umask and permission in unix.

Many one got confused with the setting of umask in unix or linux system. Whenever I was a newbie in this field I also got a bit confused with this settings. In a nutshell umask defines what will be the default permission of a file whenever a user create the file. You already are familiar with the unix permission of 754. The first digit 7(4 read +2 write+1 execute) is for permission for owner, the second digit 5 is for permission for the group and third digit 4 is for permission for others.

In brief,

User class: Owner Group Others
character representation: rwx r-x r--
binary representation: 111 101 100
octal representation: 7 5 4

Now lets a look at ls -l after creating a file without setting any umask.
-bash-3.00$ touch without_umask.txt
-bash-3.00$ ls -l
total 0
-rw-r--r-- 1 oracle oinstall 0 Aug 24 00:05 without_umask.txt

After omitting first digit as it indicates whether file or folder we get
first 3 digit after - is 110 that is 6 for user oracle permission.
second pair of 3 digits are 100 that is 4 which is for all users who are under oinstall group.
third pair of 3 digits are 100 that is 4 which is for others.

Now start with setting umask of 022. With this setting, Files (and directories) normally created with mode 777 become mode 755. Files (and directories) created with mode 666 become mode 644).

In a word umask just deducted its value(here is 022) from 777 (or other mode that a file created) and then it changes permission to the file.

-bash-3.00$ umask 022
-bash-3.00$ touch with_umask1.txt
-bash-3.00$ ls -l
total 0
-rw-r--r-- 1 oracle oinstall 0 Aug 24 00:06 with_umask1.txt
-rw-r--r-- 1 oracle oinstall 0 Aug 24 00:05 without_umask.txt

Here we see umask of 022 set permission to with_umask1.txt file to 644.

-bash-3.00$ umask 002
-bash-3.00$ touch with_umask2.txt
-bash-3.00$ ls -l
total 0
-rw-r--r-- 1 oracle oinstall 0 Aug 24 00:06 with_umask1.txt
-rw-rw-r-- 1 oracle oinstall 0 Aug 24 00:06 with_umask2.txt
-rw-r--r-- 1 oracle oinstall 0 Aug 24 00:05 without_umask.txt
Here we see umask of 002 set permission to with_umask2.txt file to 664.

-bash-3.00$ umask 000
-bash-3.00$ touch with_umask3.txt
-bash-3.00$ ls -l
total 0
-rw-r--r-- 1 oracle oinstall 0 Aug 24 00:06 with_umask1.txt
-rw-rw-r-- 1 oracle oinstall 0 Aug 24 00:06 with_umask2.txt
-rw-rw-rw- 1 oracle oinstall 0 Aug 24 00:06 with_umask3.txt
-rw-r--r-- 1 oracle oinstall 0 Aug 24 00:05 without_umask.txt

Here we see umask of 000 set permission to the file (666-000)=666.
We can chnage its settings later by chmod if we wish.
-bash-3.00$ chmod 777 with_umask3.txt
-bash-3.00$ ls -l
total 0
-rw-r--r-- 1 oracle oinstall 0 Aug 24 00:06 with_umask1.txt
-rw-rw-r-- 1 oracle oinstall 0 Aug 24 00:06 with_umask2.txt
-rwxrwxrwx 1 oracle oinstall 0 Aug 24 00:06 with_umask3.txt
-rw-r--r-- 1 oracle oinstall 0 Aug 24 00:05 without_umask.txt