The Linux Terminal

In this exercise, we will familiarize ourselves with the Linux terminal.

Starting the Terminal

1. To open the terminal, click on the terminal icon:

   

2. Create a second terminal window, either by:

   • Right-clicking on the terminal and selecting the “Open Terminal” or
   • Selecting “Open Terminal” from the “File” menu

3. Create a second terminal within the same window by pressing Ctrl+Shift+T while the terminal window is selected.

4. Close the 2nd terminal tab, either by:

   • clicking the small ‘x’ in the terminal tab (not the main terminal window)
   • typing exit and hitting enter.

5. The window will have a single line, which looks like this:

   academy@laptop-0:~$
   

6. This is called the prompt, where you enter commands. The prompt, by default, provides three pieces of information:

   1. academy is the login name of the user you are running as.
   2. laptop-0 is the host name of the computer.
   3. ~ is the directory in which the terminal is currently in. (More on this later).

7. Close the terminal window by typing exit or clicking on the red ‘x’ in the window’s titlebar.

Navigating Directories and Listing Files

Prepare your environment

1. Open the file browser, by clicking on the following icon:

   

2. Navigate to the home directory.

3. Double-click on the linux_intro folder we created in the previous step.

   • We’ll use this to illustrate various file operations in the terminal.

4. Right click in the main file-browser window and select “Open in Terminal” to create a terminal window at that location.

5. In the terminal window, type the following command to copy some sample files that we can study later:

   cp -a ~/ros_training/resources/linux_intro/. .
   

ls Command

1. Enter ls into the terminal.

   • You should see test.txt, and new listed. (If you don’t see ‘new’, go back and complete the previous exercise.
   • Directories, like new, are colored in blue.
   • The file sample_job is in green; this indicates it has its “execute” bit set, which means it can be executed as a command.

2. Type ls *.txt. This lists all the files ending with “.txt”. Only the file test.txt will be displayed.

3. Enter ls -l into the terminal.

   • Adding the -l option shows one entry per line, with additional information about each entry in the directory.
   • The first 10 characters indicate the file type and permissions
   • The first character is d if the entry is a directory.
   • The next 9 characters are the permissions bits for the file
   • The third and fourth fields are the owning user and group, respectively.
   • The second-to-last field is the time the file was last modified.
   • If the file is a symbolic link, the link’s target file is listed after the link’s file name.

4. Enter ls -a in the terminal.

   • You will now see one additional file, which is hidden (i.e. its name begins with a dot).

5. Enter ls -a -l (or ls -al) in the command.

   • You’ll now see that the file hidden_link.txt points to .hidden_text_file.txt.

   Tip

   If you check the contents of .bashrc in your home folder, you will find that several aliases (l, ll, and la) have been defined. These are real time savers when using ls with arguments often.

pwd and cd Commands

1. Enter pwd into the terminal.

   • This will show you the full path of the directory you are working in.

2. Enter cd new into the terminal.

   • The prompt should change to academy@laptop-0:~/linux_intro/new$.
   • Typing pwd will show you now in the directory /home/academy/linux_intro/new.

3. Enter cd .. into the terminal.

   • In the previous exercise, we noted that .. is the parent folder.
   • The prompt should therefore indicate that the current working directory is /home/academy/linux_intro.

4. Enter cd /bin, followed by ls.

   • This folder contains a list of the most basic Linux commands.

     Note that the programs pwd and ls are both in this folder.

5. Enter cd ~/linux_intro to return to our working directory.

   • Linux uses the ~ character as a shorthand representation for your home directory.
   • It’s a convenient way to reference files and paths in command-line commands.
   • You’ll be typing it a lot in this class… remember it!

   Hint

   If you want a full list of options available for any of the commands given in this section, type man <command> (where <command> is the command you want information on) in the command line. This will provide you with built-in documentation for the command. Use the arrow and page up/down keys to scroll, and q to exit.

Altering Files

mv Command

The mv command can be used to move and rename files.

1. Type mv test.txt test2.txt, followed by ls.

   • You will notice that the file has been renamed to test2.txt.

2. Type mv test2.txt new, then ls.

   • The file will no longer be present in the folder.

3. Type cd new, then ls.

   • You will see that test2.txt has been moved into the new folder.

4. Type mv test2.txt ../test.txt, then ls.

   • test2.txt will no longer be there.

5. Type cd .., then ls.

   This shows how mv can move and rename files in one step. The file test.txt is present again in our working directory.

cp Command

1. Type cp test.txt new/test2.txt, then ls new.

   • You will see test2.txt is now in the new folder.

2. Type cp test.txt "test copy.txt", then ls -l.

   • You will see that test.txt has been copied to test copy.txt.

   Note

   The quotation marks are necessary when spaces or other special characters are included in the file name.

rm Command

1. Type rm "test copy.txt", then ls -l.
   • You will notice that test copy.txt is no longer there.

mkdir Command

1. Type mkdir new2, then ls.

   • You will see there is a new folder new2.

   Tip

   You can use the -i flag with cp, mv, and rm commands to prompt you when a file will be overwritten or removed.

Job management

Stopping Jobs

1. Type ./sample_job.
   • The program will start running.
2. Press Ctrl+C.
   • The program should exit.
3. Type ./sample_job sigterm.
   • The program will start running.
4. Press Ctrl+C.
   • This time the program will not die.

Stopping “Out of Control” Jobs

1. Open a new terminal window.

2. Type ps ax.

3. Scroll up until you find python ./sample_job sigterm.

   • This is the job that is running in the first window.
   • The first field in the table is the ID of the process (use man ps to learn more about the other fields).

4. Type ps ax | grep sample.

   • You will notice that only a few lines are returned.
   • This is useful if you want to find a particular process

   Note

   This is an advanced technique called “piping”, where the output of one program is passed into the input of the next. This is beyond the scope of this class, but is useful to learn if you intend to use the terminal extensively.

5. Type kill <id>, where <id> is the job number you found with the ps ax.

6. In the first window, type ./sample_job sigterm sigkill.

   • The program will start running.

7. In the second window, type ps ax | grep sample to get the id of the process.

8. Type kill <id>.

   • This time, the process will not die.

9. Type kill -SIGKILL <id>.

   • This time the process will exit.

Showing Process and Memory usage

1. In a terminal, type top.
   • A table will be shown, updated once per second, showing all of the processes on the system, as well as the overall CPU and memory usage.
2. Press the Shift+P key.
   • This will sort processes by CPU utilization and shows which processes are using too much CPU time.
3. Press the Shift+M key.
   • This will sort processes by memory utilization and shows which processes are using too much memory.
4. Press q or Ctrl+C to exit the program.

Editing Text (and Other GUI Commands)

1. Type gedit test.txt.
   • You will notice that a new text editor window will open, and test.txt will be loaded.
   • The terminal will not come back with a prompt until the window is closed.
2. There are two ways around this limitation. Try both…
3. Starting the program and immediately returning a prompt:
   1. Type code:gedit test.txt &.
      • The & character tells the terminal to run this command in “the background”, meaning the prompt will return immediately.
   2. Close the window, then type ls.
      • In addition to showing the files, the terminal will notify you that gedit has finished.
4. Moving an already running program into the background:
   1. Type gedit test.txt.
      • The window should open, and the terminal should not have a prompt waiting.
   2. In the terminal window, press Ctrl+Z.
      • The terminal will indicate that gedit has stopped, and a prompt will appear.
   3. Try to use the gedit window.
      • Because it is paused, the window will not run.
   4. Type bg in the terminal.
      • The gedit window can now run.
   5. Close the gedit window, and type ls in the terminal window.
      • As before, the terminal window will indicate that gedit is finished.

Running Commands as Root

1. In a terminal, type ls -a /root.
   • The terminal will indicate that you cannot read the folder /root.
   • Many times you will need to run a command that cannot be done as an ordinary user, and must be done as the “super user”
2. To run the previous command as root, add sudo to the beginning of the command.
   • In this instance, type sudo ls -a /root instead.
   • The terminal will request your password (in this case, academy) in order to proceed.
   • Once you enter the password, you should see the contents of the /root directory.

Warning

sudo is a powerful tool which doesn’t provide any sanity checks on what you ask it to do, so be VERY careful in using it.