Building Analysis Pipelines

Parts of this lesson borrow heavily from “Bioinformatics Data Skills” by Vince Buffalo. You can buy it here: And you should because it’s awesome

The python sys module

The sys module interacts with the python interpreter. This module primarily comes with certain variables (rather than functions) which are particularly useful, two of which are described below.

Passing command-line arguments sys.argv

Often, you’ll want to pass input arguments to a script. All input arguments are stored in the variable sys.argv (note that you must import the sys module!). For example, you might have a script called which performs certain calculations on a sequence data file, but each time you run the script, you might want to process a different file. One option to pass the file name as an input argument: python inputfile.fasta, where inputfile.fasta is the single command-line argument. If you’ve loaded the sys module, all command line arguments will be stored in sys.argv:

Assume the following code is included in the script

import sys
print sys.argv

From the command line you run python inputfile.fasta and get back:
['', 'inputfile.fasta']

Notice that the first entry in sys.argv is the name of the script. After this come all command line arguments! In addition, all sys.argv entries will be strings. So remember that if you want to use an input argument as a number, you must convert it to a float or integer.

Generally, you should save the input arguments to a new variable inside the script:

import sys
infile = sys.argv[1]

Error checking is often very useful here; you might have a script which requires two command line arguments. For instance, let’s say you have a script (which does something..) which takes a file name and a number as its two arguments. To ensure that this always happens, help yourself out with assertion statements:

import sys
# sys.argv must be of length 3 (script name, inputfile, number)
# print a usage statement if arguments not provided or provided incorrectly
assert( len(sys.argv) == 3 ), "Usage: python <inputfile> <number>"
infile = sys.argv[1]
number = int( sys.argv[2] ) # remember to convert from string to int, as needed!

Editing the path with sys.path

You can view everything in python’s path by printing the contents of the list sys.path. This variable will you tell which directories on your computer that the current python interpretter is able to access. To edit this path in place, for instance by adding a directory to the path, simply use .append():

import sys

## The python os and shutil modules

The os and shutil modules are useful for interacting with your computer’s operating system (typically UNIX). With these modules, you can run commands from your python script which are analogous to UNIX commands like cd and pwd.
Some examples:

Module Command Description Unix equivalent Example
os os.listdir List all items in a given directory ls os.listdir("/directory/of/interest/")
os os.remove Remove a file rm os.remove("i_hate_this_file.txt")
os os.rmdir Remove a directory rm -r os.rmdir("/i/hate/this/directory/")
os os.mkdir Create a new directory mkdir os.mkdir("/path/to/brand/new/directory/")
os os.mkdirs Create many new directories mkdir os.mkdir("/path/to/a/brand/new/directory/", "/path/to/another/brand/new/directory/")
os os.chdir Change directory where python is running cd os.chdir("/another/directory/where/i/want/to/be/")
shutil shutil.copy Copy a file cp shutil.copy("old_file.txt", "new_file.txt")
shutil shutil.move Move a file mv shutil.move("old_file.txt", "new_file.txt")

Running external commands with os

You will often want to use Python scripting to automate analyses which use external programs or softwares. You can actually call these programs directly from your python script using the function os.system(). This function takes a single argument: the command you want to run (as a string). Anything that you could type into the command line can be given to os.system!

# Create a multiple sequence alignment in MAFFT from python
import os
# Define input, output files
infile = "unaligned.fasta"
outfile = "aligned.fasta"
command = "mafft " + infile + " > " + outfile

We can also incorporate sys to provide the input/output file names as arguments!

# Create a multiple sequence alignment in MAFFT from python
import os
import sys

# Check and save input arguments
assert( len(sys.argv) == 3 ), "Usage: python <inputfile> <outputfile>"
infile = sys.argv[1]
outfile = sys.argv[2]

# Run the alignment
command = "mafft " + infile + " > " + outfile

Finally, you can check that the command has run properly by saving the output of os.system() (basically, save it to a variable). In UNIX, a returned value of 1 means an error occurred, but a returned value of 0 means everything went fine. Therefore, we want to make sure that os.system() returns a value of 0, by editing the last few lines:

command = "mafft " + infile + " > " + outfile
aligned_properly = os.system(command)
assert(aligned_properly == 0), "MAFFT didn't work!"

Python is pretty great, if you’re into that kind of thing. We just talked about how to interface with bash from python. Now lets talk about some of the amazing things you can do with bash.

Part 2: useful UNIX/Bash one-liners: pipes, sed, awk, and more!

Why is Unix the language of bioinformatics? Read about the Unix philosophy here:

Do one thing and do it Well.

Bash is great for getting a quick look at your data and answering questions easily. While you can do all of these things in python with what you’ve already learned, it’s often far easier and quicker to use the huge ecosystem of Unix tools already available and heavily optimized (albeit quirky).
We’re going to look at a lot of commands, so remember, you can always use man to look up the specifics of bash commands. Try man ls to look at all the options for ls. Pressing q will return you to your terminal.

Lets review some basic bash commands:

We can use head to quickly look at the first lines in a file

head Mus_musculus.GRCm38.75_chr1.bed

You can change the number of lines returned with the -n argument.

tail is just like head but looks at the end of a file

tail Mus_musculus.GRCm38.75_chr1.bed

history shows you your recent shell commands


and grep is a powerful way to search files.

But Stephanie will cover grep in GREAT detail later, so we’ll mostly just think of it as “find” for now.

history | grep tail

Welcome to the new world. The new world of | or “pipes”

Piping allows you to redirect the output of one unix command to be the input of another command. This is a really powerful idea and critical to effectively using Unix. We’ll come back to it.

less is a great way to inspect files.

Less actually starts a program that allows you to view text at the console. You won’t be able to edit it. To quit less just press q. space takes you to the next page, and b goes back.

less contaminated.fasta

wc outputs the number of words, lines, and characters in the supplied file, or files

The argument -l restricts this to just lines, what we usually care about.

wc Mus_musculus.GRCm38.75_chr1.bed
wc Mus_musculus.GRCm38.75_chr1.bed contaminated.fasta   

So of course, you can imagine using this to answer some questions.

cut pulls out specific columns (“fields”) from files

It uses tab as the separator by default

cut -f 2 Mus_musculus.GRCm38.75_chr1.bed | head -n 3

The -f argument specifies which column to keep

column then makes that output pretty to look at in the console

cat Mus_musculus.GRCm38.75_chr1.gtf | column -t

Only use for looking at data in the terminal. If you output column to a file the variable number of spaces is a mess for other programs to parse.

Now lets get to some Good Stuff

sort will, of course, “sort” plain text data. But its syntax can be confusing, so get familiar with the man page.

cat example.bed
sort example.bed
sort -k1,1 -k2,2n example.bed
Command Meaning Example
-b ignore leading blanks sort -b filename > filename.sorted
-r reverse sort -r filename > filename.sortedr
-k POS1 sort by field/character indicated by POS1 sort by field 2: sort -k 2 filename
sort by second character in field 2: sort -k 2.2 filename
-k POS1,POS2 sort based on the characters from POS1 to POS2 sort by characters in fields 2 and 3: sort -k 2,3 filename
sort starting with second character in field 2 up to and including field 3: sort -k 2.2,3 filename

uniq takes a file or standard input (from a pipe) and removes consecutive duplicates. It’s very simple, but you’ll use it a lot.

cat letters.txt
uniq letters.txt
sort letters.txt | uniq
sort letters.txt | uniq -c
Command Meaning Example
-c prefixes lines with the number of times they occur uniq -c filename
-d prints only repeated lines uniq -d filename
-u prints only unique lines uniq -u filename > filename.unique
-f N skips N number of lines uniq -f 30 filename

sort | uniq and sort | uniq -c are very commonly used in bioinformatics. Combined with other unix tools like grep and cut they are outrageously powerful.

grep -v "^#" Mus_musculus.GRCm38.75_chr1.gtf | cut -f3 | sort | uniq -c
grep -v "^#" Mus_musculus.GRCm38.75_chr1.gtf | cut -f3 | sort | uniq -c | sort -rn

awk is a programming language onto itself. We’ll keep it (pretty) simple, though, and show how simple awk “one-liners” can be used as integral components of analysis pipelines.

awk statements follow the syntax of pattern { action }. They can be very simple:

awk `{print $0}` example.bed

awk is designed to process columnar data (data separated by fields). $1 represents the first field, $2 the second, etc.

awk '$3 - $2 > 18' example.bed
# example using HW csv file
# print columns 2 and 3
cat WEEK_06_python5_HW.csv | awk -F, '{ print ($2,$3) }'
# print columns 2 and 3, add a `tab` between the items
cat WEEK_06_python5_HW.csv | awk -F, '{ print ($2"\t"$3) }'
# print columns 1 and 2, adding in a `tab` between, a new field "newline", and a new line at the end
cat WEEK_06_python5_HW.csv | awk -F, '{print ($1"\t"$2 "\tnewline\n")}'
# more complicated, a for loop that prints each item
cat WEEK_06_python5_HW.csv | awk -F, '{for (i=1;i<=4;i++) {print ($i)}}'
# for each line, from items 1 until 4: `for (i=1;i<=4;i++)`
# print each item: `{print $i}`

Last but not least, lets talk about sed. The Stream editor. Sed reads data from a file or standard input and can edit it a line at a time.

This is mostly based on regular expression (which Stephanie will explain further), so again just think of it as a simple Find and Replace.

head -n 3 chroms.txt
sed 's/chrom/chr/' chroms.txt | head -n 3
# replace first instance of XX with YY for each line
sed s/XX/YY/ filename > newfile
# replace all instances of XX with YY
# - `g` flag means 'global' and searches for all instances of the pattern
sed s/XX/YY/g filename > newfile
# replace all instances of XX with YY and of AA with ZZ
# - `-e` flag lets you execute multiple sed commands at once
sed -e s/XX/YY/g -e s/AA/ZZ/g filename > newfile
# keep only letter and space characters ([a-zA-Z' ']*) that come before a different type of character in each line
# - must escape all () using `\`, ie: \([regex]\)
# - must put whitespace and replacement ('\1\2') in quotes, or else it is interpreted as a separate command
sed -E s/\([a-zA-Z' ']*\)\(.*\)/'\1'/ example
sed -E s/\([0-9]\)/'\1\
'/ examplefile.txt

Reminder: Unix and pipes are awesome. There are computationally efficient and easy to debug. They deserve to be a part of your workflow. But what about when need to do something more? A lot of things to a lot of files… lets talk about scripting in bash.

Part 3: Bash Scripts for Pipelines

Calling all of your shell scripts from Python isn’t really efficient. There’s a fair bit of computational overhead when you ‘call’ the shell from within python. If you have a python analysis script you want to run on a lot of files, it’s better to write a script in bash than to wrap it all in python.

A bash script is really just a list of commands. By convention bash scripts end in .sh and can be run by typing:


Okay take a look at head my_first_script. How do you want to take a look? What do you see? A similar shebang to a python script, and then some parameters. After the parameters, each line is a separate bash command.

Now lets look at using command line arguments in bash. Try running the second script.

#you can also run scripts like this, but you have to set them to be executable
chmod u+x
./ firstarg secondarg thirdarg

So command line arguments are pretty easy. What else is nice about bash scripts?

sh 15

Now take a look at that script. You can do conditionals and for loops in bash! You can also do while and until loops, and complex conditional statements. The syntax is weird, though (as in -ne instead of python’s !=), so pay attention.

Okay lets get a little bit more real. Lets say you have a pipeline of 10 commands you need to run on a bunch of files. Instead of going command by command and file by file, you can wrap all the commands into a single bash script and call that for each file. Look at this:

sh contaminated.fastq
sh chroms.txt

You can of course write new files inside bash scripts using > or >> and even clean up intermediate files with rm.

Why do we care? It makes it much easier to make your analysis reproducible and consistent- by using scripts and pipelines, you can ensure you’re treating your data in the same way each time. And, mostly importantly: It’s easier!

There are many POWERFUL ways to expand your bash scripting- globbing, find, xargs, file arrays, and more! Play around with it. You’ll be glad you did!