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Linux Permissions

A quick reference

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The Tech Guy
·May 31, 2022·

4 min read

Linux Permissions
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Required Reads: How to manage users in Linux Ubuntu.

The intention of this article is to provide you with a quick summary on the most important topics you should focus to successfully manage file and directory permissions.

Standard Access Permissions: Exist as the default method which works on individual files and directories. (covered in this article)

Access Control Lists (ACLs): Allow the administrator to enforce extended security attributes on files and directories. (will be discussed on another article)

User access rights are form of Permission Classes and Types

Classes: user (u), group (g), other (o)

Types: read(r), write(w), execute(x)

Permission Modes: add (+), revoke (-), assign (=)

The following is the structure of permissions:

Type of FileUserGroupOther
d (dir)
- (file)
rwxrwxrwx
OctalSymDescription
0---No permissions
1--xExecute Only
2-w-Write only
3-wxWrite Execute
4r--Read Only
5r-wRead Execute
6rw-Read Write
7rwxRead Write Execute
XXX
421

You can modify permissions in two ways by using the command chmode which sets the permissions for files and directories

Consider the following file:

ls -ll | grep file

-r--r--r-- 1 server server 0 May 31 20:29 file

Symbolic Form

server@server:~$ chmod u+x file -v
mode of 'file' changed from 0444 (r--r--r--) to 0544 (r-xr--r--)
server@server:~$

In the same way, you can use a combination of Class, Type and Mode

i.e ugo=x o=r g-x ... etc.

Octal Form

server@server:~$ chmod 544 file -v
mode of 'file' changed from 0777 (rwxrwxrwx) to 0544 (r-xr--r--)

The concept of UMASK:

Linux assigns default permissions to new files and directories based on the umask value.

The umask is a three-digit octal value that refers to the u,g,o, the default Values are:

  • 0022 - Root
  • 0002 - All normal users
server@server:~$ umask
0002

server@server:~$ umask -S
u=rwx,g=rwx,o=rx

The default inital values that Linux assigned are:

  • Files: 666 (rw-rw-rw-)
  • Directories: 777 (rwxrwxrwx)

Because of the umask value, your file will be created as 664 and your directory as 775. The way you calculate this is by subtracting the umask value to the default initial values.

For example, if you want no changes on any new file or directory you create then you apply the following command:

server@server:~$ umask -S
u=rwx,g=rwx,o=rx
server@server:~$ umask 000
server@server:~$ umask -S
u=rwx,g=rwx,o=rwx

Notice that now our files and directories will be created with the default initial values.

I don't recommend umask 000, in fact you should set your umask based on your security needs.

Special File Permissions:

Lastly, there are three more Bits that need to be taken in consideration:

Bit
setuidSet on binary executable files at the owner levelAllows the file to be executed by no-owners with the same privileges as the user-rwSrwxrwx
setgidSet on binary executable files at the group levelAllows to execute the file by no-owners with the same privileges as the group-rwxrwSrwx
stickySet on public and shared writable directoriesProtect files and subdirectories from being deleted or moved by other usersdrwxrwxrwT

Example: setuid

server@server:~$ chmod -v u+s file 
mode of 'file' changed from 0544 (r-xr--r--) to 4544 (r-sr--r--)

server@server:~$ chmod -v +4000 file 
mode of 'file' retained as 4544 (r-sr--r--)

Example: setgid

server@server:~$ chmod -v g+s file
mode of 'file' changed from 4544 (r-sr--r--) to 6544 (r-sr-Sr--)

server@server:~$ chmod -v +2000 file
mode of 'file' retained as 6544 (r-sr-Sr--)

Example: sticky

server@server:~$ ll | grep dir
drwxrwxrwx  2 server server     4096 May 31 21:16 testdir/
server@server:~$ chmod -v +1000 testdir/
mode of 'testdir/' changed from 0777 (rwxrwxrwx) to 1777 (rwxrwxrwt)

This is it, I hope you enjoyed it !!!

 
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