File Permissions
chmod is used to change file permissions.
Permission Groups
Each file in Linux has 3 permission groups.
- Owner - Apply to the owner of the file
- Group - Apply to the user group assigned to the file
- Others - Apply to all other users
Viewing the Permissions
A simple way to view the file permissions is to use ls -l command whereas -l flag stands for
use a long listing format.
$ ls -l
total 16
drwxrwxr-x 5 wtongze wtongze 4096 Sep 3 08:42 docs
-rw-rw-r-- 1 wtongze wtongze 1554 Sep 3 09:54 mkdocs.yml
-rw-rw-r-- 1 wtongze wtongze 524 Aug 28 08:21 requirements.txt
drwxrwxr-x 7 wtongze wtongze 4096 Sep 3 11:44 site
-rwxrw-r-x
│││││││└┴┴ Others (x1)
││││└┴┴ Group (x10)
│└┴┴ User (x100)
└ File type
| Permission | Meaning | Value |
|---|---|---|
r |
Read | 4 |
w |
Write | 2 |
x |
Execute | 1 |
The above permission can also be writen as 0765 because:
$$
(4 + 2 + 1) * 100 + (4 + 2) * 10 + (4 + 1) * 1 = 0765
$$
Info
Permission for directories often permit execute for all users in order to allow search for access to directory meta information and change working directory.
Leftmost digit in the 4-digit permission can also have the following attributes1:
| Meaning | Value |
|---|---|
| Set User ID | 4 |
| Set Group ID | 2 |
| Sticky | 1 |
Changing the Permissions
chmod command is used to change the permissions for files.
Permission Group Shorthand
u- Userg- Groupo- Othersa- All users
Examples
chmod a+x example.sh # Allow all uses to execute
chmod o-x example.sh # Disallow others to execute
chmod 700 private.txt # Only file owner can read, write and execute
chmod -R 600 folder/ # Change permission recursively