vsbuffalo / bioawk-tutorial

A Quick bioawk tutorial

There was some interest in bioawk, a useful awk fork for handling bioinformatics formats at the UC Davis Software Carpentry course, so here is a quick tutorial.

Concepts

Don't write your own FASTA/FASTQ parsers! FASTA is much easier, but code reuse is important here. FASTQ is a very hard format to parse safely and quickly. See this post for more info on how tricky it can be to parse FASTQ.

Heng Li (author of samtools, bwa) has written a nice set of parsers for different languages called readfq. bioawk is also a tool of Heng Li's too.

Installing

For this quick tutorial, let's just clone bioawk and install it to your /usr/local/bin/:

git clone git://github.com/lh3/bioawk.git && cd bioawk && make && mv awk bioawk && sudo cp bioawk /usr/local/bin/

What's awk?

awk is an old programming language that is useful for processing streams of data. Very quickly, there are three parts of each quick awk program: BEGIN, middle, and END. BEGIN is loaded before the data, the middle is run on each line, and the END is run after the lines are done processing. For example, we could do:

$ cat animals.txt
2011-04-22 21:06 Grizzly 36
2011-04-23 14:12 Elk 25
2011-04-23 10:24 Elk 26
2011-04-23 20:08 Wolverine 31
2011-04-23 18:46 Muskox 20

$ awk '{ if ($4 > 26) print $3 }' animals.txt
Grizzly
Wolverine

So awk maps these columns to column numbers, like $1 and $3. $0 is the whole line. Delimiters (field separators) can be set with -F.

Bioawk

Bioawk is just like awk, but instead of working with mapping columns to variables for you, it maps bioinformatics field formats (like FASTA/FASTQ name and sequence).

You can count sequences very effectively with bioawk, because awk updates the built-in variable NR (number of records):

bioawk -cfastx 'END{print NR}' test.fastq

But this is just the beginning; what if you wanted to use it to make a tab-delimited table of names and sequence lengths, you could do:

bioawk -cfastx '{print $name, length($seq)}' test-trimmed.fastq

Or maybe you want to see how many sequences are shorter (less than 80bp) now?

bioawk -cfastx 'BEGIN{ shorter = 0} {if (length($seq) < 80) shorter += 1} END {print "shorter sequences", shorter}' test-trimmed.fastq

bioawk can also take other input formats:

bed:
     1:chrom 2:start 3:end 4:name 5:score 6:strand 7:thickstart 8:thickend 9:rgb 10:blockcount 11:blocksizes 12:blockstarts
sam:
    1:qname 2:flag 3:rname 4:pos 5:mapq 6:cigar 7:rnext 8:pnext 9:tlen 10:seq 11:qual
vcf:
    1:chrom 2:pos 3:id 4:ref 5:alt 6:qual 7:filter 8:info
gff:
    1:seqname 2:source 3:feature 4:start 5:end 6:score 7:filter 8:strand 9:group 10:attribute
fastx:
	1:name 2:seq 3:qual 4:comment