grant read ("R") and write ("W") but not execute ("X"), you sum the weight for read (4) and write (2) to get the octal digit 6.

umask

The umask command changes an environment setting that controls which permissions newly created files will have.  This command will never change the permissions of any existing files.  And unless extra steps (not discussed here) are taken, the new setting will be forgotten as soon as you log off.

To view the current setting, enter the command "umask".  This will report the current setting as a set of three octal digits.  (See note 2.)  To change the setting, enter the command umask new_value, where new_value is three octal digits.  The first digit is the mask for the file owner (or user), the second is the mask for the group, and the third is the mask for all others.  Each octal digit is expanded to three binary digits, to set the value for each individual permission bit of the nine permission bits (three groups of three).

It is important to understand that the umask setting doesn't enforce a set of permissions for new files.  Instead it blocks permissions from files.  To complicate matters ever more (if possible), a zero binary digit will allow that permisison while a one will block that permission.  This is backwards from they way people expect after learning chmod.  Consider the following examples: 

    /home/user> umask
    027
    /home/user> umask 000 # allow any permissions
    /home/user> umask 077 # block all permissions for group, others
    /home/user>

The first example show how to view the current setting, which block write permission ("W") for the group and all permissions for others.  The second example doesn't block any permissions, and the last example blocks all permissions.

Different commands in Unix will create files using a Unix API (a system call) called creat.  The programmer tells creat what permissions are desired for the new file.  For example the vi command attempts to create files with read and write permissions for all users, whereas the C compiler attemps to create files with all permissions for all users.  The request permissions will be granted unless blocked by umask.

To understand this better, try setting the umask value to 000, then create a new file using vi.  Now see what permissions the file has by using the ls -l command.  Try other values and then create new files and see what permissions the files end up with.

Footnotes:

Note 1:

Actually up to four octal digits can be entered.  This document does not discuss all the features and the concepts of either permissions or of the chmod and umask commands.   (Back)

Note 2:

On some older Unix systems, leading zeros in the numeric output of commands such as umask were suppressed.  So an output of 0 for 000 or 02 for 002 was common to see.   (Back)