The-Linux-Terminal.md

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:

   ros-industrial@ros-i-melodic-vm:~$

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

   1. ros-industrial is the login name of the user you are running as.
   2. ros-i-melodic-vm 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 your home folder in the file browser.
2. Double-click on the ex0.3 folder we created in the previous step.
   • We'll use this to illustrate various file operations in the terminal.
3. Right click in the main file-browser window and select "Open in Terminal" to create a terminal window at that location.
4. In the terminal window, type the following command to create some sample files that we can study later:
   • cp -a ~/industrial_training/exercises/0.3/. .

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. 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.
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.

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 ros-industrial@ros-i-melodic-vm:~/ex0.3/new$.
   • Typing pwd will show you now in the directory /home/ros-industrial/ex0.3/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/ros-industrial/ex0.3.
4. Enter cd /bin, followed by ls.
   • This folder contains a list of the most basic Linux commands.
     Note that pwd and ls are both in this folder.
5. Enter cd ~/ex0.3 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!

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

1. Type mv test.txt test2.txt, followed by ls.
   • You will notice that the file has been renamed to test2.txt.
     This step shows how mv can rename files.
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 test2.txt in the folder.
     These steps show how mv can move files.
4. Type mv test2.txt ../test.txt, then ls.
   • test2.txt will no longer be there.
5. Type cd .., then ls.
   • You will notice that test.txt is present again.
     This shows how mv can move and rename files in one step.

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 that 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.

touch Command

1. Type touch ~/Templates/"Untitled Document".
   • This will create a new Document named "Untitled Document"

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 Control+C.
   • The program should exit.
3. Type ./sample_job sigterm.
   • The program will start running.
4. Press Control+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.
     This can be used to determine which processes are using too much CPU time.
3. Press the Shift+M key.
   • This will sort processes by memory utilization
     This can be used to determine 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 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, rosindustrial) 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.