Introduction to Linux in HPC/Users and permissions

Slide Layout

   page 1: 
       Linux is a multi-user system.
       A user is logged in with an account and certain permissions.
   page 2: 
       Each file and directory has certain permissions.
       root has top permissions.
       Normal users may belong to multiple groups.
   page 3: 
       Read permission
       Write permission
       Execute permission for i) file and ii) directory
   page 4 - 12: 
       Example of ls -l (animation)
   page 13 - 16: 
       Permissions for user, group and other (animation)
   page 17: 
       chown to modify owner/group
       chmod to modify permission
   page 18: 
       Process is an instance of a program
       top and pstree
       Each process has permissions of that user.
       Each process has a unique ID (PID).
   page 19: 
       Start command in background command &.
       Ctrl-Z and bg
       fg brings a job to foreground.
   page 20 - 21: 
       Example of top (animation)
   page 22: 
       Single-letter commands for top

Quiz

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

Info:

no tips in this section

Warning:

no warnings in this section

Exercises in Terminal (slide 89)

1. 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.

2. Start a process (e.g. sleep 10m)

Answer:

     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.

Hint:

     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).