CLI basics

Creative Commons License: CC-BY Questions:
  • What is a command shell and why would I use one?

  • How can I move around on my computer?

  • How can I see what files and directories I have?

  • How can I specify the location of a file or directory on my computer?

  • How can I create, copy, and delete files and directories?

  • How can I edit files?

  • Explain how the shell relates to the keyboard, the screen, the operating system, and users’ programs.

  • Explain when and why command-line interfaces should be used instead of graphical interfaces.

  • Explain the similarities and differences between a file and a directory.

  • Translate an absolute path into a relative path and vice versa.

  • Construct absolute and relative paths that identify specific files and directories.

  • Use options and arguments to change the behaviour of a shell command.

  • Demonstrate the use of tab completion and explain its advantages.

  • Create a directory hierarchy that matches a given diagram.

  • Create files in that hierarchy using an editor or by copying and renaming existing files.

  • Delete, copy and move specified files and/or directories.

Time estimation: 1 hour
Level: Introductory Introductory
Supporting Materials:
Published: Sep 30, 2021
Last modification: Oct 23, 2023
License: Tutorial Content is licensed under Creative Commons Attribution 4.0 International License. The GTN Framework is licensed under MIT
purl PURL:
version Revision: 9
Best viewed in a Jupyter Notebook

This tutorial is best viewed in a Jupyter notebook! You can load this notebook one of the following ways

Launching the notebook in Jupyter in Galaxy

  1. Instructions to Launch JupyterLab
  2. Open a Terminal in JupyterLab with File -> New -> Terminal
  3. Run wget
  4. Select the notebook that appears in the list of files on the left.

Downloading the notebook

  1. Right click one of these links: Jupyter Notebook (With Solutions), Jupyter Notebook (Without Solutions)
  2. Save Link As..

This tutorial will walk you through the basics of how to use the Unix command line.


This tutorial is significantly based on the Carpentries “The Unix Shell” lesson, which is licensed CC-BY 4.0. Adaptations have been made to make this work better in a GTN/Galaxy environment.


In this tutorial, we will cover:

  1. Background
    1. Setup
    2. The Shell
    3. Nelle’s Pipeline: A Typical Problem
  2. Navigating Files and Directories
    1. General Syntax of a Shell Command
  3. Working with Files and Directories
    1. Step one: see where we are and what we already have
    2. Create a directory
    3. Creating a text file
    4. Moving files and directories
    5. Copying files and directories
    6. Removing files and directories
    7. Operations with multiple files and directories


Humans and computers commonly interact in many different ways, such as through a keyboard and mouse, touch screen interfaces, or using speech recognition systems. The most widely used way to interact with personal computers is called a graphical user interface (GUI). With a GUI, we give instructions by clicking a mouse and using menu-driven interactions.

While the visual aid of a GUI makes it intuitive to learn, this way of delivering instructions to a computer scales very poorly. Imagine the following task: for a literature search, you have to copy the third line of one thousand text files in one thousand different directories and paste it into a single file. Using a GUI, you would not only be clicking at your desk for several hours, but you could potentially also commit an error in the process of completing this repetitive task. This is where we take advantage of the Unix shell. The Unix shell is both a command-line interface (CLI) and a scripting language, allowing such repetitive tasks to be done automatically and fast. With the proper commands, the shell can repeat tasks with or without some modification as many times as we want. Using the shell, the task in the literature example can be accomplished in seconds.


Before we do anything, we’ll get you setup with some test data which can help guide your exploration of the CLI.

cd ~/
mkdir -p Desktop/
cd Desktop/
wget -c
unzip -u

The Shell

The shell is a program where users can type commands. With the shell, it’s possible to invoke complicated programs like climate modeling software or simple commands that create an empty directory with only one line of code. The most popular Unix shell is Bash (the Bourne Again SHell — so-called because it’s derived from a shell written by Stephen Bourne). Bash is the default shell on most modern implementations of Unix and in most packages that provide Unix-like tools for Windows.

Using the shell will take some effort and some time to learn. While a GUI presents you with choices to select, CLI choices are not automatically presented to you, so you must learn a few commands like new vocabulary in a language you’re studying. However, unlike a spoken language, a small number of “words” (i.e. commands) gets you a long way, and we’ll cover those essential few today.

The grammar of a shell allows you to combine existing tools into powerful pipelines and handle large volumes of data automatically. Sequences of commands can be written into a script, improving the reproducibility of workflows.

In addition, the command line is often the easiest way to interact with remote machines and supercomputers. Familiarity with the shell is near essential to run a variety of specialized tools and resources including high-performance computing systems. As clusters and cloud computing systems become more popular for scientific data crunching, being able to interact with the shell is becoming a necessary skill. We can build on the command-line skills covered here to tackle a wide range of scientific questions and computational challenges.

Let’s get started.

When the shell is first opened, you are presented with a prompt, indicating that the shell is waiting for input.


The shell typically uses $ as the prompt, but may use a different symbol. In the examples for this lesson, we will not include this prompt!

Most importantly: when typing commands, either from these lessons or from other sources, do not type the prompt, only the commands that follow it. Also note that after you type a command, you have to press the Enter key to execute it.

The prompt is followed by a text cursor, a character that indicates the position where your typing will appear. The cursor is usually a flashing or solid block, but it can also be an underscore or a pipe. You may have seen it in a text editor program, for example.

So let’s try our first command, ls which is short for listing.

Hands-on: Open a Jupyter Terminal

This tutorial will let you accomplish almost everything from this view, running code in the cells below directly in the training material. You can choose between running the code here, or opening up a terminal tab in which to run it.

Here are some instructions for how to do this on various environments.

Jupyter on UseGalaxy.* and

  1. Use the File → New → Terminal menu to launch a terminal.

    screenshot of jupyterlab showing the File menu expanded to show new and terminal option.

  2. Disable “Simple” mode in the bottom left hand corner, if it activated.

    screenshot of jupyterlab showing a toggle labelled simple.

  3. Drag one of the terminal or notebook tabs to the side to have the training materials and terminal side-by-side

    screenshot of jupyterlab with notebook and terminal side-by-side.


  1. Use the Split View functionality of cocalc to split your view into two portions.

    screenshot of cocalc button to split views.

  2. Change the view of one panel to a terminal

    screenshot of cocalc swapping view port to that of a terminal.

This command will list the contents of the current directory:


If the shell can’t find a program whose name is the command you typed, it will print an error message such as:

Input: Bash
$ ks
ks: command not found

This might happen if the command was mis-typed or if the program corresponding to that command is not installed.

Nelle’s Pipeline: A Typical Problem

Nelle Nemo, a marine biologist, has just returned from a six-month survey of the North Pacific Gyre, where she has been sampling gelatinous marine life in the Great Pacific Garbage Patch. She has 1520 samples that she’s run through an assay machine to measure the relative abundance of 300 proteins. She needs to run these 1520 files through an imaginary program called she inherited. On top of this huge task, she has to write up results by the end of the month so her paper can appear in a special issue of Aquatic Goo Letters.

The bad news is that if she has to run by hand using a GUI, she’ll have to select and open a file 1520 times. If takes 30 seconds to run each file, the whole process will take more than 12 hours of Nelle’s attention. With the shell, Nelle can instead assign her computer this mundane task while she focuses her attention on writing her paper.

The next few lessons will explore the ways Nelle can achieve this. More specifically, they explain how she can use a command shell to run the program, using loops to automate the repetitive steps of entering file names, so that her computer can work while she writes her paper.

As a bonus, once she has put a processing pipeline together, she will be able to use it again whenever she collects more data.

In order to achieve her task, Nelle needs to know how to:

  • navigate to a file/directory
  • create a file/directory
  • check the length of a file
  • chain commands together
  • retrieve a set of files
  • iterate over files
  • run a shell script containing her pipeline

Navigating 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, we’ll go to our open shell window.

First, let’s find out where we are by running a command called pwd (which stands for ‘print working directory’). Directories are like places, at any time while we are using the shell, we are in exactly one place called our current working directory. Commands mostly read and write files in the current working directory, i.e. ‘here’, so knowing where you are before running a command is important. pwd shows you where you are:


Try running it now.

The home directory path will look different on different operating systems.

Linux OSX Jupyter (UseGalaxy/Binder) CoCalc
/home/nelle /Users/nelle /home/joyvan /projects/<id>

On Windows, it will be similar to C:\Documents and Settings\nelle or C:\Users\nelle. (Note that it may look slightly different for different versions of Windows.) In future examples, we’ve used Mac output as the default - Linux and Windows output may differ slightly but should be generally similar.

We will also assume that your pwd command returns your user’s home directory. If pwd returns something different, you may need to navigate there using cd or some commands in this lesson will not work as written. See Exploring Other Directories for more details on the cd command.

To understand what a ‘home directory’ is, let’s have a look at how the file system as a whole is organized. For the sake of this example, we’ll be illustrating the filesystem on our scientist Nelle’s computer. After this illustration, you’ll be learning commands to explore your own filesystem, which will be constructed in a similar way, but not be exactly identical.

On Nelle’s computer, the filesystem looks like this:

The file system is made up of a root directory that contains sub-directories titled bin, data, users, and tmp.

At the top is the root directory that holds everything else. We refer to it using a slash character, /, on its own; this character 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.

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 /.

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 path, it’s just a separator.

Underneath /Users, we find one directory for each user with an account on Nelle’s machine, her colleagues imhotep and larry.

Like other directories, home directories are sub-directories underneath '/Users' like '/Users/imhotep', '/Users/larry' or '/Users/nelle'.

The user imhotep’s files are stored in /Users/imhotep, user larry’s in /Users/larry, and Nelle’s in /Users/nelle. Because Nelle is the user in our examples here, therefore we get /Users/nelle as our home directory. Typically, when you open a new command prompt, you will be in your home directory to start.

Now let’s learn the command that will let us see the contents of our own filesystem. We can see what’s in our home directory by running ls:


ls prints the names of the files and directories in the current directory. We can make its output more comprehensible by using the -F option (also known as a switch or a flag), which tells ls to classify the output by adding a marker to file and directory names to indicate what they are:

  • a trailing / indicates that this is a directory
  • @ indicates a link
  • * indicates an executable

Depending on your default options, the shell might also use colors to indicate whether each entry is a file or directory.

ls -F

Here, we can see that our home directory contains only sub-directories. Any names in our output that don’t have a classification symbol are plain old files.

If your screen gets too cluttered, you can clear your terminal using the clear command. You can still access previous commands using and to move line-by-line, or by scrolling in your terminal.

Getting help

ls has lots of other options. There are two common ways to find out how to use a command and what options it accepts:

  1. We can pass a --help option to the command,
  2. We can read its manual with man

The --help option

Many bash commands, and programs that people have written that can be run from within bash, support a --help option to display more information on how to use the command or program.

ls --help

If you try to use an option (flag) that is not supported, ls and other commands will usually print an error message similar to:

Input: Bash
ls -j
ls: invalid option -- 'j'
Try 'ls --help' for more information.

The man command

The other way to learn about ls is to type

Input: Bash
$ man ls
LS(1)                                                          User Commands                                                          LS(1)

       ls - list directory contents

       ls [OPTION]... [FILE]...

       List information about the FILEs (the current directory by default).  Sort entries alphabetically if none of -cftuvSUX nor --sort is

       Mandatory arguments to long options are mandatory for short options too.

       -a, --all
              do not ignore entries starting with .

       -A, --almost-all
              do not list implied . and ..

              with -l, print the author of each file

       -b, --escape
              print C-style escapes for nongraphic characters

              with -l, scale sizes by SIZE when printing them; e.g., '--block-size=M'; see SIZE format below

       -B, --ignore-backups
              do not list implied entries ending with ~

       -c     with -lt: sort by, and show, ctime (time of last modification of file status information); with -l: show ctime  and  sort  by
              name; otherwise: sort by ctime, newest first

       -C     list entries by columns

              colorize the output; WHEN can be 'always' (default if omitted), 'auto', or 'never'; more info below

This command will turn your terminal into a page with a description of the ls command and its options.

To navigate through the man pages, you may use and to move line-by-line, or try B and Spacebar to skip up and down by a full page. To search for a character or word in the man pages, use / followed by the character or word you are searching for. Sometimes a search will result in multiple hits. If so, you can move between hits using N (for moving forward) and Shift+N (for moving backward).

To quit the man pages, press Q.

Of course, there is a third way to access help for commands: searching the internet via your web browser. When using internet search, including the phrase unix man page in your search query will help to find relevant results.

GNU provides links to its manuals including the core GNU utilities, which covers many commands introduced within this lesson.

Question: Exploring More `ls` Flags

You can also use two options at the same time. What does the command ls do when used with the -l option? What about if you use both the -l and the -h option?

Some of its output is about properties that we do not cover in this lesson (such as file permissions and ownership), but the rest should be useful nevertheless.

The -l option makes ls use a long listing format, showing not only the file/directory names but also additional information, such as the file size and the time of its last modification. If you use both the -h option and the -l option, this makes the file size ‘human readable’, i.e. displaying something like 5.3K instead of 5369.

Question: Listing in Reverse Chronological Order

By default, ls lists the contents of a directory in alphabetical order by name. The command ls -t lists items by time of last change instead of alphabetically. The command ls -r lists the contents of a directory in reverse order. Which file is displayed last when you combine the -t and -r flags? Hint: You may need to use the -l flag to see the last changed dates.

The most recently changed file is listed last when using -rt. This can be very useful for finding your most recent edits or checking to see if a new output file was written.

# Explore the possible solutions here!

Exploring Other Directories

Not only can we use ls on the current working directory, but we can use it to list the contents of a different directory. Let’s take a look at our current directory by running ls -F ~/, i.e., the command ls with the -F option and the argument /. The argument ~/ tells ls that we want a listing of something other than our current working directory:

ls -F ~/

As you may now see, using a bash shell is strongly dependent on the idea that your files are organized in a hierarchical file system. 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.

Now that we know the shell-lesson-data directory is located in current working directory, we can do two things.

First, we can look at its contents, using the same strategy as before, passing a directory name to ls:

ls -F shell-lesson-data

Second, we can actually change our location to a different directory, so we are no longer located in our home directory.

The command to change locations is 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. The cd command is akin to double clicking a folder in a graphical interface to get into a folder.

Let’s say we want to move to the data directory we saw above. We can use the following series of commands to get there:

cd shell-lesson-data

These commands will move us into the shell-lesson-data directory, then into the data directory. You will notice that cd doesn’t print anything. This is normal. Many shell commands will not output anything to the screen when successfully executed. But if we run pwd after it, we can see that we are now in /Users/nelle/Desktop/shell-lesson-data/data. If we run ls -F without arguments now, it lists the contents of /Users/nelle/Desktop/shell-lesson-data/data, because that’s where we now are:

ls -F data/

We now know how to go down the directory tree (i.e. how to go into a subdirectory), but how do we go up (i.e. how do we leave a directory and go into its parent directory)? We might try the following:

cd shell-lesson-data

But we get an error! Why is this?

With our methods so far, cd can only see sub-directories inside your current directory. There are different ways to see directories above your current location; we’ll start with the simplest.

There is a shortcut in the shell to move up one directory level that looks like this:

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/Desktop/shell-lesson-data:


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

ls -F -a

-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.

Note that in most command line tools, multiple options can be combined with a single - and no spaces between the options: ls -F -a is equivalent to ls -Fa.

In addition to the hidden directories .. and ., you may also see a file called .bash_profile. This file usually contains shell configuration settings. You may also see other files and directories beginning with .. These are usually files and directories that are used to configure different programs on your computer. The prefix . is used to prevent these configuration files from cluttering the terminal when a standard ls command is used.

These three commands are the basic commands for navigating the filesystem on your computer: pwd, ls, and cd. Let’s explore some variations on those commands. What happens if you type cd on its own, without giving a directory?


How can you check what happened? pwd gives us the answer!


It turns out that cd without an argument will return you to your home directory, which is great if you’ve gotten lost in your own filesystem.

Let’s try returning to the data directory from before. Last time, we used three commands, but we can actually string together the list of directories to move to data in one step:

cd ~/Desktop/shell-lesson-data/data

Check that we’ve moved to the right place by running pwd and ls -F.

If we want to move up one level from the data directory, we could use cd ... But there is another way to move to any directory, regardless of your current location.

So far, when specifying directory names, or even a directory path (as above), we have been using relative paths. When you use a relative path with a command like ls or cd, it tries to find that location from where we are, rather than from the root of the file system.

However, it is possible to specify the absolute path to a directory by including its entire path from the root directory, which is indicated by a leading slash. 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.

This allows us to move to our shell-lesson-data directory from anywhere on the filesystem (including from inside data). To find the absolute path we’re looking for, we can use pwd and then extract the piece we need to move to shell-lesson-data.

Input: Bash
$ pwd

Then we could run something like:

$ cd /Users/nelle/shell-lesson-data

Run pwd and ls -F to ensure that we’re in the directory we expect. If we’re not, edit the command block below to make sure we’re in the correct location (~/Desktop/shell-lesson-data/)

ls -F

The shell interprets a tilde (~) character at the start of a path to mean “the current user’s home directory”. For example, if Nelle’s home directory is /Users/nelle, then ~/data is equivalent to /Users/nelle/data. This only works if it is the first character in the path: here/there/~/elsewhere is not here/there/Users/nelle/elsewhere.

Another shortcut is the - (dash) character. cd will translate - into the previous directory I was in, which is faster than having to remember, then type, the full path. This is a very efficient way of moving back and forth between two directories – i.e. if you execute cd - twice, you end up back in the starting directory.

The difference between cd .. and cd - is that the former brings you up, while the latter brings you back.

Try it out below!

First navigate to ~/Desktop/shell-lesson-data (you should already be there).

cd ~/Desktop/shell-lesson-data

Then cd into the creatures directory

cd creatures

Now if you run

cd -

you’ll see you’re back in ~/Desktop/shell-lesson-data. Run cd - again and you’re back in ~/Desktop/shell-lesson-data/creatures

Question: Absolute vs Relative Paths

Starting from /Users/amanda/data, which of the following commands could Amanda use to navigate to her home directory, which is /Users/amanda?

  1. cd .
  2. cd /
  3. cd /home/amanda
  4. cd ../..
  5. cd ~
  6. cd home
  7. cd ~/data/..
  8. cd
  9. cd ..
  1. No: . stands for the current directory.
  2. No: / stands for the root directory.
  3. No: Amanda’s home directory is /Users/amanda.
  4. No: this command goes up two levels, i.e. ends in /Users.
  5. Yes: ~ stands for the user’s home directory, in this case /Users/amanda.
  6. No: this command would navigate into a directory home in the current directory, if it exists.
  7. Yes: unnecessarily complicated, but correct.
  8. Yes: shortcut to go back to the user’s home directory.
  9. Yes: goes up one level.
Question: Relative Path Resolution

Using the filesystem diagram below, if pwd displays /Users/thing, what will ls -F ../backup display?

  1. ../backup: No such file or directory
  2. 2012-12-01 2013-01-08 2013-01-27
  3. 2012-12-01/ 2013-01-08/ 2013-01-27/
  4. original/ pnas_final/ pnas_sub/

A directory tree below the Users directory where '/Users' contains the directories 'backup' and 'thing'; '/Users/backup' contains 'original', 'pnas_final' and 'pnas_sub'; '/Users/thing' contains 'backup'; and '/Users/thing/backup' contains '2012-12-01', '2013-01-08' and '2013-01-27'.

  1. No: there is a directory backup in /Users.
  2. No: this is the content of Users/thing/backup, but with .., we asked for one level further up.
  3. No: see previous explanation.
  4. Yes: ../backup/ refers to /Users/backup/.
Question: `ls` Reading Comprehension

Using the filesystem diagram below, if pwd displays /Users/backup, and -r tells ls to display things in reverse order, what command(s) will result in the following output:

pnas_sub/ pnas_final/ original/

A directory tree below the Users directory where '/Users' contains the directories 'backup' and 'thing'; '/Users/backup' contains 'original', 'pnas_final' and 'pnas_sub'; '/Users/thing' contains 'backup'; and'/Users/thing/backup' contains '2012-12-01', '2013-01-08' and '2013-01-27'.

  1. ls pwd
  2. ls -r -F
  3. ls -r -F /Users/backup
  1. No: pwd is not the name of a directory.
  2. Yes: ls without directory argument lists files and directories in the current directory.
  3. Yes: uses the absolute path explicitly.

General Syntax of a Shell Command

We have now encountered commands, options, and arguments, but it is perhaps useful to formalise some terminology.

Consider the command below as a general example of a command, which we will dissect into its component parts:

$ ls -F ~/Desktop/shell-lesson-data/

ls is the command, with an option -F and an argument /. We’ve already encountered options (also called switches or flags) which either start with a single dash (-) or two dashes (--), and they change the behavior of a command. Arguments tell the command what to operate on (e.g. files and directories). Sometimes options and arguments are referred to as parameters. A command can be called with more than one option and more than one argument, but a command doesn’t always require an argument or an option.

Each part is separated by spaces: if you omit the space between ls and -F the shell will look for a command called ls-F, which doesn’t exist. Also, capitalization can be important. For example, ls -s will display the size of files and directories alongside the names, while ls -S will sort the files and directories by size, as shown below:

$ ls -s Desktop/shell-lesson-data/data
total 116
 4 amino-acids.txt   4 animals.txt   4 morse.txt  12 planets.txt  76 sunspot.txt
 4 animal-counts     4 elements      4 pdb         4 salmon.txt
$ ls -S Desktop/shell-lesson-data/data
sunspot.txt  animal-counts  pdb        amino-acids.txt  salmon.txt
planets.txt  elements       morse.txt  animals.txt

Putting all that together, our command above gives us a listing of files and directories in our shell-lesson-data folder.

Nelle’s Pipeline: Organizing Files

Knowing 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’). Similarly, putting the year first means that June 2012 will come before June 2013.

Each of her physical samples is labelled according to her lab’s convention with a unique ten-character ID, such as ‘NENE01729A’. This ID 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.

Now in her current directory shell-lesson-data, Nelle can see what files she has using the command:

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

This command 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 19 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.

Working with Files and Directories

We now know how to explore files and directories, but how do we create them in the first place?

Step one: see where we are and what we already have

Let’s go back to our shell-lesson-data directory on the Desktop and use ls -F to see what it contains:

ls -F

You should see folders like the following, if not, you’re in the wrong place!

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

Create a directory

Let’s create a new directory called thesis using the command mkdir thesis (which has no output):

mkdir thesis

As you might guess from its name, mkdir means ‘make directory’. Since thesis is a relative path (i.e., does not have a leading slash, like /what/ever/thesis), the new directory is created in the current working directory:

ls -F

Since we’ve just created the thesis directory, there’s nothing in it yet:

ls -F thesis

Note that mkdir is not limited to creating single directories one at a time. The -p option allows mkdir to create a directory with nested subdirectories in a single operation:

mkdir -p project/data project/results

The -R option to the ls command will list all nested subdirectories within a directory. Let’s use ls -FR to recursively list the new directory hierarchy we just created in the project directory:

ls -FR project

It should look like:

data/  results/



Using the shell to create a directory is no different than using a file explorer. If you open the current directory using your operating system’s graphical file explorer, the thesis directory will appear there too. While the shell and the file explorer are two different ways of interacting with the files, the files and directories themselves are the same.

Complicated names of files and directories can make your life painful when working on the command line. Here we provide a few useful tips for the names of your files and directories.

  1. Don’t use spaces.

    Spaces can make a name more meaningful, but since spaces are used to separate arguments on the command line it is better to avoid them in names of files and directories. You can use - or _ instead (e.g. north-pacific-gyre/ rather than north pacific gyre/). To test this out, try typing mkdir north pacific gyreand see what directory (or directories!) are made when you check with ls -F.

  2. Don’t begin the name with - (dash).

    Commands treat names starting with - as options.

  3. Stick with letters, numbers, . (period or ‘full stop’), - (dash) and _ (underscore).

    Many other characters have special meanings on the command line. We will learn about some of these during this lesson. There are special characters that can cause your command to not work as expected and can even result in data loss.

If you need to refer to names of files or directories that have spaces or other special characters, you should surround the name in quotes ("").

Creating a text file

Let’s change our working directory to thesis using cd, then run a text editor called Nano to create a file called draft.txt:

$ cd thesis
$ nano draft.txt
Warning: Nano won't work in a Jupyter Notebook!

Nano is an interactive, full screen editor. You’ll need to switch to a proper console to do this step, if you haven’t already.

In JupyterLab you can find this under the top menu by clicking File -> New -> Terminal

When we say, “nano is a text editor” we really do mean “text”: it can only work with plain character data, not tables, images, or any other human-friendly media. We use it in examples because it is one of the least complex text editors. However, because of this trait, it may not be powerful enough or flexible enough for the work you need to do after this workshop. On Unix systems (such as Linux and macOS), many programmers use Emacs or Vim (both of which require more time to learn), or a graphical editor such as Gedit. On Windows, you may wish to use Notepad++. Windows also has a built-in editor called notepad that can be run from the command line in the same way as nano for the purposes of this lesson.

No matter what editor you use, you will need to know where it searches for and saves files. If you start it from the shell, it will (probably) use your current working directory as its default location. If you use your computer’s start menu, it may want to save files in your desktop or documents directory instead. You can change this by navigating to another directory the first time you “Save As…”

Let’s type in a few lines of text. Once we’re happy with our text, we can press Ctrl+O (press the Ctrl or Control key and, while holding it down, press the O key) to write our data to disk (we’ll be asked what file we want to save this to: press Return to accept the suggested default of draft.txt).

screenshot of nano text editor in action.

Once our file is saved, we can use Ctrl+X to quit the editor and return to the shell.

The Control key is also called the ‘Ctrl’ key. There are various ways in which using the Control key may be described. For example, you may see an instruction to press the Control key and, while holding it down, press the X key, described as any of:

  • Control-X
  • Control+X
  • Ctrl-X
  • Ctrl+X
  • ^X
  • C-x

In nano, along the bottom of the screen you’ll see ^G Get Help ^O WriteOut. This means that you can use Control-G to get help and Control-O to save your file.

nano doesn’t leave any output on the screen after it exits, but ls now shows that we have created a file called draft.txt:

Question: Creating Files a Different Way

We have seen how to create text files using the nano editor. Now, try the following command:

touch my_file.txt
  1. What did the touch command do? When you look at your current directory using the GUI file explorer, does the file show up?

  2. Use ls -l to inspect the files. How large is my_file.txt?

  3. When might you want to create a file this way?

  1. The touch command generates a new file called my_file.txt in your current directory. You can observe this newly generated file by typing ls at the command line prompt. my_file.txt can also be viewed in your GUI file explorer.

  2. When you inspect the file with ls -l, note that the size of my_file.txt is 0 bytes. In other words, it contains no data. If you open my_file.txt using your text editor it is blank.

  3. Some programs do not generate output files themselves, but instead require that empty files have already been generated. When the program is run, it searches for an existing file to populate with its output. The touch command allows you to efficiently generate a blank text file to be used by such programs.

# Explore the possible solutions here!

You may have noticed that all of Nelle’s files are named ‘something dot something’, and in this part of the lesson, we always used the extension .txt. This is just a convention: we can call a file mythesis or almost anything else we want. However, most people use two-part names most of the time to help them (and their programs) tell different kinds of files apart. The second part of such a name is called the filename extension and indicates what type of data the file holds: .txt signals a plain text file, .pdf indicates a PDF document, .cfg is a configuration file full of parameters for some program or other, .png is a PNG image, and so on.

This is just a convention, albeit an important one. Files contain bytes: it’s up to us and our programs to interpret those bytes according to the rules for plain text files, PDF documents, configuration files, images, and so on.

Naming a PNG image of a whale as whale.mp3 doesn’t somehow magically turn it into a recording of whale song, though it might cause the operating system to try to open it with a music player when someone double-clicks it.

Moving files and directories

Returning to the shell-lesson-data directory,

cd ~/Desktop/shell-lesson-data/

In our thesis directory we have a file draft.txt which isn’t a particularly informative name, so let’s change the file’s name using mv, which is short for ‘move’:

mv thesis/draft.txt thesis/quotes.txt

The first argument tells mv what we’re ‘moving’, while the second is where it’s to go. In this case, we’re moving thesis/draft.txt to thesis/quotes.txt, which has the same effect as renaming the file. Sure enough, ls shows us that thesis now contains one file called quotes.txt:

ls thesis

One must be careful when specifying the target file name, since mv will silently overwrite any existing file with the same name, which could lead to data loss. An additional option, mv -i (or mv --interactive), can be used to make mv ask you for confirmation before overwriting.

Note that mv also works on directories.

Let’s move quotes.txt into the current working directory. We use mv once again, but this time we’ll use just the name of a directory as the second argument to tell mv that we want to keep the filename but put the file somewhere new. (This is why the command is called ‘move’.) In this case, the directory name we use is the special directory name . that we mentioned earlier.

mv thesis/quotes.txt .

The effect is to move the file from the directory it was in to the current working directory. ls now shows us that thesis is empty:

ls thesis

Alternatively, we can confirm the file quotes.txt is no longer present in the thesis directory by explicitly trying to list it:

ls thesis/quotes.txt

ls with a filename or directory as an argument only lists the requested file or directory. If the file given as the argument doesn’t exist, the shell returns an error as we saw above. We can use this to see that quotes.txt is now present in our current directory:

ls quotes.txt
Question: Moving Files to a new folder

After running the following commands, Jamie realizes that she put the files sucrose.dat and maltose.dat into the wrong folder. The files should have been placed in the raw folder.

$ ls -F
 analyzed/ raw/
$ ls -F analyzed
fructose.dat glucose.dat maltose.dat sucrose.dat
$ cd analyzed

Fill in the blanks to move these files to the raw/ folder (i.e. the one she forgot to put them in)

mv sucrose.dat maltose.dat ____/____
$ mv sucrose.dat maltose.dat ../raw

Recall that .. refers to the parent directory (i.e. one above the current directory) and that . refers to the current directory.

Copying files and directories

The cp command works very much like mv, except it copies a file instead of moving it. We can check that it did the right thing using ls with two paths as arguments — like most Unix commands, ls can be given multiple paths at once:

cp quotes.txt thesis/quotations.txt
ls quotes.txt thesis/quotations.txt

We can also copy a directory and all its contents by using the recursive option -r, e.g. to back up a directory:

cp -r thesis thesis_backup

We can check the result by listing the contents of both the thesis and thesis_backup directory:

ls thesis thesis_backup
Question: Renaming Files

Suppose that you created a plain-text file in your current directory to contain a list of the statistical tests you will need to do to analyze your data, and named it: statstics.txt

After creating and saving this file you realize you misspelled the filename! You want to correct the mistake, which of the following commands could you use to do so?

  1. cp statstics.txt statistics.txt
  2. mv statstics.txt statistics.txt
  3. mv statstics.txt .
  4. cp statstics.txt .
  1. No. While this would create a file with the correct name, the incorrectly named file still exists in the directory and would need to be deleted.
  2. Yes, this would work to rename the file.
  3. No, the period(.) indicates where to move the file, but does not provide a new file name; identical file names cannot be created.
  4. No, the period(.) indicates where to copy the file, but does not provide a new file name; identical file names cannot be created.
Question: Moving and Copying

What is the output of the closing ls command in the sequence shown below?

Input: Bash
Input: Bash
Input: Bash
mkdir recombined
mv proteins.dat recombined/
cp recombined/proteins.dat ../proteins-saved.dat
  1. proteins-saved.dat recombined
  2. recombined
  3. proteins.dat recombined
  4. proteins-saved.dat

We start in the /Users/jamie/data directory, and create a new folder called recombined. The second line moves (mv) the file proteins.dat to the new folder (recombined). The third line makes a copy of the file we just moved. The tricky part here is where the file was copied to. Recall that .. means ‘go up a level’, so the copied file is now in /Users/jamie. Notice that .. is interpreted with respect to the current working directory, not with respect to the location of the file being copied. So, the only thing that will show using ls (in /Users/jamie/data) is the recombined folder.

  1. No, see explanation above. proteins-saved.dat is located at /Users/jamie
  2. Yes
  3. No, see explanation above. proteins.dat is located at /Users/jamie/data/recombined
  4. No, see explanation above. proteins-saved.dat is located at /Users/jamie

Removing files and directories

Returning to the shell-lesson-data directory, let’s tidy up this directory by removing the quotes.txt file we created. The Unix command we’ll use for this is rm (short for ‘remove’):

rm quotes.txt

We can confirm the file has gone using ls:

ls quotes.txt

The Unix shell doesn’t have a trash bin that we can recover deleted files from (though most graphical interfaces to Unix do). Instead, when we delete files, they are unlinked from the file system so that their storage space on disk can be recycled. Tools for finding and recovering deleted files do exist, but there’s no guarantee they’ll work in any particular situation, since the computer may recycle the file’s disk space right away.

Question: Using `rm` Safely

What happens when we execute rm -i thesis_backup/quotations.txt? Why would we want this protection when using rm?

$ rm: remove regular file 'thesis_backup/quotations.txt'? y

The -i option will prompt before (every) removal (use Y to confirm deletion or N to keep the file). The Unix shell doesn’t have a trash bin, so all the files removed will disappear forever. By using the -i option, we have the chance to check that we are deleting only the files that we want to remove.

If we try to remove the thesis directory using rm thesis, we get an error message:

rm thesis

This happens because rm by default only works on files, not directories.

rm can remove a directory and all its contents if we use the recursive option -r, and it will do so without any confirmation prompts:

rm -r thesis

Given that there is no way to retrieve files deleted using the shell, rm -r should be used with great caution (you might consider adding the interactive option rm -r -i).

Operations with multiple files and directories

Oftentimes one needs to copy or move several files at once. This can be done by providing a list of individual filenames, or specifying a naming pattern using wildcards.

Question: Copy with Multiple Filenames

For this exercise, you can test the commands in the shell-lesson-data/data directory.

In the example below, what does cp do when given several filenames and a directory name?

Input: Bash
$ mkdir backup
$ cp amino-acids.txt animals.txt backup/

In the example below, what does cp do when given three or more file names?

Input: Bash
$ ls -F
amino-acids.txt  animals.txt  backup/  elements/  morse.txt  pdb/
planets.txt  salmon.txt  sunspot.txt
$ cp amino-acids.txt animals.txt morse.txt

If given more than one file name followed by a directory name (i.e. the destination directory must be the last argument), cp copies the files to the named directory.

If given three file names, cp throws an error such as the one below, because it is expecting a directory name as the last argument.

cp: target 'morse.txt' is not a directory
# Explore the possible solutions here!

Using wildcards for accessing multiple files at once

* is a wildcard, which matches zero or more characters. Let’s consider the shell-lesson-data/molecules directory: *.pdb matches ethane.pdb, propane.pdb, and every file that ends with ‘.pdb’. On the other hand, p*.pdb only matches pentane.pdb and propane.pdb, because the ‘p’ at the front only matches filenames that begin with the letter ‘p’.

? is also a wildcard, but it matches exactly one character. So ?ethane.pdb would match methane.pdb whereas *ethane.pdb matches both ethane.pdb, and methane.pdb.

Wildcards can be used in combination with each other e.g. ???ane.pdb matches three characters followed by ane.pdb, giving cubane.pdb ethane.pdb octane.pdb.

When the shell sees a wildcard, it expands the wildcard to create a list of matching filenames before running the command that was asked for. As an exception, if a wildcard expression does not match any file, Bash will pass the expression as an argument to the command as it is. For example, typing ls *.pdf in the molecules directory (which contains only files with names ending with .pdb) results in an error message that there is no file called *.pdf. However, generally commands like wc and ls see the lists of file names matching these expressions, but not the wildcards themselves. It is the shell, not the other programs, that deals with expanding wildcards.

Question: List filenames matching a pattern

When run in the molecules directory, which ls command(s) will produce this output?

ethane.pdb methane.pdb

  1. ls *t*ane.pdb
  2. ls *t?ne.*
  3. ls *t??ne.pdb
  4. ls ethane.*

The solution is 3.

1. shows all files whose names contain zero or more characters (*) followed by the letter t, then zero or more characters (*) followed by ane.pdb. This gives ethane.pdb methane.pdb octane.pdb pentane.pdb.

2. shows all files whose names start with zero or more characters (*) followed by the letter t, then a single character (?), then ne. followed by zero or more characters (*). This will give us octane.pdb and pentane.pdb but doesn’t match anything which ends in thane.pdb.

3. fixes the problems of option 2 by matching two characters (??) between t and ne. This is the solution.

4. only shows files starting with ethane..

# Explore the possible solutions here!
Question: More on Wildcards

Sam has a directory containing calibration data, datasets, and descriptions of the datasets:

├── 2015-10-23-calibration.txt
├── 2015-10-23-dataset1.txt
├── 2015-10-23-dataset2.txt
├── 2015-10-23-dataset_overview.txt
├── 2015-10-26-calibration.txt
├── 2015-10-26-dataset1.txt
├── 2015-10-26-dataset2.txt
├── 2015-10-26-dataset_overview.txt
├── 2015-11-23-calibration.txt
├── 2015-11-23-dataset1.txt
├── 2015-11-23-dataset2.txt
├── 2015-11-23-dataset_overview.txt
├── backup
│   ├── calibration
│   └── datasets
└── send_to_bob
    ├── all_datasets_created_on_a_23rd
    └── all_november_files

Before heading off to another field trip, she wants to back up her data and send some datasets to her colleague Bob. Sam uses the following commands to get the job done:

$ cp *dataset* backup/datasets
$ cp ____calibration____ backup/calibration
$ cp 2015-____-____ send_to_bob/all_november_files/
$ cp ____ send_to_bob/all_datasets_created_on_a_23rd/

Help Sam by filling in the blanks.

The resulting directory structure should look like this

├── 2015-10-23-calibration.txt
├── 2015-10-23-dataset1.txt
├── 2015-10-23-dataset2.txt
├── 2015-10-23-dataset_overview.txt
├── 2015-10-26-calibration.txt
├── 2015-10-26-dataset1.txt
├── 2015-10-26-dataset2.txt
├── 2015-10-26-dataset_overview.txt
├── 2015-11-23-calibration.txt
├── 2015-11-23-dataset1.txt
├── 2015-11-23-dataset2.txt
├── 2015-11-23-dataset_overview.txt
├── backup
│   ├── calibration
│   │   ├── 2015-10-23-calibration.txt
│   │   ├── 2015-10-26-calibration.txt
│   │   └── 2015-11-23-calibration.txt
│   └── datasets
│       ├── 2015-10-23-dataset1.txt
│       ├── 2015-10-23-dataset2.txt
│       ├── 2015-10-23-dataset_overview.txt
│       ├── 2015-10-26-dataset1.txt
│       ├── 2015-10-26-dataset2.txt
│       ├── 2015-10-26-dataset_overview.txt
│       ├── 2015-11-23-dataset1.txt
│       ├── 2015-11-23-dataset2.txt
│       └── 2015-11-23-dataset_overview.txt
└── send_to_bob
    ├── all_datasets_created_on_a_23rd
    │   ├── 2015-10-23-dataset1.txt
    │   ├── 2015-10-23-dataset2.txt
    │   ├── 2015-10-23-dataset_overview.txt
    │   ├── 2015-11-23-dataset1.txt
    │   ├── 2015-11-23-dataset2.txt
    │   └── 2015-11-23-dataset_overview.txt
    └── all_november_files
        ├── 2015-11-23-calibration.txt
        ├── 2015-11-23-dataset1.txt
        ├── 2015-11-23-dataset2.txt
        └── 2015-11-23-dataset_overview.txt
cp *calibration.txt backup/calibration
cp 2015-11-* send_to_bob/all_november_files/
cp *-23-dataset* send_to_bob/all_datasets_created_on_a_23rd/
# Explore the possible solutions here!
Question: Organizing Directories and Files

Jamie is working on a project and she sees that her files aren’t very well organized:

$ ls -F
analyzed/  fructose.dat    raw/   sucrose.dat

The fructose.dat and sucrose.dat files contain output from her data analysis. What command(s) covered in this lesson does she need to run so that the commands below will produce the output shown?

$ ls -F
analyzed/   raw/
$ ls analyzed
fructose.dat    sucrose.dat
mv *.dat analyzed

Jamie needs to move her files fructose.dat and sucrose.dat to the analyzed directory. The shell will expand *.dat to match all .dat files in the current directory. The mv command then moves the list of .dat files to the ‘analyzed’ directory.

# Explore the possible solutions here!
Question: Reproduce a folder structure

You’re starting a new experiment and would like to duplicate the directory structure from your previous experiment so you can add new data.

Assume that the previous experiment is in a folder called ‘2016-05-18’, which contains a data folder that in turn contains folders named raw and processed that contain data files. The goal is to copy the folder structure of the 2016-05-18-data folder into a folder called 2016-05-20 so that your final directory structure looks like this:

└── data
   ├── processed
   └── raw

Which of the following set of commands would achieve this objective? What would the other commands do?

Input: Option 1
$ mkdir 2016-05-20
$ mkdir 2016-05-20/data
$ mkdir 2016-05-20/data/processed
$ mkdir 2016-05-20/data/raw
Input: Option 2
$ mkdir 2016-05-20
$ cd 2016-05-20/
$ mkdir data/
$ cd data
$ mkdir raw processed
Input: Option 3
$ mkdir 2016-05-20/data/raw
$ mkdir 2016-05-20/data/processed
Input: Option 4
$ mkdir -p 2016-05-20/data/raw
$ mkdir -p 2016-05-20/data/processed
Input: Option 5
$ mkdir 2016-05-20
$ cd 2016-05-20
$ mkdir data
$ mkdir raw processed

The first two sets of commands achieve this objective. The first set uses relative paths to create the top-level directory before the subdirectories.

The third set of commands will give an error because the default behavior of mkdir won’t create a subdirectory of a non-existent directory: the intermediate level folders must be created first.

The fourth set of commands achieve this objective. Remember, the -p option, followed by a path of one or more directories, will cause mkdir to create any intermediate subdirectories as required.

The final set of commands generates the ‘raw’ and ‘processed’ directories at the same level as the ‘data’ directory.

# Explore the possible solutions here!