## Files

 1. As you know from the lecture, files are pretty fundamental in most
    operating systems, especially in those from the POSIX family.
 2. There is a rich set of commands for working with files. You already met
    ‹cat› which prints files. It is called ‹cat› because it can con-cat-enate
    multiple files together. Like this:
    
        $ cat intro.txt tricks.txt
    
    I will remind you that you can learn more about cat by typing:
    
        $ man cat

 3. Another very useful command is ‹ls›. It is short for ‘list files’. Many
    commands are shorthands and they are easier to remember if you know what
    the shorthands mean. You can try ‹ls› like this:
    
        $ ls ..
    
    Of course, you can also consult the man page. I'm sure you already know
    the command to do that?

 4. Then there is ‹cp› for copy and ‹rm› for remove. You can use them like
    this:
    
        $ cp intro.txt ~/pb152-intro.txt
        $ cat ~/pb152-intro.txt
        $ rm ~/pb152-intro.txt

 5. It would be a good time to create a directory in which you could play
    without messing up your home directory. Let's use ‹mkdir› (from ‘make
    directory’) for that, and let's also enter it right away using ‹cd› (for
    ‘change directory’):

        $ mkdir scratch
        $ cd scratch

 6. You can create an empty file using ‹touch›. This command will also update
    the modification timestamp on a file if it already exists. This is not
    very important to us for now. Try:
    
        $ touch file.txt

 7. If you forget where you are, you can always ask the system to ‹pwd› (print
    working directory):
    
        $ pwd
        /home/yourlogin/pb152/01/scratch
    
    (when a line in an example like above does not start with ‹$›, it is not a
    command -- it is what you should see in the terminal after typing the
    command that's marked with ‹$›)

 8. You can refer to the working directory using ‹.› or often nothing at all.
    Try the following commands:
    
        $ ls .
        $ ls
        $ cp intro.txt /tmp/yourlogin.txt
        $ ls
        $ cat ./intro.txt
        $ cat intro.txt
        $ rm /tmp/yourlogin.txt

What follows is a few philosophical remarks about using the command line. We
often shorten ‘command line interface’ to CLI or cli. Hence, continue to:

    $ cat cli.txt

