Numerical permissions with chmod

The chmod command is used to manage file system access permissions on Unix and Linux systems. There are three basic file system permissions applied to files and directories:

read (r)
write (w)
execute (x)

Each of these permissions can be applied to these three classes of users:

user (u)
group (g)
other (o)

The user is the account that owns the file and the group is generally a collection of users. Other is everyone else, including the public outside world.

Checking permissions

Type in ls -la in any directory which contains files. An example output looks like this, showing the file permissions, owner and group:

$ ls -la
total 44
drwxr-xr-x  2 user group  4096 Apr  7 13:13 .
drwxr-xr-x 23 user group  4096 Apr 17 09:42 ..
-rw-r--r--  1 user group  9632 Feb 19  2022 file-one.txt
-rw-r--r--  1 user group 21731 Jul 11  2019 file-two.txt

Concerning the two files, file-one.txt and file-one.txt, we see the permissions represented as -rw-r--r--.

The order of permissions from left-to-right is Owner + Group + Other. Therefore, -rw-r--r-- means:

-rw- r-- r-- (Owner can Read and Write)
-rw- r-- r-- (Group members can Read)
-rw- r-- r-- (Other users can Read)

Numerical values

Having grown up on the Internet, my brain is hardwired to think in FTP-applied permissions for HTML directories and files.

0 = ---     [No permissions]
1 = --x     [Can execute]
2 = -w-     [Can write]
3 = -wx     [Can write and execute]
4 = r--     [Can read]
5 = r-x     [Can read and execute]
6 = rw-     [Can read and write]
7 = rwx     [Can read, write and execute]

Directories are always 755, represented as:

drwxr-xr-x  2 user group  4096 Apr  7 13:13 .

Files are always 644, represented as:

-rw-r--r--  1 user group  9632 Feb 19  2022 file-one.txt

Changing permissions

Where files (such as scripts) require execution permissions, 740 works quite well. This allows only the owner to execute the file.

Script.sh shows a permission of value of 644:

$ ls -la
-rw-r--r--  1 user group     0 Apr 17 10:55 script.sh

To apply 740 permissions, run:

$ chmod 740 script.sh

Confirming the change:

$ ls -la
-rwxr-----  1 user group     0 Apr 17 10:55 script.sh