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The Shell: Loops & Scripts
Bob Freeman, Mary Piper, Radhika Khetani, Meeta Mistry
Wednesday, October 28, 2015

Learning Objectives

  • Capture previous commands into a script to re-run in one single command
  • Understanding variables and storing information
  • Learn how to use variables to operate on multiple files

Now that you've been introduced to a number of commands to interrogate your data, wouldn't it be great if you could do this for each set of data that comes in, without having to manually re-type the commands?

Welcome to the beauty and purpose of shell scripts.

Shell scripts

Shell scripts are text files that contain commands we want to run. As with any file, you can give a shell script any name and usually have the extension .sh. For historical reasons, a bunch of commands saved in a file is usually called a shell script, but make no mistake, this is actually a small program.

We are going to take the commands we repeat frequently and save them into a file so that we can re-run all those operations again later by typing one single command. Let's write a shell script that will do two things:

  1. Tell us what is our current working directory
  2. Lists the contents of the directory

First let's move into the ~/dc_sample_data directory and create a new file using nano:

$ cd ~/dc_sample_data
$ nano

Type in the following lines in the file:

echo "Your current working directory is:"
echo "These are the contents of this directory:"
ls -l 

The echo command is a utility for writing to standard output. By providing text in quotations after the command we indicated what it is we wanted written

Save the file and exit nano. Now let's run the new script we have created. To run a shell script you usually use the bash or sh command.

$ sh

Did it work like you expected?

Were the echo commands helpful in letting you know what came next?

This is a very simple shell script. In this lesson and in upcoming lessons, we will be learning how to write more complex ones. You will see how the power of scripts can make our lives much easier.

Bash variables

A variable is a common concept shared by many programming languages. Variables are essentially a symbolic/temporary name for, or a reference to, some information. Variables are analogous to "buckets", where information can be stored, maintained and modified without too much hassle.

Extending the bucket analogy: the bucket has a name associated with it, i.e. the name of the variable, and when referring to the information in the bucket, we use the name of the bucket, and do not directly refer to the actual data stored in it.

In the example below, we define a variable or a 'bucket' called file. We will put a filename SRR098026.fastq as the value inside the bucket.

$ file=SRR098026.fastq

Once you press return, you should be back at the command prompt. How do we know that we actually created the bash variable? We can use the echo command to list what's inside file:

$ echo $file

What do you see in the terminal? If the variable was not created, the command will return nothing. Did you notice that when we created the variable we just typed in the variable name, but when using it as an argument to the echo command, we explicitly use a $ in front of it ($file)? Why?

Well, in the former, we're setting the value, while in the latter, we're retrieving the value. This is standard shell notation (syntax) for defining and using variables. Don't forget the $ when you want to retrieve the value of a variable!

Let's try another command using the variable that we have created. In the last lesson, we introduced the wc -l command which allows us to count the number of lines in a file. We can count the number of lines in SRR098026.fastq by referencing the file variable, but first move into the untrimmed_fastq directory:

$ cd ~/dc_sample_data/untrimmed_fastq
$ wc -l $file

Ok, so we know variables are like buckets, and so far we have seen that bucket filled with a single value. Variables can store more than just a single value. They can store multiple values and in this way can be useful to carry out many things at once. Let's create a new variable called filenames and this time we will store both of the filenames in the untrimmed_fastq directory as values.

To list all the filenames in the directory that have a .fastq extension, we know the command is:

$ ls *.fastq

Now we want to assign the output of ls to the variable. We will give that variable the name filenames:

$ filenames=`ls *.fastq`

Check and see what's stored inside our newly created variable using echo:

$ echo $filenames

Let's try the wc -l command again, but this time using our new variable filenames as the argument:

$ wc -l $filenames

What just happened? Because our variable contains multiple values, the shell runs the command on each value stored in filenames and prints the results to screen.

Try using some of the other commands we learned in previous lessons (i.e head, tail) on the filename variable.


Another powerful concept in the Unix shell and useful when writing scripts is the concept of "Loops". We have just shown you that you can run a single command on multiple files by creating a variable whose values are the filenames that you wish to work on. But what if you want to run a sequence of multiple commands, on multiple files? This is where loop come in handy!

Looping is a concept shared by several programming languages, and its implementation in bash is very similar to other languages.

The structure or the syntax of (for) loops in bash is as follows:

for (variable_name) in (list)
   (commands $variable_name) 

where the variable_name defines (or initializes) a variable that takes the value of every member of the specified list one at a time. At each iteration, the loop retrieves the value stored in the variable (which is a member of the input list) and runs through the commands indicated between the do and done one at a time. This syntax/structure is virtually set in stone.

For example, we can run the same commands (echo and wc -l) used in the "Bash variables" section but this time run them sequentially on each file:

$ ls  *.fastq       # list all files ending in .fastq

$ for var in *.fastq
   echo $var
   wc -l $var

What does this loop do?

Most simply, it writes to the terminal (echo) the name of the file and the number of lines (wc -l) for each files that end in .fastq in the current directory. The output is almost identical to what we had before.

In this case the list of files is specified using the asterisk wildcard: *.fastq, i.e. all files that end in .fastq. Then, we execute 2 commands between the do and done. With a loop, we execute these commands for each file at a time. Once the commands are executed for one file, the loop then executes the same commands on the next file in the list.

Essentially, the number of loops == the number of items in the list, in our case that is 2 times since we have 2 files in ~/dc_sample_data/untrimmed_fastq that end in .fastq. This is done by changing the value of the var variable 2 times.

Of course, var is a useless variable name. But since it doesn't matter what variable name we use, we can make it something more intuitive.

$ for filename in *.fastq
   echo $filename
   wc -l $filename

In the long run, it's best to use a name that will help point out a variable's function, so your future self will understand what you are thinking now.

Pretty simple and cool, huh?

Automating with Scripts

Now that you've learned how to use loops and variables, let's put this processing power to work. Imagine, if you will, a series of commands that would do the following for us each time we get a new data set:

  • Use the for loop to iterate over each FASTQ file
  • Dump out bad reads into a new file
  • Get the count of the number of bad reads and generate a summary file
  • And after all the FASTQ files are processed, write the summary to a log file

You might not realize it, but this is something that you now know how to do. Let's get started...

Rather than doing all of this in the terminal we are going to create a script file with all relevant commands. Use nano to create our new script file:

$ nano

We always want to start our scripts with a shebang line:


This line is the absolute path to the Bash interpreter. The shebang line ensures that the bash shell interprets the script even if it is executed using a different shell.

After the shebang line, we enter the commands we want to execute. First we want to move into our untrimmed_fastq directory:

cd ~/dc_sample_data/untrimmed_fastq

And now we loop over all the FASTQs:

for filename in *.fastq

and we execute the commands for each loop:

  # tell us what file we're working on
  echo $filename

  # grab all the bad read records into new file
  grep -B1 -A2 NNNNNNNNNN $filename > $filename-badreads.fastq

We'll also count the number of these reads and put that in a new file, using the count flag of grep:

  # grab the number of bad reads and write it to a summary file
  grep -cH NNNNNNNNNN $filename >> bad-reads.count.summary

If you've noticed, we slipped a new grep flag -H in there. This flag will report the filename along with the match string. This is useful for when we generate the summary file.

And now, as a best practice of capturing all of our work into a running summary log:

# and add this summary to our run log
cat bad-reads.count.summary >> ../runlog.txt

Save and exit nano, and voila! You now have a script you can use to assess the quality of all your new datasets. Your finished script, complete with comments, should look like the following:


# enter directory with raw FASTQs
cd ~/dc_sample_data/untrimmed_fastq

# count bad reads for each FASTQ file in our directory
for filename in *.fastq 
  # tell us what file we're working on  
  echo $filename

  # grab all the bad read records
  grep -B1 -A2 NNNNNNNNNN $filename > $filename-badreads.fastq

  # grab the number of bad reads and write it to a summary file
  grep -cH NNNNNNNNNN $filename >> bad-reads.count.summary

# and add this summary to our run log
cat bad-reads.count.summary >> ../runlog.txt

To run this script, we simply enter the following command:

$ sh

To keep your data organized, let's move all of the bad read files out of our untrimmed_fastq directory into a new directory called other.

$ mkdir ../other

$ mv *bad* ../other

This lesson has been developed by members of the teaching team at the Harvard Chan Bioinformatics Core (HBC). These are open access materials distributed under the terms of the Creative Commons Attribution license (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.