================== Extended Linux Lab ================== **While you will be able to do subsequent labs on your Virtual Machine, you must complete this lab in the Computer Science Instructional Lab (CSIL).** Objectives ---------- #. Become familiar with the Linux environment #. Learn basic terminal commands and how to work with a text editor #. Learn to run a Python program from the command-line #. Learn about file permissions #. Learn about redirection and pipes #. Learn about remote access tools Linux ----- Linux is an operating system much like OS X 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. (Macs provide 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 the course. We show many examples of sample output below. The output you see when you run the commands may vary a bit. For example, most of you are not named "Gustav Martin Larsson". Terminal/Shell -------------- 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 CSIL machine, click the Application button (at the top left) and type "terminal" in the input box. Click the "terminal" icon to open the terminal window. Alternatively, you can use the keyboard shortcut ``Ctrl-Alt-t`` to open a terminal window. A terminal window will open and you will see text of the form:: username@computer:~$ where ``username`` has been replaced by your CNetID and ``computer`` is the name of the machine you happen to be using.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. Navigating the File System -------------------------- Files in Linux are stored in directories/folders, just like in OS X/Windows. Directories can hold files or other subdirectories and there are special directories for your personal files, your Desktop, etc.: +------------------+------------------+-------------------+----------------------------------------+ | Name | Linux | Mac | Windows | +==================+==================+===================+========================================+ | Root directory | / | / | C:\\ | +------------------+------------------+-------------------+----------------------------------------+ | Home directory | /home/username | /Users/username | C:\\Documents and Settings\\username | +------------------+------------------+-------------------+----------------------------------------+ .. image:: img/filesystem.username.svg :align: center :width: 650 :height: 250 All of the computers in the lab are connected through a network file system. Effectively there is one very large shared hard drive. Your files are available from any lab computer and all of our directories live in the same space. The figure above illustrates how Linux organizes the file system. Your own computer might have a slightly different organization (e.g., you might replace ``/`` with ``C:``), but the idea is the same. For the above and from this point forward, consider that the text "username" is replaced with your own actual username, which is just your CNetID. Show Files ---------- The terminal will start in your home directory, ``/home/username/``, which is a special directory assigned to your user account. Any computer that you use in CSIL will automatically connect to your home directory and all files that you created or changed in previous sessions in CSIL will be available to you. Two very useful commands are ``pwd`` and ``ls``: +---------+--------------------------------------------------------------+ | ``pwd`` | Prints your current working directory - tells you where you | | | are in your directory tree. | +---------+--------------------------------------------------------------+ | ``ls`` | Lists all of the files in the current directory. | +---------+--------------------------------------------------------------+ The following is an example using these two commands in a terminal window:: username@computer:~$ pwd /home/username/ username@computer:~$ ls Desktop Documents Downloads Music Pictures Public Templates Videos username@computer:~$ Try these commands yourself to verify that everything looks similar. Notice that the directory path and list of files that you see if you open your home folder graphically are identical to those provided by ``pwd`` and ``ls``, respectively. The only difference is how you get the information and how the information is displayed. Change Directory ---------------- +-------------------+--------------------------------------------------------------+ |``cd `` | change to the directory path-name | +-------------------+--------------------------------------------------------------+ | ``cd ..`` | move up/back one directory | +-------------------+--------------------------------------------------------------+ | ``cd`` | move to your home directory | +-------------------+--------------------------------------------------------------+ How can we move around in the file system? If we were using a graphical system, we would double click on folders and occasionally click the "back" arrow. In order to change directories in the terminal, we use ``cd`` (change directory) followed by the name of the destination directory. (A note about notation: we will use text inside angle brackets, such as ```` as a place holder. The text informally describes the type of value that should be supplied. In the case of ````, the desired value is the path-name for a file. More about path-names later.) For example if we want to change to the ``Desktop`` directory, we type the following in the terminal:: cd Desktop Here is an example of changing to the desktop directory in the terminal. We use ``pwd`` and ``ls`` to verify where we are and where we can go:: username@computer:~$ pwd /home/username/ username@computer:~$ ls Desktop Documents Downloads Music Pictures Public Templates Videos username@computer:~$ cd Desktop username@computer:~/Desktop$ pwd /home/username/Desktop/ username@computer:~/Desktop$ ls username@computer:~/Desktop$ Notice that after we ``cd`` into the ``Desktop`` the command ``pwd`` now prints out:: /home/username/Desktop/ rather than:: /home/username/ In the beginning, there are no files in the Desktop directory, which is why the output of ``ls`` in this directory is empty. We can move up one step in the directory tree (e.g., from ``/home/username/Desktop`` to ``/home/username`` or from ``/home/username`` to ``/home``) by typing ``cd ..`` Here "up" is represented by "``..``" In this context, this command will move us up one level back to our home directory:: username@computer:~/Desktop$ pwd /home/username/Desktop/ username@computer:~/Desktop$ cd .. username@computer:~$ pwd /home/username/ Notice that the current working directory is also shown in the prompt string. +-------------------+--------------------------------------------------------------+ | ``~`` | shortcut for your home directory | +-------------------+--------------------------------------------------------------+ | ``.`` | shortcut for the current working directory | +-------------------+--------------------------------------------------------------+ | ``..`` |shortcut for one level up from your current working directory | +-------------------+--------------------------------------------------------------+ The tilde (~) directory is the same as your home directory: that is, ``~`` is shorthand for ``/home/username``. Here's another useful shorthand: a single dot (``.``) refers to the current directory. Usually when you use ``cd``, you will specify what is called a *relative* path, that is, you are telling the computer to take you to a directory where the location of the directory is described relative to the current directory. The only reason that the computer knows that we can ``cd`` to ``Desktop`` is because ``Desktop`` is a folder within the ``/home/username`` directory. But, if we use a ``/`` at the *beginning* of our path, we are specifying the path relative to the the "root" or top of the file system. For example:: username@computer:~$ pwd /home/username/ username@computer:~$ cd /home/username/Desktop username@computer:~/Desktop$ pwd /home/username/Desktop username@computer:~/Desktop$ cd /home/username username@computer:~$ pwd /home/username These commands achieve the same thing as the ones above: we ``cd`` into ``Desktop``, a folder within our home directory, and then back to our home directory. Paths that start with a ``/`` are known as *absolute paths* because they always lead to the same place, regardless of your current working directory. Running ``cd`` without an argument will take you back to your home directory without regard to your current location in the file system. For example:: username@computer:~/Desktop$ cd username@computer:~$ pwd /home/username To improve the readability of our examples, we will use ``$`` as the prompt rather than the full text ``username@computer:~$`` in the rest of this lab and, more generally, in the course going forward. Keep in mind, though, that the prompt shows your current working directory. .. _setting_up_your_course_directory: Setting Up Your CAPP 30121 Directory ------------------------------------ We need a set of files to practice our commands on. Unfortunately, your home directories are mostly empty. In this section, we're going to download a set of files for you to work with. We will do this using Git, a version control system and code-sharing tool. Git will be described in more depth in another lab. For now, please execute the following: - Make sure that your departmental Git account is correctly set up. Using a browser, go to https://mit.cs.uchicago.edu/ and try to log in with your CNetID and password. When you type in your username, use only your CNetID username (without "@uchicago.edu"). If you are unable to log in, please tell a TA. - Back in a terminal window, make sure that you are in your home directory ``/home/username`` using the ``pwd`` command. If you are not in that directory then use ``cd`` to navigate to it. - Run the following command in the terminal if you are on a CSIL computer:: cs-setup-script capp30121-aut-19 .. note:: Copy-Paste: In Windows (Mac) you usually copy-paste with ``Ctrl-C`` (``Command-C``) and ``Ctrl-V`` (``Command-V``). These short-cuts are available in graphical programs in Linux but not in the Terminal. Instead you can copy text just by selecting it with your mouse. Select the line that starts with ``cs-setup-script...`` above to copy it. You can paste by clicking the middle mouse button where you want the copied text to go. Middle click in the terminal. You should also be able to use ``Ctrl-Shift-C`` and ``Ctrl-Shift-V``, but you may find that this method does not reliably work. - The setup script will ask you first to enter your CNetID:: Enter your CNetID [username]: Where your CNetID will appear in place of ``username``. You can either type in your CNetID or hit enter to accept the username in brackets. Next, you will be asked for your CNetID password:: Enter your CNetID password: The script needs your password to access your information on the CS department's Git server and will handle it securely. .. note:: You may be accustomed to seeing an asterisk character appear for each password character that you type on a web browser. This will not happen when you type passwords into the terminal. The password is not "echoed" back in any way (not even with asterisks), so don't be alarmed if it looks as if you're password isn't being typed in. - Next, the script will print this:: You are a member of the following repositories. Please select the one you want to use: [1] username [X] Exit Choose one: Choose ``1``. Later in the quarter, you will become a member of other repositories as you work in groups with other students. - If successful, the script will print out the following:: Setting up your Git repository... Your git repository has been created in /home/username/capp30121-aut-19-username Setting up chisubmit... chisubmit has been set up. You can use chisubmit commands inside /home/username/capp30121-aut-19-username Where, once again, your CNetID will appear in place of ``username``. This output indicates that your Git repository has been correctly set up. The script also configured your ``capp30121`` directory for ``chisubmit``, a tool you will use to submit your programming assignments. We'll discuss ``chisubmit`` in another lab. - After running the setup script, list the files in your home directory. You should see a new directory ``capp30121-aut-19-username``. This directory will contain all of your work for this class. It contains a subdirectory, ``lab1``, that has some files for us to play with. You will learn how to manipulate these files in the next section. Note that you will also see subdirectories named ``pa0`` and ``pa1``. You can ignore these for now. Use ``pwd``, ``ls``, and ``cd`` to navigate to the ``lab1`` subdirectory. Using an Editor --------------- .. figure:: img/ubuntu-sublime-1.png :align: center :width: 780px :alt: List the files in the ``lab1`` directory. You should see the following:: my_echo.py hello_world.py my_input.txt 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 `Sublime Text `_, which is good for writing code. You can open a specific file, say ``test.txt``, using the ``subl`` command from the Linux command-line by typing:: subl test.txt (Don't use the ``sublime_text`` command directly. On the VM, it generates a bunch of irritating error messages and grabs hold of your terminal window.) When you run this command, you will get a new window that displays the following text:: Lab 1 Test file =============== Author: Firstname Lastname If the file is blank, quit ``subl`` 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 ``lab1`` subdirectory inside the ``capp30121-aut-19-username`` directory. For now, we will use sublime (``subl``) in a very basic way. You can navigate to a particular place in a file using the arrow keys 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 ``sublime-text`` from the application launcher: simply click the Application button (at the top left of your screen), type "sublime-text" in the input box, and then hit enter. You can then use the ``file`` menu to navigate the correct file. Later on you may find it useful to have access to a graphical tree view in your editor. You can access this by launching folder view of sublime rather than just a single file via ``subl .`` Exercises ~~~~~~~~~ Make sure that you are comfortable with this level of usage: #. Add your name after ``Author:`` in this file #. Save the file #. Close and reopen the file in sublime and ensuring that your name is still there #. Finally, close sublime Copy (``cp``), Move (``mv``), Remove (``rm``), and Make Directory (``mkdir``) ----------------------------------------------------------------------------- +---------------------------------+----------------------------------------------+ | ``cp`` | copy the source file to the new destination | +---------------------------------+----------------------------------------------+ | ``mv`` | move the source file to the new destination | +---------------------------------+----------------------------------------------+ | ``rm`` | remove or delete a file | +---------------------------------+----------------------------------------------+ | ``mkdir`` | make a new empty directory | +---------------------------------+----------------------------------------------+ Sometimes it is useful to make a copy of a file. To copy a file, use the command:: cp where ```` is replaced by the name of the file you want to copy and ```` is replaced by the desired name for the copy. An example of copying the file ``test.txt`` to ``copy.txt`` is below:: $ cp test.txt copy.txt ```` can also be replaced with a path to a directory. In this case, the copy will be stored in the specified directory and will have the same name as the source. Move (``mv``) has exactly the same syntax, but does not keep the original file. Remove (``rm``) will delete the file from your directory. If you want to copy or remove an entire directory along with its the files, the normal ``cp`` and ``rm`` commands will not work. Use ``cp -r``instead of ``cp`` or ``rm -r`` instead of ``rm`` to copy or remove directories (the ``r`` stands for "recursive"): Make sure you want to remove *everything* in the named directory, including subdirectories, *before* you use ``rm -r``. You can make a new directory with ``mkdir directoryname``, where ``directoryname`` is the desired name for the new directory. Exercises ~~~~~~~~~ Try the following tasks to practice and check your understanding of these terminal commands. 1. Execute the above copy command and use ``ls`` to ensure that both files exist. 2. Move the file ``copy.txt`` to the name ``copy2.txt``. Use ``ls`` to verify that this command worked. 3. Make a new directory named ``backups`` using the ``mkdir`` command. 4. Copy the file ``copy2.txt`` to the ``backups`` directory. 5. Verify that step (4) was successful by listing the files in the ``backups`` directory. 6. Now that we have a copy of ``test.txt`` in the backups directory we no longer need ``copy2.txt``. Remove the file ``copy2.txt`` in this directory. It can be tedious (and, when you are tired, challenging) to spell directory or file names exactly, so the terminal provides an auto-complete mechanism to guide you through your folder explorations. To access this functionality simply start typing whatever name you are interested in the context of a command and then hit tab. If there is only one way to finish that term hitting tab will fill in the rest of the term, for instance, if we typed ``ls b`` and then hit tab it would automatically finish the word ``ls backups`` and then await our hitting enter. If there is MORE than one way to finish a term, like if we had another folder called ``backups-old``, then hitting tab twice with cause the terminal to display all of the options available. Training yourself to use auto-completion (aka, tab completion) will save you time and reduce the inevitable frustration that arises from mistyping filenames when you are tired or distracted. Run a Python Program -------------------- +----------------------+------------------------------------+ | ``python3 file.py`` | runs the python program file.py | +----------------------+------------------------------------+ 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 ``hello_world.py`` in your ``lab1`` directory. Now, run the program in ``hello_world.py`` by typing (don't forget about auto-complete!):: python3 hello_world.py This program is a very simple. It just prints "Hello, World!" to the screen. .. note:: 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 CSIL 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 ``hello_world.py``. This change is very simple but goes through all the mechanical steps needed to program. Open the file ``hello_world.py`` with the command:: subl hello_world.py 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: #. Save the file ``hello_world.py`` in sublime (forgetting to save is a surprisingly common error) #. Rerun the program using ``python3`` Let's reinforce the steps to programming in Python with the terminal: #. Change your ``.py`` file with an editor #. Save the file #. Run the file with ``python3`` Forgetting to save the file (step 2) is a very common mistake! 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. Users that fall into each scope can have three specific permissions. **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 You'll get output of the form:: 1 owner group 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``. These 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. Exercises ~~~~~~~~~ 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 set to 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. #. Verify this claim by running ``ls -l backups/copy2.txt`` and ``ls -ld backups`` in your ``lab1`` 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`, which tells us that ``backups`` is directory. A regular file would have a ``-`` in that spot. Once you have verified the claim, go ahead and remove the ``backups`` directory using the command: ``rm -r backups``. Changing Permissions, Owner, & Group ------------------------------------ +-----------------------------------------+----------------------------------------------+ | ``chmod`` | set the permissions for a file/directory | +-----------------------------------------+----------------------------------------------+ | ``chmod`` | update the permissions for a file/directory | +-----------------------------------------+----------------------------------------------+ | ``chown`` | change the owner of a file to username | +-----------------------------------------+----------------------------------------------+ | ``chgrp`` | change the group of a file | +-----------------------------------------+----------------------------------------------+ | ``cat`` | print the contents of a file to the terminal | +-----------------------------------------+----------------------------------------------+ Each permission has a unique value: read = 4, write = 2, execute = 1. You can describe the permissions of a 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. To change permissions, we use the ``chmod`` command. This command can be used in two ways. We can set the permissions for a file using the three-digit number described in the previous paragraph or by adding to and/or removing permissions from the current settings. To use the latter approach, 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 (we'll make use of an unfamiliar operator, >, but bear with us):: $ echo "Hello!" > testfile $ ls -l testfile -rw-rw---- 1 username username 7 Aug 23 11:22 testfile $ cat testfile Hello! $ chmod 222 testfile #set only write permissions for all scopes $ ls -l testfile --w--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``. By the way, don't get hung up on memorizing the permission numbers. If the symbolic version makes more sense to you, use it. To change the owner of a file or directory (if you are the owner or root), use the command:: chown To change a file's group (if you are the owner or root), use the command:: chgrp It is unlikely that you will need to use these two commands for this course. Exercises ~~~~~~~~~ #. Run ``echo "Hello!" > testfile`` to construct ``testfile``. Look at the permissions using ``ls -l``. #. Change the permissions on ``testfile`` to allow and read access for others. Run ``ls -l testfile`` to check the new permissions. #. Remove user and group write access from ``testfile``. Check the corrected permissions. #. Remove ``testfile`` using ``rm``. 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! Exercises ~~~~~~~~~ #. Navigate to your ``capp30121-aut-19-username`` directory. What do you see when you run ``ls pa*``? What about ``ls pa*/*``? #. What do you expect to see when you run the command ``ls ../pa*`` from within your ``capp30121-aut-19-username/lab1`` directory? Environment Variables (Skip if short on time) --------------------------------------------- +------------------------------------+------------------------------------------------------+ | ``printenv`` | print the current state of the environment variables | +------------------------------------+------------------------------------------------------+ | ``export ="some value"`` | define an environment variable | +------------------------------------+------------------------------------------------------+ Sometimes, when we have text (a path, for example) that we use often, we give it a name by assigning it to a variable for convenience. The command:: $ printenv gives a long list of defined variables. Try it in your terminal to see what happens! When we want to define new variables, we use the ``=`` operator and ``export`` command. As you found from using printenv, is typical to name environment variables in all capital letters, so we could define a new variable as simply:: $ NEWVARIABLE=~/Downloads It is important **not to add spaces**; the commands:: $ NEWVARIABLE =~/Downloads $ NEWVARIABLE= ~/Downloads $ NEWVARIABLE = ~/Downloads would all be misinterpreted by the terminal; spaces within quotes are allowed, if accurate. We add a ``$`` before the name of an environment variable in a command to use its value:: $ NEWVARIABLE=~/Downloads $ cd NEWVARIABLE error $ cd $NEWVARIABLE $ pwd /home/username/Downloads A variable created as above is only available to the current shell. It is a local variable, so future shells (such as those that you create when you open a new terminal window using ``Ctrl-Shift-N``) will not have access to it. In order to save variables, we need to export them using the export command. Variables that are exported are called environment variables, which are generally declared as follows:: $ export NEWVARIABLE=~/Desktop For this environment variable to take effect on all future shells, the ``export`` statement needs to be added to a special file called ``.bashrc`` in your home directory (this file gets run every time you start a new shell) Exercises ~~~~~~~~~ #. Define an environment variable and use ``echo``, which takes a list of values as command-line arguments and echos them to the screen, to see its value. Note that environment variables defined at the command-line are only available until you log out. They will not be available the next time you log in. 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 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. 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 /home/username/ $ ls Desktop Documents Downloads Music Pictures Public Templates Videos You could also run them together, like so:: $ pwd ; ls /home/username/ 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. .. note:: 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 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. 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 current terminal (your env variables will still be there, but the text display will be gone) 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 as save time. Redirection ----------- 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 Desktop Documents Downloads Music Pictures Public Templates test-0.txt test-1.txt Videos $ 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: #. If you redirect to a file that does not exist, that file will be created. #. 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_echo.py < my-input.txt (Change back to your ``lab1`` 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 ``my_echo.py`` 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. Piping ------ 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 Nice. Exercises ~~~~~~~~~ #. Use piping to chain together the ``printenv`` and ``tail`` commands to display the last 10 lines of output from ``printenv``. #. Replicate the above functionality without using the ``|`` operator. (hint: Use a temporary file.) Remote Access (optional, skip if short on time) ------------------------------------------------- We'll finish up with a description of some useful commands. If you run out of time, you can skip this part and return to it later. There are two main tools for accessing a remote computer through the command line: one for running commands on the remote computer, and one for file transfer. The first of these commands is much more likely to be useful in this class. SSH ~~~ SSH allows you to open a terminal session on a computer remotely, and is a major motivation for becoming proficient with the terminal. The following command:: $ ssh username@domain begins an SSH session, and allows you to access all of your files and programs on the remote computer (as long as these programs can be executed through the shell). The command for SSHing into CSIL Linux computers is:: $ ssh CNETID@linux.cs.uchicago.edu You should try this now and ask a question if you have trouble, as it is something you may have to do for your CS classes here. To exit an SSH session, simply use the command ``exit``. ``ssh`` is installed by default on Linux and OSX. `PuTTY `__ is a popular SSH client for Windows. SCP ~~~ While SSH allows you to log in to another computer, SCP provides the ability to transfer files between computers. ``scp`` is useful, but do **not** use it to move files in your ``capp30121-aut-19`` repository between machines. It is much safer to use Git, which you will learn about in a separate lab, to manage the files in your repository. In general, SCP is called as:: $ scp user@host1:/path/to/file1 user@host2:path/to/file2 and copies a file from one computer (the first argument) and places it in the second computer (the second argument). If you want to copy a file to your local computer, you can simply specify the second argument as a file path, without the username or domain name; the same principle applies for copying from your local computer. If you want to leave the file named as it was, you don't need to specify the file name in the second argument. An example of using SCP would be:: $ ls Desktop Downloads $ scp username@linux.cs.uchicago.edu:~/capp30121/assignment1/Grade.txt . $ ls Desktop Downloads Grade.txt Recall that a single dot (``.``) refers to the current directory. 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 your name 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). **Make sure to always log out from your machine before you leave CSIL.**