The Unix Shell

Files and Directories

The part of the operating system responsible for managing files and directories is called the file system. It organizes our data into files, which hold information, and directories (also called “folders”), which hold files or other directories.

Several commands are frequently used to create, inspect, rename, and delete files and directories. To start exploring them, let’s open a shell window:

$

The dollar sign is a prompt, which shows us that the shell is waiting for input; your shell may show something more elaborate.

Type the command whoami, then press the Enter key (sometimes marked Return) to send the command to the shell. The command’s output is the ID of the current user, i.e., it shows us who the shell thinks we are:

$ whoami

nelle

More specifically, when we type whoami the shell:

1. finds a program called whoami,
2. runs that program,
3. displays that program’s output, then
4. displays a new prompt to tell us that it’s ready for more commands.

Next, let’s find out where we are by running a command called pwd (which stands for “print working directory”). At any moment, our current working directory is our current default directory, i.e., the directory that the computer assumes we want to run commands in unless we explicitly specify something else. Here, the computer’s response is /Users/nelle, which is Nelle’s home directory:

$ pwd

/Users/nelle

To understand what a “home directory” is, let’s have a look at how the file system as a whole is organized. At the top is the root directory that holds everything else. We refer to it using a slash character / on its own; this is the leading slash in /Users/nelle.

Inside that directory are several other directories: bin (which is where some built-in programs are stored), data (for miscellaneous data files), users (where users’ personal directories are located), tmp (for temporary files that don’t need to be stored long-term), and so on:

The File System

We know that our current working directory /Users/nelle is stored inside /Users because /Users is the first part of its name. Similarly, we know that /Users is stored inside the root directory / because its name begins with /.

Underneath /Users, we find one directory for each user with an account on this machine. The Mummy’s files are stored in /Users/imhotep, Wolfman’s in /Users/larry, and ours in /Users/nelle, which is why nelle is the last part of the directory’s name.

Home Directories

Notice that there are two meanings for the / character. When it appears at the front of a file or directory name, it refers to the root directory. When it appears inside a name, it’s just a separator.

Let’s see what’s in Nelle’s home directory by running ls, which stands for “listing”:

$ ls

creatures  molecules           pizza.cfg
data       north-pacific-gyre  solar.pdf
Desktop    notes.txt           writing

Nelle’s Home Directory

ls prints the names of the files and directories in the current directory in alphabetical order, arranged neatly into columns. We can make its output more comprehensible by using the flag -F, which tells ls to add a trailing / to the names of directories:

$ ls -F

creatures/  molecules/           pizza.cfg
data/       north-pacific-gyre/  solar.pdf
Desktop/    notes.txt            writing/

Here, we can see that /Users/nelle contains six sub-directories. The names that don’t have trailing slashes, like notes.txt, pizza.cfg, and solar.pdf, are plain old files. And note that there is a space between ls and -F: without it, the shell thinks we’re trying to run a command called ls-F, which doesn’t exist.

Now let’s take a look at what’s in Nelle’s data directory by running ls -F data, i.e., the command ls with the arguments -F and data. The second argument — the one without a leading dash — tells ls that we want a listing of something other than our current working directory:

$ ls -F data

amino-acids.txt   elements/     morse.txt
pdb/              planets.txt   sunspot.txt

The output shows us that there are four text files and two sub-sub-directories. Organizing things hierarchically in this way helps us keep track of our work: it’s possible to put hundreds of files in our home directory, just as it’s possible to pile hundreds of printed papers on our desk, but it’s a self-defeating strategy.

Notice, by the way that we spelled the directory name data. It doesn’t have a trailing slash: that’s added to directory names by ls when we use the -F flag to help us tell things apart. And it doesn’t begin with a slash because it’s a relative path, i.e., it tells ls how to find something from where we are, rather than from the root of the file system.

If we run ls -F /data (with a leading slash) we get a different answer, because /data is an absolute path:

$ ls -F /data

access.log    backup/    hardware.cfg
network.cfg

The leading / tells the computer to follow the path from the root of the file system, so it always refers to exactly one directory, no matter where we are when we run the command.

What if we want to change our current working directory? Before we do this, pwd shows us that we’re in /Users/nelle, and ls without any arguments shows us that directory’s contents:

$ pwd

/Users/nelle

$ ls

creatures  molecules           pizza.cfg
data       north-pacific-gyre  solar.pdf
Desktop    notes.txt           writing

We can use cd followed by a directory name to change our working directory. cd stands for “change directory”, which is a bit misleading: the command doesn’t change the directory, it changes the shell’s idea of what directory we are in.

$ cd data

cd doesn’t print anything, but if we run pwd after it, we can see that we are now in /Users/nelle/data. If we run ls without arguments now, it lists the contents of /Users/nelle/data, because that’s where we now are:

$ pwd

/Users/nelle/data

$ ls -F

amino-acids.txt   elements/     morse.txt
pdb/              planets.txt   sunspot.txt

We now know how to go down the directory tree: how do we go up? We could use an absolute path:

$ cd /Users/nelle

but it’s almost always simpler to use cd .. to go up one level:

$ pwd

/Users/nelle/data

$ cd ..

.. is a special directory name meaning “the directory containing this one”, or more succinctly, the parent of the current directory. Sure enough, if we run pwd after running cd .., we’re back in /Users/nelle:

$ pwd

/Users/nelle

The special directory .. doesn’t usually show up when we run ls. If we want to display it, we can give ls the -a flag:

$ ls -F -a

./                  creatures/          notes.txt
../                 data/               pizza.cfg
.bash_profile       molecules/          solar.pdf
Desktop/            north-pacific-gyre/ writing/

-a stands for “show all”; it forces ls to show us file and directory names that begin with ., such as .. (which, if we’re in /Users/nelle, refers to the /Users directory). As you can see, it also displays another special directory that’s just called ., which means “the current working directory”. It may seem redundant to have a name for it, but we’ll see some uses for it soon. Finally, we also see a file called .bash_profile. This file usually contains settings to customize the shell (terminal). For this lesson material it does not contain any settings. There may also be similar files called .bashrc or .bash_login. The . prefix is used to prevent these configuration files from cluttering the terminal when a standard ls command is used.

Nelle’s Pipeline: Organizing Files

Knowing just this much about files and directories, Nelle is ready to organize the files that the protein assay machine will create. First, she creates a directory called north-pacific-gyre (to remind herself where the data came from). Inside that, she creates a directory called 2012-07-03, which is the date she started processing the samples. She used to use names like conference-paper and revised-results, but she found them hard to understand after a couple of years. (The final straw was when she found herself creating a directory called revised-revised-results-3.)

Nelle names her directories “year-month-day”, with leading zeroes for months and days, because the shell displays file and directory names in alphabetical order. If she used month names, December would come before July; if she didn’t use leading zeroes, November (‘11’) would come before July (‘7’).

Each of her physical samples is labelled according to her lab’s convention with a unique ten-character ID, such as “NENE01729A”. This is what she used in her collection log to record the location, time, depth, and other characteristics of the sample, so she decides to use it as part of each data file’s name. Since the assay machine’s output is plain text, she will call her files NENE01729A.txt, NENE01812A.txt, and so on. All 1520 files will go into the same directory.

If she is in her home directory, Nelle can see what files she has using the command:

$ ls north-pacific-gyre/2012-07-03/

This is a lot to type, but she can let the shell do most of the work through what is called tab completion. If she types:

$ ls nor

and then presses tab (the tab key on her keyboard), the shell automatically completes the directory name for her:

$ ls north-pacific-gyre/

If she presses tab again, Bash will add 2012-07-03/ to the command, since it’s the only possible completion. Pressing tab again does nothing, since there are 1520 possibilities; pressing tab twice brings up a list of all the files, and so on. This is called tab completion, and we will see it in many other tools as we go on.

File System for Challange Questions

pnas-sub/ pnas-final/ original/