Users and Permissions

This tutorial covers the concept of users (and groups) in the multi-user operating system Linux. It will also explain the various permissions that a file or directory can have, how to determine and change them. As an example, it will show how to make a Python script executable by the user.

Video

( Slides as pdf)

Quiz

Who can change the permissions of a file?

	the owner of this file
	every user
	users in the same group

How is the top-level admin (or superuser) called in Linux?

	god
	guru
	root

Which one of the following is not a file permission?

	Write
	Read
	Execute
	Size

Who can change the owner or group of a file?

	the owner of that file
	everyone in this group
	admin (root)

Which command can prevent other users not in your group from writing to input.dat?

	chmod u-w input.dat
	chmod o-w input.dat
	chmod g-w input.dat

Exercises in Terminal (slide 89)

1. List your home-directory, so that you can see users and permissions. Pick a file of your choice and explain 1) whom it belongs, 2) to which group it belongs and 3) what permissions all other users have for this file.

Answer:

     List your home-directory: ls -l $HOME or ll ~
     The owner of a file can be found in the middle, right to the first small number.
     The group follows to the right of the owner.
     The permissions for other users are in the third rwx-grouping, i.e., left to the first small number.
     If these permission state e.g., "r--", then the file is readable from everyone else, but neither writeable nor executable.

2. Create an empty file, make it read-only, make it executable.

Answer:

     Create an empty file: touch file.txt.
     make it read-only: chmod a-w file.txt; chmod a-x file.txt; chmod a+r file.txt.
     make it executable: chmod a+x file.txt.

     Explanation: The touch command can be used to create an empty file. In this example it is file.txt.
          chmod a-w file.txt; chmod a-x file.txt; chmod a+r file.txt means to execute the three commands subsequently. 
          chmod a-w file.txt removes the write permission for all users. 
          chmod a-x file.txt removes the executable permission for all users.
          chmod a+r file.txt adds the read permission for all users.
          chmod a+x file.txt makes file.txt executable for all users.

<< Text Display and Search

Overview

Processes >>