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The Unix Shell

Overview

Teaching: 60 min
Exercises: 15 min
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?

Objectives

  • Explain how the shell relates to users’ programs.

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

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

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

  • Create a directory hierarchy that matches a given diagram.

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

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

Contents

  1. Introducing the Shell
  2. Working with files and directories
  3. Glossary of terms

Introducing the Shell

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Motivation

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Usually you move around your computer and run programs through graphical user interfaces (GUIs). For example, Finder for Mac and Explorer for Windows. These GUIs are convenient because you can use your mouse to navigate to different folders and open different files. However, there are some things you simply can’t do from these GUIs.

The Unix Shell (or the command line) allows you to do everything you would do through Finder/Explorer, and a lot more. But it’s so scary! I thought so at first, too. Since then, I’ve learned that it’s just another way to navigate your computer and run programs, and it can be super useful for your work. For instance, you can use it to combine existing tools into a pipeline to automate analyses, you can write a script to do things for you and improve reproducibility, you can interact with remote machines and supercomputers that are far away from you, and sometimes it’s the only option for the program you want to run.

We’re going to use it to:

  1. Organize our R code and plots from the R plotting lesson.
  2. Perform version control using git during the rest of the workshop.

What the Shell looks like

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When you open up the terminal for the first time, it can look pretty scary - it’s basically just a blank screen. Don’t worry - we’ll take you through how to use it step by step.

The first line of the shell shows a prompt - the shell is waiting for an input. When you’re following along in the lesson, don’t type the prompt when typing commands. To make the prompt the same for all of us, run this command:

PS1='$ '

Tree Structure

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The first thing we need to learn when using the shell is how to get around our computer. The shell folder (directory) structure is the same file structure as you’re used to. We call the way that different directories are nested the “directory tree”. You start at the root directory (/) and you can move “up” and “down” the tree. Here’s an example:

Directory Tree

Now that we understand directory trees a bit, let’s check it out from the command line. We can see where we are by using the command pwd which stands for “print working directory”, or the directory we are currently in:

pwd

/home/USERNAME/

Congrats! You just ran your first command from the command line. The output is a file path to a location (a directory) on your computer.

The output will look a little different depending on what operating system you’re using:

Let’s check to see what’s in your home directory using the ls command, which lists all of the files in your working directory:

ls

Desktop     Downloads   Movies      Pictures
Documents   Library     Music       Public

You should see some files and directories you’re familiar with such as Documents and Desktop.

If you make a typo, don’t worry. If the shell can’t find a command you type, it will show you a helpful error message.

ks

ks: command not found

This error message tells us the command we tried to run, ks, is not a command that is recognized, letting us know we might have made a mistake when typing.

Man and Help

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Often we’ll want to learn more about how to use a certain command such as ls. There are several different ways you can learn more about a specific command.

Some commands have additional information that can be found by using the -h or --help flags. Others require a special command called man to print the help manual for that command. Let’s run an experiment - try finding help with the ls command using either flags (ls --help) or the man command (man ls). What worked for you?

For Macs and Linux operating systems, we need the man command, while the bash emulator on windows uses the help flags.

On the manual page for ls, we see a section titled options. These options, also called flags, are similar to arguments in R functions, and allow us to customize how ls runs.

To get out of the man page on a Mac, click q.

Sometimes, commands will have multiple flags that we want to use at the same time. For example, ls has a flag -F that displays a slash after all directories, as well as a flag -a that includes hidden files and directories (ones that begin with a .). There are two ways to run ls using both of these flags:

ls -F -a
ls -Fa

Note that when we run the -a command, we see a . and a .. in the directory. The . corresponds to the current directory we are in and the .. corresponds to the directory directly above us in the directory tree. We’ll learn more about why this is useful in a bit.

Using the Manual Pages

Use man to open the manual for the command ls.

What flags would you use to…

  1. Print files in order of size?
  2. Print files in order of the last time they were edited?
  3. Print more information about the files?
  4. Print more information about the files with unit suffixes?
  5. Print files in order of size AND also print more information about the files?

Solution

  1. ls -S
  2. ls -t
  3. ls -l
  4. ls -lh
  5. ls -lS

Next, let’s move to our Desktop. To do this, we use cd to change directories.

Run the following command:

cd Desktop

Copy and Paste in Windows Bash Emulator

One of my biggest frustrations when I began using a bash emulator on windows is that my normal commands for paste (Ctrl + v) didn’t work! In Git Bash (our bash emulator), we can instead use Ctrl + Shift + Insert.

Let’s see if we’re in the right place:

pwd

/home/USERNAME/Desktop

We just moved down the directory tree into the Desktop directory.

What files and directories do you have on your Desktop? How can you check?

ls

list.txt
un-report
notes.pdf
Untitled.png

Your Desktop will likely look different, but the important thing is that you see the folder we worked in for the R plotting lesson. Is the un-report directory listed on your Desktop?

Finding Your Desktop on OneDrive

Many windows use OneDrive to backup their files to cloud storage (this is a great idea!). However, it can make navigating file paths a little more complicated. When you type ls in your home directory, do you see something like OneDrive - Cornell University ?

If so, we may need to explicitly navigate to the Desktop directory that lives in our Cornell OneDrive. We can do so with the following command:

cd 'OneDrive - Cornell University/Desktop'

The apostrophes are necessary, because our file path has spaces in it, which Unix will otherwise interpret as separate arguments. If you run ls, do you see the un-report folder now?

How can we get into the un-report directory?

cd un-report

We just went down the directory tree again.

Let’s see what files are in un-report:

ls

awesome_plot.jpg
awesome_violin_plot.jpg
base_R.R
co2-un-data.csv
gapminder_1997.csv
gapminder_data.csv
gdp_population.R
rnd-un-data.csv

Is it what you expect? Are the files you made in the R plotting lesson there?

Now let’s move back up the directory tree. First, let’s try this command:

cd Desktop

cd: Desktop: No such file or directory

This doesn’t work because the Desktop directory is not within the directory that we are currently in.

To move up the directory tree, you can use .., which is the parent of the current directory:

cd ..
pwd

/home/USERNAME/Desktop
/c/Users/USERNAME/Desktop
/c/Users/USERNAME/OneDrive - Cornell University/Desktop

Everything that we’ve been doing is working with file paths. We tell the computer where we want to go using cd plus the file path. We can also tell the computer what files we want to list by giving a file path to ls:

ls un-report

awesome_plot.jpg
awesome_violin_plot.jpg
base_R.R
co2-un-data.csv
gapminder_1997.csv
gapminder_data.csv
gdp_population.R
rnd-un-data.csv

ls ..

list.txt
un-report
notes.pdf
Untitled.png

What happens if you just type cd without a file path?

cd
pwd

/home/USERNAME
/c/home/USERNAME

It takes you back to your home directory!

To get back to your projects directory you can use the following commands, depending on your operating system:

cd Desktop/un-report
cd "OneDrive - Cornell University/Desktop/un-report"

We have been using relative paths, meaning you use your current working directory to get to where you want to go.

You can also use the absolute path, or the entire path from the root directory. What’s listed when you use the pwd command is the absolute path:

pwd

You can also use ~ for the path to your home directory:

cd ~
pwd

/home/USERNAME
/c/Users/USERNAME

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 ..

Solution

  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 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 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.

Working with files and directories

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Now that we know how to move around your computer using the command line, our next step is to organize the project that we started in the R plotting lesson You might ask: why would I use the command line when I could just use the GUI? My best response is that if you ever need to use a high-performance computing cluster (such as Great Lakes at the University of Michigan), you’ll have no other option. You might also come to like it more than clicking around to get places once you get comfortable, because it’s a lot faster!

First, let’s make sure we’re in the right directory (the un-reports directory):

pwd

/home/USERNAME/Desktop/un-report
/c/Users/USERNAME/Desktop/un-report
/c/Users/USERNAME/OneDrive - Cornell University/Desktop/un-report

If you’re not there, cd to the correct place.

Next, let’s remind ourselves what files are in this directory:

ls

awesome_plot.jpg
awesome_violin_plot.jpg
base_R.R
co2-un-data.csv
gapminder_1997.csv
gapminder_data.csv
gdp_population.R
rnd-un-data.csv

You can see that right now all of our files are in our main directory. However, it can start to get crazy if you have too many different files of different types all in one place! We’re going to create a better project directory structure that will help us organize our files. This is really important, particularly for larger projects. If you’re interested in learning more about structuring computational biology projects in particular, here is a useful article.

What do you think good would be a good way to organize our files?

One way is the following:

.
├── code
│   ├── base_R.R
    └── gdp_population.R
├── data
│   ├── co2-un-data.csv
    └── gapminder_1997.csv
    └── gapminder_data.csv
    └── rnd-un-data.csv
└── figures
    ├── awesome_plot.jpg
    └── awesome_violin_plot.jpg

The R script goes in the code directory, the gapminder datasets go in the data directory, and the figures go in the figures directory. This way, all of the files are organized into a clearer overall structure.

A few notes about naming files and directories:

So how do we make our directory structure look like this?

First, we need to make a new directory. Let’s start with the code directory. To do this, we use the command mkdir plus the name of the directory we want to make:

mkdir code

Now, let’s see if that directory exists now:

ls

awesome_plot.jpg
awesome_violin_plot.jpg
base_R.R
code
co2-un-data.csv
gapminder_1997.csv
gapminder_data.csv
gdp_population.R
rnd-un-data.csv

How can we check to see if there’s anything in the code directory?

ls code

Nothing in there yet, which is expected since we just made the directory.

The next step is to move the .R file into the code directory. To do this, we use the mv command. The first argument after mv is the file you want to move, and the second argument is the place you want to move it:

mv gdp_population.R code

Okay, let’s see what’s in our current directory now:

ls

awesome_plot.jpg
awesome_violin_plot.jpg
base_R.R
code
co2-un-data.csv
gapminder_1997.csv
gapminder_data.csv
rnd-un-data.csv

gdp_population.R is no longer there! Where did it go? Let’s check the code directory, where we moved it to:

ls code

gdp_population.R

There it is! Let’s also move our base_R.R file.

mv base_R.R code

Creating directories and moving files

Create a data directory and move all your csv files into the newly created data directory.

Solution

From the un-report directory:

mkdir data
mv co2-un-data.csv data
mv gapminder_data.csv data
mv gapminder_1997.csv data
mv rnd-un-data.csv data

Okay, now we have the code and data in the right place. But we have several figures that should still be in their own directory.

First, let’s make a figures directory:

mkdir figures

Next, we have to move the figures. But we have so many figures! It’d be annoying to move them one at a time. Thankfully, we can use a wildcard to move them all at once. Wildcards are used to match files and directories to patterns.

One example of a wildcard is the asterisk, *. This special character is interpreted as “multiple characters of any kind”.

Let’s see how we can use a wildcard to list only files with the extension .jpg:

ls *jpg

awesome_plot.jpg
awesome_violin_plot.jpg

See how only the files ending in .jpg were listed? The shell expands the wildcard to create a list of matching file names before running the commands. Can you guess how we move all of these files at once to the figures directory?

mv *jpg figures

We can also use the wildcard to list all of the files in all of the directories:

ls *

code:
base_R.R gdp_population.R

data:
co2-un-data.csv gapminder_1997.csv  gapminder_data.csv rnd-un-data.csv

figures:
awesome_plot.jpg    awesome_violin_plot.jpg

This output shows each directory name, followed by its contents on the next line. As you can see, all of the files are now in the right place!

Working with Wildcards

Suppose we are in a directory containing the following files:

cubane.pdb
ethane.pdb
methane.pdb
octane.pdb
pentane.pdb
propane.pdb
README.md

What would be the output of the following commands?

  1. ls *
  2. ls *.pdb
  3. ls *ethane.pdb
  4. ls *ane
  5. ls p*

Solution

  1. cubane.pdb ethane.pdb methane.pdb octane.pdb pentane.pdb propane.pdb README.md
  2. cubane.pdb ethane.pdb methane.pdb octane.pdb pentane.pdb propane.pdb
  3. ethane.pdb methane.pdb
  4. None. None of the files end in only ane. This would have listed files if ls *ane* were used instead.
  5. pentane.pdb propane.pdb

Viewing Files

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To view and navigate the contents of a file we can use the command less. This will open a full screen view of the file.

For instance, we can run the command less on our gapminder_data.csv file:

less data/gapminder_data.csv

To navigate, press spacebar to scroll to the next page and b to scroll up to the previous page. You can also use the up and down arrows to scroll line-by-line. Note that less defaults to line wrapping, meaning that any lines longer than the width of the screen will be wrapped to the next line. To exit less, press the letter q.

One particularly useful flag for less is -S which cuts off really long lines (rather than having the text wrap around):

less -S data/gapminder_data.csv

To navigate, press spacebar to scroll to the next page and b to scroll up to the previous page. You can also use the up and down arrows to scroll line-by-line. Note that less defaults to line wrapping, meaning that any lines longer than the width of the screen will be wrapped to the next line, (to disable this use the option -S when running less, ex less -S file.txt). To exit less, press the letter q.

Note that not all file types can be viewed with less. While we can open PDFs and excel spreadsheets easily with programs on our computer, less doesn’t render them well on the command line. For example, if we try to less a .pdf file we will see a warning.

less figures/awesome_plot.jpg

figures/awesome_plot.jpg may be a binary file.  See it anyway?

If we say “yes”, less will render the file but it will appear as a seemingly random display of characters that won’t make much sense to us.

Editing Files

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Beyond viewing the content of files, we may want to be able to edit or write files on the command line. There are many different text editors you can use to edit files on the command line, but we will talk about nano since it is a bit easier to learn. To edit a file with nano type nano file.txt. If the file exists, it will open the file in a nano window, if the file does not exist it will be created. One nice feature of nano is that it has a cheat sheet along the bottom with some common commands you’ll need. When you are ready to save (write) your file, you type Ctrl+O. Along the bottom will appear a prompt for the file name to write to. The current name of the file will appear here, to keep the name as it is hit enter otherwise you can change the name of the file then hit enter. To exit nano, press Ctrl+X. If you forget to save before exiting, no worries nano will prompt you to first save the file.

Since we moved around files when we organized our project directory we will have to update our R script. The path we use to read in our dataset is no longer correct. We will use nano to update the path to our new directory structure.

nano code/gdp_population.R

gapminder_data <- read_csv("data/gapminder_data.csv")

Great! Now as an exercise we can change the paths to write out figures.

Editing file paths with nano

Use nano to edit the file paths of the figures saved in code/gdp_population.R to match our new directory structure.

Solution

nano code/gdp_population.R

Edit the lines in code/gdp_population.R where plots are saved:

ggsave("figures/awesome_plot.jpg", width=6, height=4)
ggsave("figures/awesome_histogram.jpg", width=6, height=4)

Glossary of terms

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Key Points

  • A shell is a program whose primary purpose is to read commands and run other programs.

  • Tab completion can help you save a lot of time and frustration.

  • The shell’s main advantages are its support for automating repetitive tasks and its capacity to access network machines.

  • Information is stored in files, which are stored in directories (folders).

  • Directories nested in other directories for a directory tree.

  • cd [path] changes the current working directory.

  • ls [path] prints a listing of a specific file or directory.

  • ls lists the current working directory.

  • pwd prints the user’s current working directory.

  • / is the root directory of the whole file system.

  • A relative path specifies a location starting from the current location.

  • An absolute path specifies a location from the root of the file system.

  • Directory names in a path are separated with / on Unix, but \ on Windows.

  • .. means ‘the directory above the current one’; . on its own means ‘the current directory’.

  • cp [old] [new] copies a file.

  • mkdir [path] creates a new directory.

  • mv [old] [new] moves (renames) a file or directory.

  • rm [path] removes (deletes) a file.

  • * matches zero or more characters in a filename.

  • The shell does not have a trash bin — once something is deleted, it’s really gone.

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