# Directories in UNIX

:::{note}
If you're one of those people who's only used a {abbr}`GUI (Graphical User Interface)`, or you save all of
your files on your Desktop, this sub-section is for you. There are
plenty of [web
sites](https://www.tutorialspoint.com/unix/unix-directories.htm) that
discuss directories; this is just a brief overview.
:::

The "folders" that you see when you look at your GUI are actually
directories in your operating system. That tells you what a directory
is: a container for other files, including other directories. The
separator for directory names is "/", so `a/b/c` is directory `c`
within directory `b` within directory `a`.[^f4]

[^f4]: Now you know why it's hard to put a / in a folder name: The
    operating system can't tell the difference between a / that's within
    a folder name versus a / that is a directory separator.

Everything in UNIX is within a directory. Yes, even your Desktop;
typically, that is a directory whose name is `~/Desktop`. That leads
us to common abbreviations and commands for directories when you're
using the command line:

- `~<account>` means the home directory of the user
  \<account\>[^f5]. Just plain `~` means your own home directory. So
  `~/Desktop` means a directory named Desktop within your home
  directory.

[^f5]: It's always something like "~seligman" (tilde-seligman), never
    "--seligman" (dash-seligman). Depending on the exact font used to
    print or display this tutorial, sometimes tildes look like dashes.
    On most keyboards, tilde is typed with SHIFT-\` where \` (backtick)
    is near the upper-left-hand corner of the keyboard.

- `cd` is the command to "change directory." It's the usual way to
  go from one directory to another. If there were a directory named
  `Root`[^f6] in your home directory, you could visit that directory
  with:

        > cd ~/Root

[^f6]: UNIX is normally a case-sensitive operating system. `~/Root`,
    `~/ROOT`, and `~/root` are three *different* directories.
    Exception: In Mac OS Darwin, by default file names are *case-insensitive*; all
    three of those directories would be the same. 

- `..` is a reference to your parent directory, the one "above" the
  one you're currently in. If you wanted to return to your home
  directory from `~/Root`, you could type:

        > cd ..

    If you use the `cd` command without any arguments, it will return
    you to your home directory:[^f7]

        > cd

[^f7]: Knowing this will become useful in the future, as you become more
    sophisticated in your use of UNIX. Eventually you'll learn about
    shell variables. Sooner or later, you'll make a typo in a variable
    name; e.g.,

        > cd $ROTSYS

    Instead of going to `$ROOTSYS`, your intended destination, you'll
    find yourself in your home directory. That's because `$ROTSYS`
    doesn't have a value, so UNIX interpreted this as the `cd` command
    without any arguments.

- To look at the contents of your current directory, use the `ls`
  command:

        > ls

    You can also list the contents of any other directory (for which you
    have permission to view):

        > ls ~seligman/root-class

- If you forget which directory you're in, use the `pwd` ("print
  working directory") command:

        > pwd

:::{figure-md} porn_folder-fig
:class: align-center

<img src="https://imgs.xkcd.com/comics/porn_folder.png" alt="xkcd porn_folder" width="70%">

<https://xkcd.com/981/> by Randall Munroe. 
Moral: Be careful how you organize your directories!
:::