Intro to the Shell

The Shell

Author: Tracy Teal Original contributors: Paul Wilson, Milad Fatenejad, Sasha Wood and Radhika Khetani for Software Carpentry (


  • What is the shell?
  • How do you access it?
  • How do you use it?
    • Getting around the Unix file system
    • looking at files
    • manipulating files
    • automating tasks
  • What is it good for?
  • Where are resources where I can learn more? (because the shell is awesome)

What is the shell?

The shell is a program that presents a command line interface which allows you to control your computer using commands entered with a keyboard instead of controlling graphical user interfaces (GUIs) with a mouse/keyboard combination.

There are many reasons to learn about the shell.

  • For most bioinformatics tools, you have to use the shell. There is no graphical interface. If you want to work in metagenomics or genomics you're going to need to use the shell.
  • The shell gives you power. The command line gives you the power to do your work more efficiently and more quickly. When you need to do things tens to hundreds of times, knowing how to use the shell is transformative.
  • To use remote computers or cloud computing, you need to use the shell.
  • We're going to use it in this class, for all of the reasons above.


Unix is user-friendly. It's just very selective about who its friends are. - Anonymous

Today we're going to go through how to access Unix/Linux and some of the basic shell commands.

Information on the shell

Shell cheat sheets:

Explain shell - a web site where you can see what the different components of a shell command are doing.


  • A shell use case
  • How to access the shell

An automation use case

Text Complexity in Newspapers pdf

Another newspaper case

Get the front page of the newspaper from a particular newspaper in different years

Count occurances of a particular word

How to access the shell

The shell is already available on Mac and Linux. For Windows, you'll have to download a separate program.


On Mac the shell is available through Terminal Applications -> Utilities -> Terminal Go ahead and drag the Terminal application to your Dock for easy access.


For Windows, we're going to be using gitbash. Download and install gitbash Open up the program.


Well, you should be set.

Starting with the shell

We will spend most of our time learning about the basics of the shell by manipulating some experimental data from a hearing test.

We're going to create a directory that we're going to do this work in and then download the data.

To create the directory type:

mkdir edamame

This means, make directory 'edamame'

Now we're going to go in to that directory

cd edamame

This stands for 'change directory'

Now let's look at see what's in here. There should be nothing right now, because we just made it, but let's check. Type: ls

ls stands for 'list' and it lists the contents of a directory.

Now we're going to download the data for the tutorial. For this you'll need internet access, because you're going to get it off the web. Enter the command:

git clone

This command will grab all of the data needed for this workshop from the internet. (We're not going to talk about git right now, but it's a tool for doing version control.)

Now let's check to see that we got the data. Again type: ls Now we should see a list of a lot of files, including something called data.

In there, all mixed up together are files and directories/folders. If we want to know which is which, we can type: ls -F Anything with a "/" after it is a directory. Things with a "*" after them are programs. If there's after the name, then it's a file.

You can also use the command ls -l to see whether items in a directory are files or directories. ls -l gives a lot more information too, such as the size of the file

So, we can see that we have several files, directories and a program. Great!

The Unix directory file structure (a.k.a. where am I?)

As you've already just seen, you can move around in different directories or folders at the command line. Why would you want to do this, rather than just navigating around the normal way.

When you're working with bioinformatics programs, you're working with your data and it's key to be able to have that data in the right place and make sure the program has access to the data. Many of the problems people run in to with command line bioinformatics programs is not having the data in the place the program expects it to be.

The file structure looks like this:


Moving around the file system

Let's practice moving around a bit.

We're going to work in that data directory we just downloaded.

First let's navigate there using the regular way.

First we did something like go to the folder of our username. Then we opened 'edamame', then and finally 'data'

Let's draw out how that went.

Now let's draw some of the other files and folders along that way.

This is called a hierarchical file system structure, like an upside down tree with root (/) at the base.

When you are working at your computer or log in to a remote computer, you are on one of the branches of that tree, your home directory (/home/username)

Now let's go do that same navigation at the command line.

Type cd This put's you in your home directory. This folder here.

Now using cd and ls, go in to the 'data' directory and list its contents.

Let's also check to see where we are. Sometimes when we're wandering around in the file system, it's easy to lose track of where we are and get lost.

If you want to know what directory you're currently in, type pwd This stands for 'print working directory'. The directory you're currently working in.

What if we want to move back up and out of the 'data' directory? Can we just type 'edamame'? Try it and see what happens.

To go 'back up a level' we need to use ..

Type cd ..

Now do ls and pwd. See now that we went back up in to the 'edamame' directory. .. just means go back up a level.


Now we're going to try a hunt. Move around in the 'hidden' directory and try to find the file youfoundit.txt


Most programs take additional arguments that control their exact behavior. For example, -F and -l are arguments to ls. The ls program, like many programs, take a lot of arguments. But how do we know what the options are to particular commands?

Most commonly used shell programs have a manual. You can access the manual using the man program. Try entering:

man ls

This will open the manual page for ls. Use the space key to go forward and b to go backwards. When you are done reading, just hit q to quit.

Programs that are run from the shell can get extremely complicated. To see an example, open up the manual page for the find program. No one can possibly learn all of these arguments, of course. So you will probably find yourself referring back to the manual page frequently.

Examining the contents of other directories

By default, the ls commands lists the contents of the working directory (i.e. the directory you are in). You can always find the directory you are in using the pwd command. However, you can also give ls the names of other directories to view. Navigate to the home directory if you are not already there.



Then enter the command:

ls msu_al340

This will list the contents of the msu_al340 directory without you having to navigate there.

The cd command works in a similar way. Try entering:

cd msu_al340/hidden

and you will jump directly to hidden without having to go through the intermediate directory.

Try finding the anotherfile.txt file without changing directories.

Full vs. Relative Paths

The cd command takes an argument which is the directory name. Directories can be specified using either a relative path a full path. The directories on the computer are arranged into a hierarchy. The full path tells you where a directory is in that hierarchy. Navigate to the home directory. Now, enter the pwd command and you should see:


which is the full name of your home directory. This tells you that you are in a directory called username, which sits inside a directory called home which sits inside the very top directory in the hierarchy. The very top of the hierarchy is a directory called / which is usually referred to as the root directory. So, to summarize: username is a directory in home which is a directory in /.

Now enter the following command:

cd /home/username/msu_al340/hidden

This jumps to hidden. Now go back to the home directory (cd). We saw earlier that the command:

cd msu_al340/hidden

had the same effect - it took us to the hidden directory. But, instead of specifying the full path (/home/swc/boot-camps/shell), we specified a relative path. In other words, we specified the path relative to our current directory. A full path always starts with a /. A relative path does not. You can usually use either a full path or a relative path depending on what is most convenient. If we are in the home directory, it is more convenient to just enter the relative path since it involves less typing.

Over time, it will become easier for you to keep a mental note of the structure of the directories that you are using and how to quickly navigate amongst them.

Short Exercise

Now, list the contents of the /bin directory. Do you see anything familiar in there?

Saving time with shortcuts, wild cards, and tab completion


There are some shortcuts which you should know about. Dealing with the home directory is very common. So, in the shell the tilde character, ~, is a shortcut for your home directory. Navigate to the msu_al340 directory:

cd msu_al340/hidden

Then enter the command:

ls ~

This prints the contents of your home directory, without you having to type the full path. The shortcut .. always refers to the directory above your current directory. Thus:

ls ..

prints the contents of the /home/username/msu_al340. You can chain these together, so:

ls ../../

prints the contents of /home/username which is your home directory. Finally, the special directory . always refers to your current directory. So, ls, ls ., and ls ././././. all do the same thing, they print the contents of the current directory. This may seem like a useless shortcut right now, but we'll see when it is needed in a little while.

To summarize, while you are in the shell directory, the commands ls ~, ls ~/., ls ../../, and ls /home/username all do exactly the same thing. These shortcuts are not necessary, they are provided for your convenience.

Our data set: Interview Subjects

We want to interview people about their experience in the Polar Vortex of 2014 to create a story of survival. A few different undergraduate interview people on campus and ask their date of birth, gender, Critical Internal temperature type (CI type), Volume of snow experienced, Range they had to walk and their self-reported ability to Discriminate temperature.

Each interview was written out to a text file, one set per file. Each participant has a unique subject ID, and a made-up subject name. Each interview has a unique experiment ID. The undergraduates have collected 351 files so far.

The data is a bit of a mess! There are inconsistent file names, there are extraneous "NOTES" files that we'd like to get rid of, and the data is spread across many directories. We are going to use shell commands to look at this dat and get it into shape.

Wild cards

Navigate to the ~/msu_al340/data/THOMAS directory. This directory contains our hearing test data for THOMAS. If we type ls, we will see that there are a bunch of files which are just four digit numbers. By default, ls lists all of the files in a given directory. The * character is a shortcut for "everything". Thus, if you enter ls *, you will see all of the contents of a given directory. Now try this command:

ls *1

This lists every file that ends with a 1. This command:

ls /usr/bin/*.sh

Lists every file in /usr/bin that ends in the characters .sh. And this command:

ls *4*1

lists every file in the current directory whose name contains the number 4, and ends with the number 1. There are four such files: 0241, 0341, 0431, and 0481.

So how does this actually work? Well…when the shell (bash) sees a word that contains the * character, it automatically looks for filenames that match the given pattern. In this case, it identified four such files. Then, it replaced the *4*1 with the list of files, separated by spaces. In other words, the two commands:

ls *4*1
ls 0241 0341 0431 0481

are exactly identical. The ls command cannot tell the difference between these two things.

Short Exercise

Do each of the following using a single ls command without navigating to a different directory.

  1. List all of the files in /bin that start with the letter 'c
  2. List all of the files in /bin that contain the letter 'a'
  3. List all of the files in /bin that end with the letter 'o'

BONUS: List all of the files in '/bin' that contain the letter 'a' or 'c'

Tab Completion

Navigate to the home directory. Typing out directory names can waste a lot of time. When you start typing out the name of a directory, then hit the tab key, the shell will try to fill in the rest of the directory name. For example, enter:

cd m<tab>

The shell will fill in the rest of the directory name for msu_al340. Now go to msu_al340/data/THOMAS

ls 02<tab><tab>

When you hit the first tab, nothing happens. The reason is that there are multiple directories in the home directory which start with 02. Thus, the shell does not know which one to fill in. When you hit tab again, the shell will list the possible choices.

Tab completion can also fill in the names of programs. For example, enter e<tab><tab>. You will see the name of every program that starts with an e. One of those is echo. If you enter ec<tab> you will see that tab completion works.

Command History

You can easily access previous commands. Hit the up arrow. Hit it again. You can step backwards through your command history. The down arrow takes your forwards in the command history.

^-C will cancel the command you are writing, and give you a fresh prompt.

^-R will do a reverse-search through your command history. This is very useful.

You can also review your recent commands with the history command. Just enter:


to see a numbered list of recent commands, including this just issues history command. You can reuse one of these commands directly by referring to the number of that command.

If your history looked like this:

259  ls *
260  ls /usr/bin/*.sh
261  ls *4*1

then you could repeat command #260 by simply entering:


(that's an exclamation mark).

Short Exercise

  1. Find the line number in your history for the last exercise (listing files in /bin) and reissue that command.

Which program?

Commands like ls, rm, echo, and cd are just ordinary programs on the computer. A program is just a file that you can execute. The program which tells you the location of a particular program. For example:

which ls

Will return "/bin/ls". Thus, we can see that ls is a program that sits inside of the /bin directory. Now enter:

which find

You will see that find is a program that sits inside of the /usr/bin directory.

So … when we enter a program name, like ls, and hit enter, how does the shell know where to look for that program? How does it know to run /bin/ls when we enter ls. The answer is that when we enter a program name and hit enter, there are a few standard places that the shell automatically looks. If it can't find the program in any of those places, it will print an error saying "command not found". Enter the command:

echo $PATH

This will print out the value of the PATH environment variable. More on environment variables later. Notice that a list of directories, separated by colon characters, is listed. These are the places the shell looks for programs to run. If your program is not in this list, then an error is printed. The shell ONLY checks in the places listed in the PATH environment variable.

Navigate to the shell directory and list the contents. You will notice that there is a program (executable file) called hello in this directory. Now, try to run the program by entering:


You should get an error saying that hello cannot be found. That is because the directory /home/username/msu_al340 is not in the PATH. You can run the hello program by entering:


Remember that . is a shortcut for the current working directory. This tells the shell to run the hello program which is located right here. So, you can run any program by entering the path to that program. You can run hello equally well by specifying:


Or by entering:


When there are no / characters, the shell assumes you want to look in one of the default places for the program.

Examining Files

We now know how to switch directories, run programs, and look at the contents of directories, but how do we look at the contents of files?

The easiest way to examine a file is to just print out all of the contents using the program cat. Enter the following command:

cat ex_data.txt

This prints out the contents of the ex_data.txt file. If you enter:

cat ex_data.txt ex_data.txt

It will print out the contents of ex_data.txt twice. cat just takes a list of file names and writes them out one after another (this is where the name comes from, cat is short for concatenate).

Short Exercises

  1. Print out the contents of the ~/msu_al340/dictionary.txt file. What does this file contain?

  2. Without changing directories, (you should still be in msu_al340), use one short command to print the contents of all of the files in the /home/username/msu_al340/data/THOMAS directory.

cat is a terrific program, but when the file is really big, it can be annoying to use. The program, less, is useful for this case. Enter the following command:

less ~/msu_al340/dictionary.txt

less opens the file, and lets you navigate through it. The commands are identical to the man program.

Some commands in less

key action
"space" to go forward
"b" to go backwarsd
"g" to go to the beginning
"G" to go to the end
"q" to quit

less also gives you a way of searching through files. Just hit the "/" key to begin a search. Enter the name of the word you would like to search for and hit enter. It will jump to the next location where that word is found. Try searching the dictionary.txt file for the word "cat". If you hit "/" then "enter", less will just repeat the previous search. less searches from the current location and works its way forward. If you are at the end of the file and search for the word "cat", less will not find it. You need to go to the beginning of the file and search.

Remember, the man program actually uses less internally and therefore uses the same commands, so you can search documentation using "/" as well!


Let's turn to the interviews that we began with. This data is located in the ~/msu_al340/data directory. Each subdirectory corresponds to a particular participant in the study. Navigate to the Bert subdirectory in data. There are a bunch of text files which contain experimental data results. Lets print them all:

cat au*

Now enter the following command:

cat au* > ../all_data

This tells the shell to take the output from the cat au* command and dump it into a new file called ../all_data. To verify that this worked, examine the all_data file. If all_data had already existed, we would overwritten it. So the > character tells the shell to take the output from what ever is on the left and dump it into the file on the right. The >> characters do almost the same thing, except that they will append the output to the file if it already exists.

Short Exercise

Use >>, to append the contents of all of the files whose names contain the number 4 in the directory:


to the existing all_data file. Thus, when you are done all_data should contain all of the experiment data from Bert and any experimental data file from gerdal with filenames that contain the number 4.

Creating, moving, copying, and removing

We've created a file called all_data using the redirection operator >. This file is critical - it's our interview results - so we want to make copies so that the data is backed up. Lets copy the file using the cp command. The cp command backs up the file. Navigate to the data directory and enter:

cp all_data all_data_backup

Now all_data_backup has been created as a copy of all_data. We can move files around using the command mv. Enter this command:

mv all_data_backup /tmp/

This moves all_data_backup into the directory /tmp. The directory /tmp is a special directory that all users can write to. It is a temporary place for storing files. Data stored in /tmp is automatically deleted when the computer shuts down.

The mv command is also how you rename files. Since this file is so important, let's rename it:

mv all_data all_data_IMPORTANT

Now the file name has been changed to all_data_IMPORTANT. Let's delete the backup file now:

rm /tmp/all_data_backup

The mkdir command is used to make a directory. Just enter mkdir followed by a space, then the directory name.

Short Exercise

Do the following:

  1. Rename the all_data_IMPORTANT file to all_data.
  2. Create a directory in the data directory called foo
  3. Then, copy the all_data file into foo

By default, rm, will NOT delete directories. You can tell rm to delete a directory using the -r option. Enter the following command:

rm -r foo

Count the words

The wc program (word count) counts the number of lines, words, and characters in one or more files. Make sure you are in the data directory, then enter the following command:

wc Bert/* gerdal/*4*

For each of the files indicated, wc has printed a line with three numbers. The first is the number of lines in that file. The second is the number of words. Finally, the total number of characters is indicated. The final line contains this information summed over all of the files. Thus, there were 10445 characters in total.

Remember that the Bert/* and gerdal/*4* files were merged into the all_data file. So, we should see that all_data contains the same number of characters:

wc all_data

Every character in the file takes up one byte of disk space. Thus, the size of the file in bytes should also be 10445. Let's confirm this:

ls -l all_data

Remember that ls -l prints out detailed information about a file and that the fifth column is the size of the file in bytes.

The awesome power of the Pipe

Suppose I wanted to only see the total number of character, words, and lines across the files Bert/* and gerdal/*4*. I don't want to see the individual counts, just the total. Of course, I could just do:

wc all_data

Since this file is a concatenation of the smaller files. Sure, this works, but I had to create the all_data file to do this. Thus, I have wasted a precious 10445 bytes of hard disk space. We can do this without creating a temporary file, but first I have to show you two more commands: head and tail. These commands print the first few, or last few, lines of a file, respectively. Try them out on all_data:

head all_data
tail all_data

The -n option to either of these commands can be used to print the first or last n lines of a file. To print the first/last line of the file use:

head -n 1 all_data
tail -n 1 all_data

Let's turn back to the problem of printing only the total number of lines in a set of files without creating any temporary files. To do this, we want to tell the shell to take the output of the wc Bert/* gerdal/*4* and send it into the tail -n 1 command. The | character (called pipe) is used for this purpose. Enter the following command:

wc Bert/* gerdal/Data0559 | tail -n 1

This will print only the total number of lines, characters, and words across all of these files. What is happening here? Well, tail, like many command line programs will read from the standard input when it is not given any files to operate on. In this case, it will just sit there waiting for input. That input can come from the user's keyboard or from another program. Try this:

tail -n 2

Notice that your cursor just sits there blinking. Tail is waiting for data to come in. Now type:


then CONTROL+d. You should see the lines:


printed back at you. The CONTROL+d keyboard shortcut inserts an end-of-file character. It is sort of the standard way of telling the program "I'm done entering data". The | character is replaces the data from the keyboard with data from another command. You can string all sorts of commands together using the pipe.

The philosophy behind these command line programs is that none of them really do anything all that impressive. BUT when you start chaining them together, you can do some really powerful things really efficiently. If you want to be proficient at using the shell, you must learn to become proficient with the pipe and redirection operators: |, >, >>.

A sorting example

Let's create a file with some words to sort for the next example. We want to create a file which contains the following names:


To do this, we need a program which allows us to create text files. There are many such programs, the easiest one which is installed on almost all systems is called nano. Navigate to /tmp and enter the following command:

nano toBeSorted

(If you don't have nano, you can use another text editor or vi) Now enter the four names as shown above. When you are done, press CONTROL+O to write out the file. Press enter to use the file name toBeSorted. Then press CONTROL+x to exit nano.

When you are back to the command line, enter the command:

sort toBeSorted

Notice that the names are now printed in alphabetical order.

Short Exercise

Use the echo command and the append operator, >>, to append your name to the file, then sort it and make a new file called Sorted.

Let's navigate back to ~/msu_al340/data. Enter the following command:

wc Bert/* | sort -k 3 -n

We are already familiar with what the first of these two commands does: it creates a list containing the number of characters, words, and lines in each file in the Bert directory. This list is then piped into the sort command, so that it can be sorted. Notice there are two options given to sort:

  1. -k 3: Sort based on the third column
  2. -n: Sort in numerical order as opposed to alphabetical order

Notice that the files are sorted by the number of characters.

Short Exercise

  1. Combine the wc, sort, head and tail commands so that only the wc information for the largest file is listed

Hint: To print the smallest file, use:

wc Bert/* | sort -k 3 -n | head -n 1

Printing the smallest file seems pretty useful. We don't want to type out that long command often. Let's create a simple script, a simple program, to run this command. The program will look at all of the files in the current directory and print the information about the smallest one. Let's call the script smallest. We'll use nano to create this file. Navigate to the data directory, then:

nano smallest

Then enter the following text:

wc * | sort -k 3 -n | head -n 1

Now, cd into the Bert directory and enter the command ../smallest. Notice that it says permission denied. This happens because we haven't told the shell that this is an executable file. If you do ls -l ../smallest, it will show you the permissions on the left of the listing.

Enter the following commands:

chmod a+x ../smallest

The chmod command is used to modify the permissions of a file. This particular command modifies the file ../smallest by giving all users (notice the a) permission to execute (notice the x) the file. If you enter:

ls -l ../smallest

You will see that the file name is green and the permissions have changed. Congratulations, you just created your first shell script!

Searching files

You can search the contents of a file using the command grep. The grep program is very powerful and useful especially when combined with other commands by using the pipe. Navigate to the Bert directory. Every data file in this directory has a line which says "Range". Lets list all of the ranges from the interviews that Bert conducted:

grep Range *

Short Exercise

Create an executable script called smallestrange in the data directory, that is similar to the smallest script, but prints the file containing the file with the smallest Range. Use the commands grep, sort, and tail to do this.

For Future Reference

Finding files

The find program can be used to find files based on arbitrary criteria. Navigate to the data directory and enter the following command:

find . -print

This prints the name of every file or directory, recursively, starting from the current directory. Let's exclude all of the directories:

find . -type f -print

This tells find to locate only files. Now try these commands:

find . -type f -name "*1*"
find . -type f -name "*1*" -or -name "*2*" -print
find . -type f -name "*1*" -and -name "*2*" -print

The find command can acquire a list of files and perform some operation on each file. Try this command out:

find . -type f -exec grep Volume {} \;

This command finds every file starting from .. Then it searches each file for a line which contains the word "Volume". The {} refers to the name of each file. The trailing \; is used to terminate the command. This command is slow, because it is calling a new instance of grep for each item the find returns.

A faster way to do this is to use the xargs command:

find . -type f -print | xargs grep Volume

find generates a list of all the files we are interested in, then we pipe them to xargs. xargs takes the items given to it and passes them as arguments to grep. xargs generally only creates a single instance of grep (or whatever program it is running).

Short Exercise

Navigate to the data directory. Use one find command to perform each of the operations listed below (except number 2, which does not require a find command):

  1. Find any file whose name is "NOTES" within data and delete it

  2. Create a new directory called cleaneddata

  3. Move all of the files within data to the cleaneddata directory

  4. Rename all of the files to ensure that they end in .txt (note: it is ok for the file name to end in .txt.txt

Hint: If you make a mistake and need to start over just do the following:

  1. Navigate to the shell directory

  2. Delete the data directory

  3. Enter the command: git checkout -- data You should see that the data directory has reappeared in its original state


Redo exercise 4, except rename only the files which do not already end in .txt. You will have to use the man command to figure out how to search for files which do not match a certain name.

Where can I learn more about the shell?

  • Software Carpentry tutorial - The Unix shell
  • The shell handout - Command Reference
  • man bash
  • Google - if you don't know how to do something, try Googling it. Other people have probably had the same question.
  • Learn by doing. There's no real other way to learn this than by trying it out. Write your next paper in nano (really emacs or vi), open pdfs from the command line, automate something you don't really need to automate.

What are computational resources at MSU

  • The people who run this server
  • iCER and the High Performance Computing Cluster If you're doing work with a professor, you can get a free account and storage on the HPCC. There are also people at iCER who will help get you started or answer questions.


backtick, xargs: Example find all files with certain text

alias -> rm -i

variables -> use a path example




ssh and scp

Regular Expressions


Chaining commands together