View source for Introduction to Linux in HPC/Users and permissions

[[Category:Tutorials]]<nowiki />
{{DISPLAYTITLE:<span style="position:absolute; top:-9999px;">Introduction to Linux in HPC/</span>Users and permissions}}<nowiki />
{{Syllabus Introduction to Linux}}<nowiki />
__TOC__

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.

=== Video === <!--T:5-->

<youtube width="600" height="340" right>V7laggm16mc</youtube>

([[Media:HPC.NRW_Introduction_to_Linux_in_HPC_06_Users_and_Permissions.pdf | Slides as pdf]])

=== Quiz === <!--T:5-->  


{{hidden begin 
|title = Who can change the permissions of a file?
}}
<quiz display=simple>
{
|type="()"}
+ the owner of this file
|| Explanation: Correct, the owner of a file can change the permissions of this file. However, the super user (root) can also do this.
- every user
|| Explanation: Wrong: users other than the owner of a file cannot change the permissions of a file, because it does not belong to them.
+ the super user (root)
|| Explanation: Correct, the super user (root) can change the permissions of all files. However, the owner of a file can also do this.
</quiz>
{{hidden end}}

{{hidden begin 
|title = Which command can prevent other users not in your group from writing to input.dat?
}}
<quiz display=simple>
{
|type="()"}
- chmod u-w input.dat
|| 
+ chmod o-w input.dat
|| Explanation: The chmod command changes file permission. u represents the user who owns the file. g means other users in your group and o stands for other users not in your group. -w means to remove the write permission. Therefore B is the correct answer.
- chmod g-w input.dat
||
</quiz>
{{hidden end}}


=== Exercises in Terminal (slide 89) === <!--T:5-->  

 1. Create an empty file, make it read-only, make it executable.
   {| role="presentation" class="wikitable mw-collapsible mw-collapsed"
    | <strong>Answer:</strong>
    |-
    | 
      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.
    |}

 2. Start a process (e.g. sleep 10m)
   {| role="presentation" class="wikitable mw-collapsible mw-collapsed"
    | <strong>Answer:</strong>
    |-
    | 
      Use a second console to look at it in top.
      Kill it.
      Hint:
         To kill the started process (e.g. sleep 10m): 
         1. Go to the terminal that started the process.
         2. Press Ctrl-C to kill it.
    |}

 3. Start a process, bring it into background/foreground.
   {| role="presentation" class="wikitable mw-collapsible mw-collapsed"
    | <strong>Hint:</strong>
    |-
    |
      To bring a process to background:
         Press Ctrl-Z (pause the process).
         Type bg (bring it to background).
      To bring a process to foreground:
         Type fg (bring it to foreground).
    |}

{{Tutorial Navigation
| previous = [[Introduction_to_Linux_in_HPC/Text_display_search | Text Display and Search ]]
| main = [[Introduction_to_Linux_in_HPC | Overview ]]
| next = [[Introduction_to_Linux_in_HPC/Processes | Processes ]]
}}