chmod Command with Numbers
To manage the many combinations and define the access mode of each file or directory in a more efficient way, the assigned numbers are also used for the access mode setting. The numbers are assigned for each permission as shown below.
r: 4w: 2x: 1-: 0
Using the sum of all numbers, you can represent all access modes with a single digit for one user type. To cover the three user types, you need only three digits. Here are some examples.
755 : typical access mode when the superuser creates a directory
- Owner User :
rwx - Owner Group :
r-x - Others :
r-x
775 : typical access mode when a normal user creates a directory
- Owner User :
rwx - Owner Group :
rwx - Others :
r-x
644 : typical access mode when the superuser creates a file
- Owner User :
rw- - Owner Group :
r-- - Others :
r--
664 : typical access mode when a normal user creates a file
- Owner User :
rw- - Owner Group :
rw- - Others :
r--
700 : typical .ssh directory permission
- Owner User :
rwx - Owner Group :
--- - Others :
---
600 : typical ssh private key permission
- Owner User :
rw- - Owner Group :
--- - Others :
---
Practice
1. Confirm the initial permission setting when creating new documents
To check the initial permission, create four types of documents:
- one directory created by a normal user (dir_ubuntu)
- one file created by a normal user (file_ubuntu)
- one directory created by the superuser (dir_root)
- one file created by the superuser (file_root)
To create the documents, run the commands below.
mkdir dir_ubuntu
touch file_ubuntu
sudo su
mkdir dir_root
touch file_root
Run the ls -l command to see the permission of each document. You can see that the initial permission settings are the same as the ones explained in this section.
ls -l
drwxr-xr-x 2 root root 4096 Jan 3 01:36 dir_root
drwxrwxr-x 2 ubuntu ubuntu 4096 Jan 3 01:37 dir_ubuntu
-rw-r--r-- 1 root root 0 Jan 3 01:36 file_root
-rw-rw-r-- 1 ubuntu ubuntu 0 Jan 3 01:37 file_ubuntu
2. Change access mode using numbers
To change the access mode of dir_ubuntu to 700 and file_ubuntu to 600, run the command below.
chmod 700 dir_ubuntu
chmod 600 file_ubuntu
ls -l
drwxr-xr-x 2 root root 4096 Jan 3 01:36 dir_root
drwx------ 2 ubuntu ubuntu 4096 Jan 3 01:37 dir_ubuntu
-rw-r--r-- 1 root root 0 Jan 3 01:36 file_root
-rw------- 1 ubuntu ubuntu 0 Jan 3 01:37 file_ubuntu
You can also learn this topic offline. Click 
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