K-Sato

Posted on Jun 6, 2019 • Edited on Nov 25, 2019

Understanding Linux Permissions

K-Sato · 2019-06-06T04:54:44Z

Table of contents Introduction File Permissions File Type File Mode Change File Mode Octal Mode Symbolic Mode Introduction The multi-user capability of Unix-like systems is a feature that is deeply ingrained into the design of the operating system. File Permissions On a Linux system, each file and directory is assigned access rights for the owner of the file. You can check the permission settings with ls -l . $ ls -l drwxr-xr-x. 13 root root 1027 Jan 3 12:32 bin/cat Enter fullscreen mode Exit fullscreen mode Let's explore what drwxr-xr-x. 13 root root 1077 Jan 3 12:32 bin/cat means one by one. Command Meaning d File Type rwxr-xr-x. File Mode 13 Number of links root The owner of the file root The group the file belongs to 1027 Size of the file Jan 3 12:32 Time Stamp bin/cat The name of the file/directory File Type Command File Type - File d Directory l Symbolic Link File Mode The r letter means the user has permission to read the file/directory. The w letter means the user has permission to write the file/directory. And the x letter means the user has permission to execute the file/directory. Command Meaning r read w write x execute - not allowed Let's take a look at the 9 letters in the command. The first 3 letters show the permissions for the file owner, the second 3 letters show the permissions for the group owner and the last 3 letters show the permissions for other users. rwx/ r-x/ r-x/ Owner: rwx Group: r-x Users: r-x Enter fullscreen mode Exit fullscreen mode Change File Modes The chmod command is used to change the permissions of a file or directory. Octal Mode Each permission may be specified with an octal number: read = 4 ; write = 2 ; execute = 1 ; no permission = 0 . Meaning Number read(r) 4 write(w) 2 execute(x) 1 Example The command below means giving permissions to read(4) , write(2) and execute(1) to the owner and permissions to read(4) and execute(1) to the group user and permissions to read(4) to other users. chmod 754 myfile Enter fullscreen mode Exit fullscreen mode Symbolic Mode The below is the basic syntax of chmod . % chmod who operator permission filename Enter fullscreen mode Exit fullscreen mode You can use the following commands to change modes. Command Meaning u(user) user access g(group) group access o(other) other access a(all) user, group, and other access Command Meaning + add specified permissions - remove specified permissions = set the specified permissions Example In the following example, read permission are taken away from others. % chmod o-r filea Enter fullscreen mode Exit fullscreen mode In the following example, read and execute permissions are added for user, group, and others. $ chmod a+rx fileb Enter fullscreen mode Exit fullscreen mode In the following example, read, write, and execute permissions are assigned to the group. $ chmod g=rwx filec Enter fullscreen mode Exit fullscreen mode References Learning the shell - Lesson 9: Permissions

#linux #permissions #shell

Introduction
File Permissions
File Type
File Mode
Change File Mode
- Octal Mode
- Symbolic Mode

Introduction

The multi-user capability of Unix-like systems is a feature that is deeply ingrained into the design of the operating system.

File Permissions

On a Linux system, each file and directory is assigned access rights for the owner of the file.

You can check the permission settings with ls -l.

$ ls -l 
drwxr-xr-x. 13 root  root  1027 Jan  3 12:32 bin/cat

Let's explore what drwxr-xr-x. 13 root root 1077 Jan 3 12:32 bin/cat means one by one.

Command	Meaning
d	File Type
rwxr-xr-x.	File Mode
13	Number of links
root	The owner of the file
root	The group the file belongs to
1027	Size of the file
Jan　3 12:32	Time Stamp
bin/cat	The name of the file/directory

File Type

Command	File Type
-	File
d	Directory
l	Symbolic Link

File Mode

The r letter means the user has permission to read the file/directory. The w letter means the user has permission to write the file/directory. And the x letter means the user has permission to execute the file/directory.

Command	Meaning
r	read
w	write
x	execute
-	not allowed

Let's take a look at the 9 letters in the command.
The first 3 letters show the permissions for the file owner, the second 3 letters show the permissions for the group owner and the last 3 letters show the permissions for other users.

rwx/ r-x/ r-x/

Owner: rwx
Group: r-x
Users: r-x

Change File Modes

The chmod command is used to change the permissions of a file or directory.

Octal Mode

Each permission may be specified with an octal number: read = 4; write = 2; execute = 1; no permission = 0.

Meaning	Number
read(r)	4
write(w)	2
execute(x)	1

Example

The command below means giving permissions to read(4), write(2) and execute(1) to the owner and permissions to read(4) and execute(1) to the group user and permissions to read(4) to other users.

chmod 754 myfile

Symbolic Mode

The below is the basic syntax of chmod.

% chmod who operator permission filename

You can use the following commands to change modes.

Command	Meaning
u(user)	user access
g(group)	group access
o(other)	other access
a(all)	user, group, and other access

Command	Meaning
+	add specified permissions
-	remove specified permissions
=	set the specified permissions

Example

In the following example, read permission are taken away from others.

% chmod o-r filea

In the following example, read and execute permissions are added for user, group, and others.

$ chmod a+rx fileb

In the following example, read, write, and execute permissions are assigned to the group.

$ chmod g=rwx filec

References

Learning the shell - Lesson 9: Permissions

Oldest comments (76)

David J Eddy • Jun 6 '19

When it comes to Linux I make it a point to read even intro level articles like this one. Why? 'cause I am constantly learning new things, like I did here.

Thank you for this. Keep up the good posting!

K-Sato • Jun 7 '19

Thank you for your kind words!

Arun • Jun 6 '19

From what I know, I guess you can forgo "a" in the "a+rx".

K-Sato • Jun 7 '19

Thank you for sharing your knowledge:)!

K-Sato • Jun 7 '19 • Edited

Thank you for pointing that out:) Fixed it!

Danny Priymak • Jun 8 '19 • Edited

This article did not introduce anything brand new to me, but nonetheless very concise and well written!
Thank you for sharing.

K-Sato • Jun 8 '19

Thank you for taking the time to read it!!

Jonathan Boudreau • Jun 8 '19

There's also suid/sgid permissions. Normally, when you execute a program, it will run as the user which called it. suid will instead run the program as the owner of the file. For example, if you are logged in as user developer and try to execute a suid program owned by root, it will execute as root instead of developer.

su, sudo, passwd rely on suid to work.

Thomas H Jones II • Nov 13 '19

The extension to suid-/sgid-enabled operations being that, when you run auditing services on a system, actions are logged both by actual executing-user and effective executing user.

Bryan Baldwin • Jun 9 '19

chattr +i filename

Thomas H Jones II • Nov 13 '19

If you really want to get into powerful permissioning systems, take a look at extended filesystem access control lists (via the setfacl and getfacl utilities), extended attributes (xattrs managed via the chattr and lsattr commands) and, the ultimate cause of headaches, SELinux.

If you're running an Internet-facing system, these are all security-extensions that you want to be reasonably well versed in.

Tom Cruise • Feb 25 '22

Thank you very much for your post; I fixed it. I also hope you continue to write great posts and that we can continue the conversation; Thank you very much, dear. io games wordle website

Phuong Le • Dec 22 '22 • Edited

After reading this article, I also have a better understanding of Linux permissions. This is a great article and I'm looking into some more Linux articles. wordle game free Wordle

Donna Le • May 25 '22

I have been looking for this information for a long time, thank you very much, the information in the article has helped me a lot in my work. In your free time, do you often play games to practice your foreign language? try waffle game

maika22 • Dec 28 '22 • Edited

Very good article, I just wanted to say that this article has clear programming numbers and Wordle that are easy to understand for a newbie like me. Thank you for sharing this useful information.

View full discussion (76 comments)

Some comments may only be visible to logged-in visitors. Sign in to view all comments.