Linux - File System Permissions

File permissions in Linux are required to restrict access to owners, groups and others. It secures the files and directories from unauthorized access. There are 3 types of file permissions:

Basic permission
Special permission
Access Control List (ACL permission)

Viewing Permissions

Check the file permission.

 ubuntu@ip-172-31-93-3:~$ ls -al newfile_txt
 # Permission Link Owner GroupOwner FileSize Date   Time  Filename
   -rw-rw-r--  1   ubuntu   ubuntu      0    Mar 31 20:21 newfile_txt

Check the directory permission.

 ubuntu@ip-172-31-93-3:~$ ls -ld dev 
 # Permission Link Owner GroupOwner FileSize Date   Time  directoryName
   drwxrwxr-x  2   ubuntu ubuntu     4096    Mar 31 20:29   dev

Permission Groups/ Classes:

Owner (u)	Permissions are used for the owner of the file
Group (g)	Permissions used by members of the group
Other (o)	Permissions used by all other users

Permission Types:

Permission	Access to a file	Access to a directory
Read (r)	display file contents and copy the file	view the contents of the directory
Write (w)	modify the file contents	modify the contents of the directory
Execute (x)	execute the file if it is an executable allow	allow the use of the cd command to access the directory

Permission with Numeric & Symbols :

Number	Permission Type	Symbol
0	No Permission	- - -
1	Execute	- - x
2	Write	- w -
3	Execute + Write	- w x
4	Read	r - -
5	Read + Execute	r - x
6	Read + Write	r w -
7	Read + Write + Execute	r w x

Change permissions of the file/folder :

Give execute permission to the owner/user

 # [syntax: chmod u+<permission_type> <name of file or directory>]
 ubuntu@ip-172-31-93-3:~$ chmod u+x newfile_txt

Give write and execute permissions to groups

 # [syntax: chmod g+<permission_type> <name of file or directory>]
 ubuntu@ip-172-31-93-3:~$ chmod g+wx newfile_txt

Give write permission to others

 # [syntax: chmod o+<permission_type> <name of file or directory>]
 ubuntu@ip-172-31-93-3:~$ chmod o+w newfile_txt

Remove execute permission of the owner/user

 # [syntax: chmod u-<permission_type> <name of file or directory>]
 ubuntu@ip-172-31-81-146:~$ chmod u-x newfile_txt

Change ownership of a file

 # [syntax: chown <user_name> <name of file or directory>]
 ubuntu@ip-172-31-81-146:~$ chown steve newfile_txt

Change group ownership

 # [syntax: chown <group_name> <name of file or directory>]
 ubuntu@ip-172-31-81-146:~$ chgrp steve newfile_txt

Set the permission with a numeric value of a file 'rwx' for a user, 'rw' for group, 'r' for others

 # [syntax: chmod <numeric_permissions> <name of file or directory>]
 ubuntu@ip-172-31-81-146:~$ chmod 764 newfile_txt

Access Control List (ACL)

It provides an additional, more flexible permission mechanism for file systems. It is a service that provides special permissions to specific users and groups for particular directories and files.

Also, we can provide read-and-write access to a particular user that is not a member of the group created by you using ACL.

Check ACL permission

 # [syntax: getfacl <name of file or directory>]
 ubuntu@ip-172-31-94-49:~$ getfacl newfile.txt

Set ACL permission to the user

 # [syntax: setfacl -m u:<user_name>:<permission_type> <name of file or directory>]
 ubuntu@ip-172-31-94-49:~$ setfacl -m u:steve:rwx newfile.txt

Set ACL permission to group

 # [syntax: setfacl -m g:<group_name>:<permission_type> <name of file or directory>]
 ubuntu@ip-172-31-94-49:~$ setfacl -m g:testgrp:rw newfile.txt

Remove ACL permission of the user

 # [syntax: setfacl -x u:<user_name> <name of file or directory>]
 ubuntu@ip-172-31-94-49:~$ setfacl -x u:steve newfile.txt

Remove ACL permission from the group

 # [syntax: setfacl -x g:<group_name> <name of file or directory>]
 ubuntu@ip-172-31-94-49:~$ setfacl -x g:testgrp newfile.txt

Remove all ACL permissions

 # [syntax: setfacl -b <name of file or directory>]
 ubuntu@ip-172-31-94-49:~$ setfacl -b newfile.txt