Files and Directory Permissions

Written by Babar Zahoor RHCE 5.0
baber@wbitt.com

We have three types of permissions
read = r
write = w
execute = x

we can change permissions for three categories
owner = u
group = g
others = o

if run ls -l
we see
-rw------- 1 root root 1648 Mar 13 13:47 anaconda-ks.cfg
drwx------ 11 root root 4096 Mar 20 08:56 Desktop
-rw-r--r-- 1 root root 41065 Mar 13 13:46 install.log
-rw-r--r-- 1 root root 5891 Mar 13 13:46 install.log.syslog
drwxr-xr-x 3 root root 4096 Mar 18 23:47 ispconfig
drwxr-xr-x 37 root root 4096 Mar 20 08:25 mplayer
-rw-r--r-- 1 root root 224 Mar 14 18:48 scsrun.log

please see install.log
-rw-r--r-- 1 root root 41065 Mar 13 13:46 install.log
we see here
permissions
- 1st dash show this is file (if d then directory, if we see l then indicates
this is link file or shortcut)

rw- for owner of the file
r-- for the owner's group
r-- for all others then owner and it's group

chmod command is used to change the permissions
+ is use to add
- is use to remove

if we consider file for changing permissions.

example: chmod u+rwx,g+rw,o+rw file

We have another method to implement permissions.
Number System
r = 4
w = 2
x = 1
4+2+1 = rwx = 7 for owner or group or others means
777 for all of them

example: chmod 755 file (same for directories)

We have default permissions in the system

umask is the value which can change default permissions.

umask (to see the default permissions)
0022
please subtract 022 from 777 for directories permissions.
777
-022
-----
755

please subtract 022 from 666 for files permissions.
666
-022
-----
644

these 755, 644 are default permissions for the directories and files as well.


Thanks