Lab #0: Virtual Linux Lab


  1. Learn basic terminal commands and how to work with a text editor

  2. Become familiar with the Linux environment

  3. Learn to run a Python program from the command-line

  4. Learn about file permissions

  5. Learn about redirection and pipes


Linux is an operating system much like macOS or Windows. It has windows, programs, web browsers, and so on. Files are stored in directories (folders) that, in turn, are stored in other directories. Although you can access Linux’s features using your mouse, as you perform more and more complex tasks, you will find that using the mouse is ineffective. Linux allows us to interact with the computer entirely through text using a program called the terminal. (Mac provides a similar terminal application, and there are ways to use text-based commands on Windows too. But, Linux provides the lowest barrier to entry.) In this lab you will learn how to use the terminal to perform some basic operations in Linux. You will need these skills for the rest of your time at UChicago.

We show many examples of sample output below. The output you see when you run the commands may vary a bit.

Connecting to the Virtual Desktop

You will need to work through this lab by logging into a Virtual Desktop running Linux. We explain how to do this in our Virtual Desktop page. Make sure you complete the steps listed on that page before proceeding.


On your personal computer, you probably navigate your hard drive by double clicking on icons. While convenient for simple tasks, this approach is limited. For example, imagine that you want to delete all of the music files over 5 MB that you haven’t listened to in over a year. This task is very hard to do with the standard double-click interface but is relatively simple using the terminal.

On the virtual desktop, click the Application button (at the top left) and type “terminal” in the input box. Click the “terminal” icon to open the terminal window (we show screenshots of this in our Virtual Desktop page).

A terminal window will open and you will see text of the form:


where username has been replaced by your CNetID and computer is the name of the virtual desktop server you happen to be using. This string is called the prompt. When you start typing, the characters you type will appear to the right of the $.

The program that runs within a terminal window and processes the commands the you type is called a shell. We use bash, which is the default shell on most Linux distributions, but there are other popular shells, such as ksh, tcsh, etc.

The procedure for completing this lab is as follows. For each section, read through the explanatory text and the examples. Then, try these ideas by doing the exercises listed at the bottom of the section.

Using an Editor

List the files in the lab0 directory. You should see the following:  test.txt

How do we view and edit the contents of these files? There are many high-quality text editors for Linux. We will use Visual Studio Code, which is good for writing code.

You can open a specific file, say test.txt, using the code command from the Linux command-line by typing:

code test.txt

When you run this command, you will get a new window that looks like this:

Specifically, you’ll see the following text:

Lab 1 Test file

Author: Firstname Lastname

If the file is blank, quit code and ensure that the file test.txt exists in your local directory (use ls to list the files in your local directory). If it does not, use cd to navigate to the lab0 subdirectory inside your cmsc12100-aut-20-username` directory.

Note: somewhat counterintuitively, the menu bar for Visual Studio Code is at the top of the Browser window. You need to run your mouse over the name to see the menu options.

For now, we will use Visual Studio Code (code) in a very basic way. You can navigate to a particular place in a file using the arrow keys (or your mouse) and then type typical characters and delete them as you would in a regular text editor. You can save your changes using the save option in the file menu or use the keyboard shortcut Crtl-s. To quit, you can use the file menu quit option or the keyboard shortcut Ctrl-q.

As an aside, you can also launch code from the application launcher: simply click the Application button (at the top left of your screen), type “code” in the input box, and then click on the Visual Studio Code icon. You can then use the file menu to navigate the correct file. As with the terminal application, you might want to pin the icon for launching Visual Studio Code to your launch bar (right click your mouse and choose the “Lock to Launcher” menu item.)


Make sure that you are comfortable with this level of usage:

  1. Add your name after Author: in this file

  2. Save the file

  3. Close and reopen the file in code and ensuring that your name is still there

  4. Finally, close code.

Run a Python Program


runs the python program

In this class, you will learn Python. To run a Python program, use the command python3 and the name of the file that contains your program.

Use ls to verify that there there is a file named in your lab0 directory. Now, run the program in by typing (don’t forget about auto-complete!):


This program is a very simple. It just prints “Hello, World!” to the screen.


There are several variants of Python, including Python 2.7 and Python 3. We will be using Python 3 and the corresponding python3 interpreter. The CS machines have Python 2.7 installed as the default Python. As a result, the command python runs a version of Python 2.7. There are some differences between the two languages and Python 3 programs may not run properly using a Python 2.7 interpreter.

Edit and Run a Python Program

In this section you will modify and rerun the program in This change is very simple but goes through all the mechanical steps needed to program.

Open the file with the command:


The file contains a single line of code:

print("Hello, World!")

Change this line so that it instead says “Hello ” and then your name. For example if your name were Gustav Larsson, the line would read:

print("Hello, Gustav!")

Do the following steps:

  1. Save the file in Visual Studio Code (forgetting to save is a surprisingly common error)

  2. Rerun the program using python3

Let’s reinforce the steps to programming in Python with the terminal:

  1. Change your .py file with an editor

  2. Save the file

  3. Run the file with python3

Forgetting to save the file (step 2) is a very common mistake!

Wild Cards (using an asterisk)

Sometimes when we enter a string, we want part of it to be variable, or a wildcard. A common task is to list all files that end with a given extension, such as .txt. The wildcard functionality, through an asterisk, allows to simply say:

$ ls *.txt

The wildcard can represent a string of any length consisting of any characters - including the empty string.

It is important to be careful using wildcard, especially for commands like rm which cannot be undone. A command like:

$ rm *             ### DO NOT RUN THIS COMMAND!

will delete all of the files in your working directory!


  1. Navigate to your cmsc12100-aut-20-username directory. What do you see when you run ls lab*? What about ls lab*/*?

  2. What do you expect to see when you run the command ls ../lab* from within your cmsc12100-aut-20-username/lab0 directory?

Useful Keyboard Shortcuts

Used at the Linux prompt, the keyboard shortcut Ctrl-P will roll back to the previous command. If you type Ctrl-P twice, you will roll back by two commands. If you type Ctrl-P too many times, you can use Ctrl-N to move forward. You can also use the arrow keys: up for previous (backward), down for next (forward).

Here are few more useful shortcuts:

  • Ctrl-A will move you to the beginning of a line.

  • Ctrl-E will move you to the end of a line.

  • Ctrl-U will erase everything from where you are in a line back to the beginning.

  • Ctrl-K will erase everything from where you are to the end of the line.

  • Ctrl-L will clear the text from current terminal

Play around with these commands. Being able to scroll back to, edit, and then rerun previously used commands saves time and typing! And like auto-completion, getting in the habit of using keyboard shortcuts will reduce frustration as well save time.

Man Pages

A man page (short for manual page) documents or describes topics applicable to Linux programming. These topics include Linux programs, certain programming functions, standards, and conventions, and abstract concepts.

To get the man page for a Linux command, you can type:

man <command name>

So in order to get the man page for ls, you would type:

man ls

This command displays a man page that gives information on the ls command, including a description, flags, instructions on use, and other information.

Each man page has a description. The -k flag for man allows you to search these descriptions using a keyword. For example:

man -k printf

This searches all the descriptions for the keyword printf and prints the names of the man pages with matches.

Combining Commands

Running Commands Sequentially

It is often convenient to chain together commands that you want to run in sequence. For example, recall that to print the working directory and list all of the files and directories contained inside, you would use the following commands:

$ pwd
$ ls
Desktop  Documents  Downloads  Music  Pictures  Public  Templates  Videos

You could also run them together, like so:

$ pwd ; ls
Desktop  Documents  Downloads  Music  Pictures  Public  Templates  Videos

First, pwd is executed and run to completion, and then ls is executed and run to completion. The two examples above are thus equivalent, but the ability to run multiple commands together is a small convenience that could save you some time if there is a group of commands that you want to execute sequentially.


The shell doesn’t care about white space, so it will run any of the following as well:

$ pwd;ls
$ pwd ;ls
$ pwd; ls
$ pwd       ;        ls


The examples in this section will use commands that we’ve not yet discussed. Refer to the man pages for information about unfamiliar commands.

As we already know, commands like pwd, ls, and cat will print output to screen by default. Sometimes, however, we may prefer to write the output of these commands to a file. In Linux, we can redirect the output of a program to a file of our choosing. This operation is done with the > operator.

Try the following example and compare your output with ours:

$ cd
$ touch test-0.txt
$ ls > test-1.txt
$ cat test-1.txt
$ echo "Hello World!" > test-2.txt
$ cat test-2.txt
Hello World!
$ cat test-2.txt > test-1.txt; cat test-1.txt
Hello World!
$ rm test-*

Two important things to note:

  1. If you redirect to a file that does not exist, that file will be created.

  2. If you redirect to a file that already exists, the contents of that file will be overwritten.

You can use the append operator (>>) to append the output of command to the end of an existing file rather than overwrite the contents of that file.

Not only can we redirect the output of a program to a file, we can also have a program receive its input from a file. This operation is done with the < operator. For example:

$ python3 < my-input.txt

(Change back to your lab0 directory before you try this command.)

In general, all Linux processes can perform input/output operations through, at least, the keyboard and the screen. More specifically, there are three ‘input/output streams’: standard input (or stdin), standard output (or stdout), and standard error (or stderr). The code in simply reads information from stdin and writes it back out to stdout. The redirection operators change the bindings of these streams from the keyboard and/or screen to files. We’ll discuss stderr later in the term.


In addition to the ability to direct output to and receive input from files, Linux provides a very powerful capability called piping. Piping allows one program to receive as input the output of another program, like so:

$ program1 | program2

In this example, the output of program1 is used as the input of program2. Or to put it more technically, the stdout of program1 is connected to the stdin of program2.

As another more concrete example, consider the man command with the -k option that we’ve previously discussed. Let’s assume that you hadn’t yet been introduced to the mkdir command. How would you look for the command to create a directory? First attempts:

$ man -k "create directory"
create directory: nothing appropriate
$ man -k "directory"
(a bunch of mostly irrelevant output)

As we can see, neither of these options is particularly helpful. However, with piping, we can combine man -k with a powerful command line utility called grep (see man pages) to find what we need:

$ man -k "directory" | grep "create"
mkdir (2)            - create a directory
mkdirat (2)          - create a directory
mkdtemp (3)          - create a unique temporary directory
mkfontdir (1)        - create an index of X font files in a directory
mklost+found (8)     - create a lost+found directory on a mounted Linux second extended fil...
mktemp (1)           - create a temporary file or directory
pam_mkhomedir (8)    - PAM module to create users home directory
update-info-dir (8)  - update or create index file from all installed info files in directory
vgmknodes (8)        - recreate volume group directory and logical volume special files



  1. Use piping to chain together the printenv and tail commands to display the last 10 lines of output from printenv.

  2. Replicate the above functionality without using the | operator. (hint: Use a temporary file.)

File Permissions

Sometimes we want to restrict who can access certain resources on the file system.

Most file systems assign ‘File Permissions’ (or just permissions) to specific users and groups of users. Unix is no different. File permissions dictate who can read (view), write (create/edit), and execute (run) files on a file system.

All directories and files are owned by a user. Each user can be a member of one or more groups. To see your groups, enter the command groups into the command line.

File permissions in Unix systems are managed in three distinct scopes. Each scope has a distinct set of permissions.

User - The owner of a file or directory makes up the user scope.

Group - Each file and directory has a group assigned to it. The members of this group make up the group scope.

Others - Every user who does not fall into the previous two scopes make up the others scope.

If a user falls into more than one of these scopes, their effective permissions are determined based on the first scope the user falls within in the order of user, group, and others.

Each scope has three specific permissions for each file or directory:

read - The read permission allows a user to view a file’s contents. When set for a directory, this permission allows a user to view the names of files in the directory, but no further information about the files in the directory. r is shorthand for read permissions.

write - The write permission allows a user to modify the contents of a file. When set for a directory, this permission allows a user to create, delete, or rename files. w is shorthand for write permissions.

execute - The execute permission allows a user to execute a file (or program) using the operating system. When set for a directory, this permission allows a user to access file contents and other information about files within the directory (given that the user has the proper permissions to access the file). The execute permission does not allow the user to list the files inside the directory unless the read permission is also set. x is shorthand for execute permissions.

To list information about a file, including its permissions, type:

ls -l <filepath>

You’ll get output of the form:

<permissions> 1 owner group <size in bytes> <date modified> <filepath>

For example, if we want information on /usr/bin/python3.5:

$ ls -l /usr/bin/python3.5
-rwxr-xr-x 1 root root 4460272 Aug 20 /usr/bin/python3.5

First thing we can notice is that the owner of the file is a user named root. (FYI, root is a name for an account that has access to all commands and files on a Linux system. Other accounts may also have “root” privileges.) The file’s group is also root.

The permissions are -rwxr-xr-x. The initial dash (-) indicates that /usr/bin/python3.5 is a file, not a directory. Directories have a d instead of a dash. Then the permissions are listed in user, group, and others order. In this example, the owner, root, can read (r), write (w), and execute (x) the file. Users in the root group and all other users can read and execute the files.


By default, any files or directories that you create will have your username as both the user and the group. (If you run groups, you’ll notice that there is a group with the same name as your username. You are the only member of this group.) On our Linux machines, by default, new files are give read and write permissions to user and group and no permissions to other. New directories will be set to have read, write and execute permissions for user and group.

  1. Verify this claim by running ls -l backups/copy2.txt and ls -ld  backups in your lab0 directory.

The -d flag tells ls to list the directory, instead of its contents. Notice that that the first letter in the permissions string for backups is a d, while it is a - for backups/copy2.txt.

Once you have verified the claim, go ahead and remove the backups directory using the command: rm -r backups.

Changing Permissions, Owner, & Group

chmod <permissions> <path-name>

set the permissions for a file/directory

chmod <changes> <path-name>

update the permissions for a file/directory

chown <username> <path-name>

change the owner of a file to username

chgrp <group> <path-name>

change the group of a file

cat <path-name>

print the contents of a file to the terminal

To change permissions, we use the chmod command. There are two ways to specify the permissions. We’ll describe the more accessible one first: to set the permissions you specify the scope using a combination of u, g, and o, the permission using r, w, and x, and either + or - to indicate that you want to add or remove a permission. For example uo+rw indicates that you want to add read and write permissions for the user and others groups.

We can demonstrate this using the cat command to print file contents to the terminal:

$ echo "Hello!" > testfile
$ ls -l testfile
-rw-rw---- 1 username username 7 Aug 23 11:22 testfile
$ cat testfile
$ chmod ug-r testfile #remove read and permissions from user and group
$ ls -l testfile
--w--w---- 1 username username 7 Aug 23 11:22 testfile
$ cat testfile
cat: testfile: Permission denied
$ chmod u+r testfile #give user scope read permissions

In this last example, we have added user read permissions to testfile.

In addition to the symbolic method for setting permissions, you can also use a numeric method: each permission has a unique value: read = 4, write = 2, execute = 1. As a result, you can describe the permissions of each scope using the sum of its permissions’ values. For example, if a file has read and write permissions for the user scope, its permissions can be described as 6 (4 + 2 = 6).

You can describe the permissions of a file overall using these values for each scope. For example, 761 describes the permissions for a file with read, write, and execute permissions for the user scope, read and write permissions for the group scope, and only execute permissions for the others scope.

The symbolic approach is relative: it allows you to add and remove permissions relative the the current file permissions. The numeric method is absolute: it sets the permissions to a specific configuration. We recommend starting the symbolic approach. It is easier to get right. As you get more comfortable with setting permissions, it is useful to learn how to use the numeric method.

To change the owner of a file or directory (if you are the owner or root), use the command:

chown <new owner> <path to file>

To change a file’s group (if you are the owner or root), use the command:

chgrp <new group> <path to file>

It is unlikely that you will need to use these two commands for this course.


  1. Run echo "Hello!" > testfile to construct testfile. Look at the permissions using ls -l.

  2. Change the permissions on testfile to allow and read access for others. Run ls -l testfile to check the new permissions.

  3. Remove group write access from testfile. Check the corrected permissions.

  4. Remove testfile using rm.

Final Notes

Sometimes, a program will run indefinitely or misbehave. When this happens, you can type Ctrl-C to send an interrupt signal to the running program, which usually causes it to terminate. On occasion, you may need to type Ctrl-C a few times. Typing Ctrl-D sends an end of input signal, which tells the program that no more information is coming.

Log out

Clicking on the gear icon in the top right corner of the screen will give you a menu. To log out, choose “Log Out” from this menu and then click the “Log out” option in the widget that pops up (rather than the pause option). (Again, see our Virtual Desktop page for screen shots.)