Going further with Unix

In the last session, we learned the basics of Unix and how we can use it to create, move and manipulate files. In this session, we will delve further into Unix and its associated programming language, bash in order to see how we can perform some quite powerful and sophisticated operations on files. First of all, we’ll introduce some key command line tools and how you can use them in combination with one another.

Combining commands with pipes

There are a huge number of different Unix command line utilities and tools. What is especially useful with Unix is how easy it is to combine them all. To do this, we need to use a pipe, denoted by a vertical bar - i.e. |.

We will use a very simple pipe here to demonstrate the principal but throughout the course, we will use pipes in more complicated examples, always with a detailed explanation. First of all, let’s create some files.

cd ~/home/dir2
touch file1.txt file2.txt file3.txt

So in this instance, we know we created three text files. But what if we want to actually check this? Well we already learned that ls will show us the files in a directory.

ls *.txt

But this will just show the files, it won’t count them for us. Of course in such a simple example, we can actually just count them ourselves - but this will rarely be practical in a proper bioinformatics context. So let’s use a pipe to do the work for us.

ls *.txt | wc -l

We already saw what the first part of this command does - it lists the files. This list is then piped into wc which is a utility to count words and lines (i.e. word count). Last but not least, the -l flag tells wc to count lines and not words.

Pipes can make your Unix commands extremely efficient. A little later on, you will see how we can use pipes to perform all the separate components of a SNP calling pipeline with a single line of code!

Building up your Unix toolkit

There are a huge number of extremely helpful Unix command line tools. We have already encountered some - i.e. pwd, wc, ls and so on. There are many more, and sadly we won’t be able to cover them all here. Nontheless, when we introduce new tools throughout the course, we will do our best to explain them in detail.

For now though, we will introduce six tools which are really essential and that you will encounter numerous times. These are head, tail, grep, sed, cut and awk. Each of these tools could have a tutorial dedicated to them in their own right and they take some time to get used to - but it is worth it, they can really make your life a lot, lot easier when working with bioinformatic data!

git to get data

Before we introduce you to the Unix tools, we are going to explore a little trick that we will use throughout this course - cloning and pulling repositories from the course github page. This is a way for use to easily distribute files and update exercises where necessary. Luckily, it’s also extremely simple and straightforward to use. Let’s clone a repository now to get some text files that we can demonstrate the Unix command line tools with.

# move into your home directory
cd ~
# clone the appropriate repository
git clone https://github.com/speciationgenomics/unix_exercises.git

When you run the git clone command, you will essentially clone a directory of files we have prepared (and hosted) for you on Github into your own home directory. If you use ls you should now see a directory called unix_exercises.

Take a look inside. You will see three files.

head will display the ‘head’ of a file - i.e. the first 10 lines by default. This command is essential if you are going to be working with bioinformatic data as often your files are millions of lines long and you just want to have a quick peek at what it contains.

For example, we can use head on the udhr.txt file to see the first 10 lines

head udhr.txt

We can also specify exactly how many lines we want to display. For example, if we want to see 20 lines:

head -20 udhr.txt


tail is much the same as head, except it operates at the other end - i.e. it shows you the last ten lines of a file. It is also useful for skipping the start of a file.

First of all, let’s look at the last 10 lines of the udhr.txt file.

tail udhr.txt

Or the last 20?

tail -20 udhr.txt

And if you would like to skip a line, we can use tail for this too.

Let’s first just extract the first 10 lines of the declaration using head.

head udhr.txt > my_file.txt

Now if we use tail with -n flag, we can skip lines from the start of the file. For example:

tail -n+3 my_file.txt

The -n+3 argument skips the first three lines of the file. This is very useful for removing lines you are not interested in.


grep allows you to search for text data and strings. It is very useful for checking whether a pattern occurs in your data and also counting for occurences.

As an example, let’s search the text of Moby-Dick for the word “whale”.

grep --colour "whale" mobydick.txt

This will return every line of Moby-Dick where the word whale is mentioned. We used the --colour flag (N.B. --color will. work too if you want to spell it like that!) to return the results with a highlight, which makes it a bit easier to see. Feel free to compare without this flag if you’d like.

There are a couple of useful grep tricks here. We can return all the lines without the word “whale” if we want.

grep -v "whale" mobydick.txt

Here, -v just means invert the search. Alternatively, we can look for the word whale and print lines that come after each match.

grep --colour -A 2 "whale" mobydick.txt

The -A 2 flag just says, print the two lines after each match. We can do the same for the lines before the match with -B.

grep --colour -B 2 "whale" mobydick.txt

Whereas if we use -C, we can get the number of lines we want either side of a match.

grep --colour -C 3 "whale" mobydick.txt

This will return 3 lines before and after each match, which is equivalent to -B 3 -A 3.

Finally we can also use grep to count occurrences of a word. How many times do you think the word “whale” appears in Moby-Dick? You can use grep to find out…

grep -c "whale" mobydick.txt

Actually though, this is not quite accurate since searching for "whale" does not include instances where the word starts with a capital letter. We can solve this by altering our search term a little:

grep -c "[Ww]hale" mobydick.txt

Here the [Ww] simply means we are looking for any matches where the first letter is either “W” or “w”.


sed is similar to grep in that it allows you to search through text and replace it. Again this makes it very powerful for altering text data very rapidly.

Let’s extract some text from mobydick.txt to demonstrate sed. To do this, we will grep all sentences with the name “Ishmael” (the main character in the novel) on.

grep "Ishmael" mobydick.txt > ishmael.txt

Have a look at the file we created. Quite clearly, Ishmael is mentioned a lot less than the whale he is chasing… Anyway, let’s have a look at what we can do with sed. Firstly, it is possible to look at a specific line of our text:

sed -n 3p ishmael.txt

In this command, 3p is just telling the -n flag we want to see the third line. We could also extract lines 3-5 like so:

sed -n 3-5p ishmael.txt

But sed can actually do much more than this. For example, it can replace text. Let’s replace all instances of “Ishmael” with another name, like “Dave”:

sed 's/Ishmael/Dave/g' ishmael.txt

Which certainly changes the gravitas of the text. This is just a small demonstration of what it is possible to do with sed. It is a very useful tool, especially for file conversion and well worth getting more familiar with.


cut is a command-line utility which allows you to cut a specific column out of a file. This is a particularly useful command for accessing specfic parts of datafiles.

cut is probably the more straightforward of the tools here. We can use it to get some columns of the iris_data.tsv.

cut -f 1 iris_data.tsv | head

Note that we piped the output to head to make it clearer. We could also extract multiple columns:

cut -f 3,5 iris_data.tsv | head

Note that cut expects a tab-delimited file by default. So if your file is comma-separated or uses spaces, you need to use the -d flag to specify it. You can see examples (and more information on cut by using man cut to view the manual. Note that this works for most command-line tools too.


awk is not so much a command-line tool, rather a full programming language. It is extremely flexible and can actually be used to do many of the things the previous commands do too. However, it is particularly useful for in-line editing and converting file formats. We can get an idea of how it works with the iris_data.tsv.

For example, let’s use it in a similar way to cut and just print a single column of the data.

awk '{print $1}' iris_data.tsv | head

awk essentially iterates through each row of the file we provided it. So we can also get it to print additional values next to the column we extract. For example:

awk '{print $1,"\t"50}' iris_data.tsv | head

Here we added a tab space with "\t" and told awk to print 50 for each row.

We could also do something like add 1 to each value of a specific column. For example:

awk '{print $3,"\t"$3+1}' iris_data.tsv | head

Here we printed column 3 and also column 3 but with 1 added to all the values in it.

awk can do even more than this. It is definitely worth checking out a few tutorials on it. We will leave our introduction to awk and the other tools here - but we will return to them and their uses throughout our bioinformatics training.

Bash - a Unix based programming language

Unix is the operating system we are working in with the command line, but we interact with Unix using the bash programming language. Bash is not the prettiest or most straightforward of languages to use but it can be exceptionally powerful, even in simple cases.

Since it is a programming language, more often than not we will be writing bash scripts. We will deal with this shortly but beforehand, it is worth learning about some fundamental features of the bash language. These will help us build towards creating efficient and effective scripts.

Declaring variables

Variables in programming languages are just things that we refer to in the environment we are working in. One way to think of them is like naming objects in real life. Imagine you want to tell a (very literal) colleague to move a table for you. Now, imagine trying to do that without a word for table. You might be able to get them to do what you want each time by saying, “OK, turn the four-legged object with a horizontal flat surface 90 degrees and carry it over there” or something similar, but it would be a lot easier if you could just say “Move the table”.

This is basically what you are doing when declaring variables. In otherwords, you can think of a variable as short hand for something you want your code to refer to. In bash, we declare a variable like so:

EXAMPLE="Hello world"

Note that the variable doesn’t have to be allcaps but that this is the convention in bash. It is worth sticking to this convention because all command line tools have lowercase names. This makes your code much easier to read (which trust us, is extremely important when you come back to it!)

Recalling variables is also simple, we just precede our variable name with $. To print it to the screen, we must use the utility echo:


We can also declare multiple variables and combine them together:

NAME="my name is Hal"
echo ${EXAMPLE} ${NAME}

Note that we wrap the variable names here in curly brackets in order to preserve them. This is not always necessary but it does ensure your code is interpreted properly. This makes more sense if we combine them in a string, i.e. to make them a full sentence.

echo "${EXAMPLE}, ${NAME}"

Although these examples are actual words, more often than not, you will use variables to store the names of files. Using variables in your scripting is a really useful way to make your code very efficient. Doing this means you can change what an entire script does just by changing a single variable.

One last note on variables - we have actually already encountered one before this section of the tutorial - i.e. the $HOME variable. This is one of multiple environmental variables which are stored in our Unix environment. You can see all of them using the env command. It is best to not set any variables with the same names as these.

String manipulation

Now that we have learned to create variables, we can also explore how to manipulate them. This is not always straightforward in bash, but again it is really worth learning how to do this as it can make script writing much more straightforward. In most cases we will use string manipulation on filenames and paths, so we will use this as an example now.

First, let’s declare a variable. We’ll make a dummy filename in this instance:


Let’s echo this back to the screen:

echo $FILE

An important point to note here is that the $HOME variable has been interpreted so that we now have the entire file path.

Let’s say we just want the actual filename, i.e. without the directory or path? We can use basename:

basename $FILE

Alternatively, we could remove the filename and keep only the directory or path:

dirname $FILE

For now though, we want to operate on the filename itself, so let’s redeclare the variable so it is only the filename.

FILE=$(basename $FILE)
echo $FILE

Note that here we have to wrap the basename $FILE command in $() because it is an actual command.

OK so onto some proper string manipulation. Let’s remove the .txt suffix.

echo ${FILE%.*}

What did we do here? First we have to wrap the entire variable name in curly brackets - this will not work without them. The % denotes that we want to delete everything after the next character, which in this case is .* - i.e. everything after the period. Note that the following would have also worked:

echo ${FILE%.txt}

We don’t need to limit ourselves to the suffix. We could also delete everything after the last underscore. Like so:

echo ${FILE%_*}

We could also set it so that we delete everything after the first underscore:

echo ${FILE%%_*}

We can also delete from infront of the characters in our string manipulation example. For example:

echo ${FILE#*.}

This deletes everything up to and including the period character. We could also do the same with the underscores:

echo ${FILE#*_}
echo ${FILE##*_}

Where again, a single # states we want to delete only after the last occurrence and a double ## denotes we want to delete everything after the first occurrence.

You might be wondering, what exactly is the point of this? Well altering filenames is very important in most bioinformatics pipelines. So for example, with simple string manipulation you can change the suffix of a filename quickly and easily:

echo $FILE
echo ${FILE%.*}.jpg

One last point here; string manipulation in bash is not straightforward. It takes a lot of practice to get right and remember properly. We google this excellent tutorial nearly all the time!

Bash control flow

Control flow is an important part of many different programming languages. It is essentially a way of controlling how code is carried out.

Imagine you have to perform the same operation on many different files - do you want to type out a command for each and everyone of them? Of course not! This is why you might use control flow to repeat a command multiple times. There are many different types of control flow, but for now we will focus on the most common one - a for loop.

Let’s have a look at a simple example:

for i in {1..10}
echo "This is $i"

All this is is saying is that for each number between 1 and 10, echo a “This is 1”, “This is 2” and so on to the screen. do and done initiate and stop the loop respectively. Here, the variable i is used within the loop but this is completely arbitrary - you can use whatever variable you would like. Indeed, it is often much more convenient to use a variable that makes sense to you. For example:

for NUMBER in {1..10}
echo "This is $NUMBER"

It doesn’t just have to be numbers either. You can use a loop to iterate across multiple strings too. For example:

for NAME in Mario Link Luigi Peach Zelda
echo "My name is $NAME"

Of course, this is a silly example, but you could easily substitute this with filenames - making it quite clear why control flow is an essential skill for effective bash programming in bioinformatics.

Declaring arrays

Imagine we want to run a for loop on some text files. We’ll make five of them to demonstrate - and we can actually do this with a for loop too:

for i in {1..5}
touch file_${i}.txt

Use ls after running this code and you’ll see five text files.

Now, what if we want to do something simple like go through all of them and print their names to the screen? We could do it like this:

for FILE in *.txt
echo $FILE

This works really well for this simple example. However as your code becomes more advanced, it is easy for something like this to become quite dangerous. Imagine for example that in our for loop, we create a new .txt file each time? We would be in danger of creating an infinite loop, that continually prints the names of the new files it creates.

For this reason, it is best practice in bash to use arrays. These are essentially predefined lists of variables. They are easy to make too. Let’s try a simple example.

ARRAY=(Link Zelda Gannon)

Now we can try printing this to the screen:

echo $ARRAY

This only prints the first value of our array. Actually, arrays have indexes, so we can print any value we specify like so:

echo ${ARRAY[0]}
echo ${ARRAY[1]}
echo ${ARRAY[2]}

Notice that like python (and unlike R) everything in bash is zero-indexed - i.e. the first variable is zero and so on.

What if we want to print everything in the array?

echo ${ARRAY[@]}
echo ${ARRAY[*]}

Either of these will work fine.

We can also loop through the array, like so:

for CHARACTER in ${ARRAY[@]}

The purpose of the array here is that it ensures the scope of our loop is limited and that it doesn’t get carried away, operating on things it shouldn’t do.

One last point about arrays - it is often quite cumbersome to define them by hand. Imagine if you wanted to make an array for hundreds of files? Luckily you can also declare them from for loops too:

ARRAY2=($(for i in *.txt
echo $i

This doesn’t look very neat though - you can actually write a for loop like this on a single line - i.e.:

ARRAY2=($(for i in *.txt ;do echo $i; done))

Where ; indicates a separate line (as seen in the above example).

The convenience of arrays, like much of this tutorial will become much more apparent as you become more experienced in using Unix for bioinformatics.

Writing a bash script

So far we have learned a lot about bash as a programming language - but can we use it to write a program? Well actually… yes! This is extremely easy and it is exactly what we set out to do when we write a bash script.

Let’s start with a really basic example. Type nano into the command line in order to open the nano text editor.

Then we can write a simple bash script, like so:


# a simple bash script
echo "Hello world"


Save it as my_first_script.sh. You can take another look at the output with cat or less if you want to check you saved it properly.

Let’s breakdown some of the script. First of all there is this #!/bin/sh line. You don’t need to worry too much about that - it’s just good practice to ensure the script is run in the bash language. We also have another line starting with # - this is just a comment. Here it explains something about the script. Comments are really important actually and you should fill your script with them - they are a good way of letting yourself know what you have done. Again, they can be invaluable when you come back to your scripts after some time away…

Now we can actually run the script. We do that like so:

sh my_first_script.sh

You just wrote your first program! What it said is also quite relevant.

You can also write a script that is interactive. Let’s write another script to take an input from the command line.

Open nano and create a script with the following:


# a simple bash script with input
echo "My name is ${1} and my best friend is ${2}"


Save it as name_script.sh. In this script, the ${1} and ${2} variables are just specifying that this script will take the first and second arguments to the script from the command line. Let’s see it in action.

sh name_script.sh Mark Milo

Feel free to add whatever combination you want in here. You can actually try running this without the arguments too and see that it still works, just the output doesn’t make much sense.

A more serious scripting example

Now we have learned a little about how to script, let’s write one that will do something for us. We’ll create five files (again using a for loop) and then convert them all from .txt to .jpg.

Firstly, let’s make those files:

for i in {1..5}
touch file_${i}.txt

Now, we can open up nano and write out script. We would do this like so:


# a script to rename files

# declare an array
ARRAY=($(for i in file*.txt; do echo $i; done))

# loop over array
for FILE in ${ARRAY[@]}
echo "Creating ${FILE%.*}.jpg"
mv $FILE ${FILE%.*}.jpg

Now write this script out as a file_renamer.sh.

If you run this as sh file_renamer.sh - it will print the name of each file it converts to the screen. You can then use ls to see that it has indeed converted all the .txt files to .jpg.

A script writing challenge

With all the skills we have learned in this tutorial, it is now time for you to put them to the test. Return to the unix_exercises directory you created when you used git clone and write a short script to do the following:

  • make an array of the three files
  • loop through the array and count the number of lines in the file
  • print the number of lines and the name of the file to the output
Click here to see a possible solution.

```shell #!/bin/sh # a possible solution script # declare an array ARRAY=($(for i in *.t*; do echo $i; done)) # loop over array for FILE in ${ARRAY[@]} do echo "${FILE}" wc -l ${FILE} done ```