Permissions

1. Overview

• chmod - modify file access rights
• su - temporarily become the superuser
• sudo - temporarily become the superuser
• chown - change file ownership
• chgrp - change a file's group ownership

2. Description

2.1 View permissions on a file

[me@linuxbox me]$ ls -l /bin/bash
-rwxr-xr-x 1 root root  316848 Feb 27  2000 /bin/bash

• The file "/bin/bash" is owned by user "root"
• The superuser has the right to read, write, and execute this file
• The file is owned by the group "root"
• Members of the group "root" can also read and execute this file
• Everybody else can read and execute this file

rwx rwx rwx = 111 111 111
rw- rw- rw- = 110 110 110
rwx --- --- = 111 000 000

and so on...

rwx = 111 in binary = 7
rw- = 110 in binary = 6
r-x = 101 in binary = 5
r-- = 100 in binary = 4

 

2.2 chmod

The chmod command is used to change the permissions of a file or directory. To use it, you specify the desired permission settings and the file or files that you wish to modify. There are two ways to specify permissions which are octal notation method and symbolic notation method.

2.2.1 Symbolic notation method

options	Definition
u	owner
g	group
o	other
a	all(same as ugo)
x	execute
w	write
r	read
+	add permission
-	remove permission
=	set permission

chmod a-w file (removes all writing permissions)
chmod o+x file (sets execute permissions for other (public permissions))
chmod u=rx file        (Give the owner rx permissions, not w)
chmod go-rwx file      (Deny rwx permission for group, others)
chmod g+w file         (Give write permission to the group)
chmod a+x file1 file2  (Give execute permission to everybody)
chmod g+rx,o+x file    (OK to combine like this with a comma)

u = user that owns the file
g = group that owns the file
o = other (everyone else)
a = all (everybody)

r = read aces to the file
w = write access
x = execute (run) access 

2.2.2 Octal notation method

options	Definition
#--	owner
-#-	group
--#	other
1	execute
2	write
4	read

2.3 Directory Permissions

• r - Allows the contents of the directory to be listed if the x attribute is also set.
• w - Allows files within the directory to be created, deleted, or renamed if the x attribute is also set.
• x - Allows a directory to be entered (i.e. cd dir).

Value	Meaning
777	(rwxrwxrwx) No restrictions on permissions. Anybody may list files, create new files in the directory and delete files in the directory. Generally not a good setting.
755	(rwxr-xr-x) The directory owner has full access. All others may list the directory, but cannot create files nor delete them. This setting is common for directories that you wish to share with other users.
700	(rwx------) The directory owner has full access. Nobody else has any rights. This setting is useful for directories that only the owner may use and must be kept private from others.

2.4 Switch to Superuser

[me@linuxbox me]$ su
Password:
[root@linuxbox me]#

2.5 Employ Superuser privilege

[me@linuxbox me]$ sudo some_command
Password:
[me@linuxbox me]$

3. Changing Ownership

3.1 Changing User Ownership

A file's owner can be changed using the chown command. For example, to change the foobar file's owner to tux:

user@host:/home/user$ sudo chown tux foobar

3.2 Changing Group Ownership

To change the foobar file's group to penguins, you could use either chgrp or chown with special syntax:

user@host:/home/user$ sudo chgrp penguins foobar
user@host:/home/user$ sudo chown :penguins foobar

3.3 Changing User and Group Ownership

To change the foobar file's owner to tux and the group to penguins with a single command, the syntax would be:

user@host:/home/user$ sudo chown tux:penguins foobar

4. Reference

https://en.wikipedia.org/wiki/Chmod

http://linuxcommand.org/lc3_lts0090.php

https://help.ubuntu.com/community/FilePermissions

https://askubuntu.com/questions/303593/how-can-i-chmod-777-all-subfolders-of-var-www

https://docs.nersc.gov/filesystems/unix-file-permissions/

https://unix.stackexchange.com/questions/101073/how-to-change-permissions-from-root-user-to-all-users

https://askubuntu.com/questions/1081036/chmod-777-in-terminal-the-command-to-make-all-changes-affect-every-file-and-fo