Sort chromosome names with Linux sort

One common task that comes up regularly when analyzing genomic data is to sort a data file by chromosome names. See the following example.txt file:

chr21	Any
chr2	Data
chr3	Or
chrY	Information
chr1	Some
chr3	Annotation
chr10	Or
chrX	Other

I would like the data to be in this order:

chr1	Some
chr2	Data
chr3	Or
chr3	Annotation
chr10	Or
chr21	Any
chrX	Other
chrY	Information

i.e. Chromosome names with a number come first in numerical order, followed by chromosomes names with a letter.

Let’s use Linux sort command:

sort example.txt

Then the data is ordered as follows:

chr1	Some
chr10	Or
chr2	Data
chr21	Any
chr3	Annotation
chr3	Or
chrX	Other
chrY	Information

However, this is not what we want. We’d like the chromosome names sorted numerically when possible. That means, chr10 should come after chr3.

Then, let’s try:

sort -k 1.4,1 -n -s example.txt

The options are saying that to sort via a key (-k) starting at the 4th character in the 1st field and ending at the end of the 1st field (i.e. 1, 2, 3, 10, 21, X, Y), numerically (-n), and stably (-s).

The result, however, is not quite what we want:

chrX	Other
chrY	Information
chr1	Some
chr2	Data
chr3	Or    
chr3	Annotation
chr10	Or
chr21	Any

It correctly sorted the chromosome names with numbers, but it placed the chromosomes with letters at the beginning.

I haven’t figured out how to precede letters by numbers along this path, but it turns out another option of sort -V comes to our rescue:

sort -k1,1 -V -s example.txt

Voilà! It amazingly works:

chr1	Some
chr2	Data
chr3	Or
chr3	Annotation
chr10	Or
chr21	Any
chrX	Other
chrY	Information	

What -V means is “natural sort of (version) numbers within text” (type man sort to find out), and it magically orders numbers and texts.

A side note: the Mac OS X native sort does not support -V, you’ll have to install GNU core utilities and use gsort instead.