My Best Awk Tricks

June 29, 2016

This is a wrap-up of my AWK tutorial series.
You can start with why learn awk.
Or you can just straight to part 1 of the tutorial.

If you’ve read the tutorial, the amount of magic should be down to a minimum.

Disclaimers

Before you ask: I have a cheatsheet and that’s where I keep the recipes that follow. I’m just human – I copy and paste what I need.

I’m not the author of these recipes, I only collected them over time.

Allow me to skip the cat FILE | or awk 'YOUR SCRIPT' FILE parts. By now, I trust you to figure that out.

Which specific columns make sense for your specific needs will depend on you. I might use $0 or $1, but you’ll have to fix those. That’s what I do after I paste.

uniq without sort

I posted about this before, but it’s still my favorite:

$ awk '!seen[$0]++'

The action is print, of course. The condition is true the first time a string is put into the array. It follows that subsequent appearances won’t be printed.

Related to the above, print duplicates (without sort):

$ awk '++seen[$0] == 2'

Print the 2nd time you see a string.

Group counts or sums

This was covered in the tutorial, but it’s damn useful:

$ awk '{ groups[$0]++ }     END { for (k in groups) print groups[k], k }' # count
$ awk '{ groups[$1] += $2 } END { for (k in groups) print groups[k], k }' # sum

Accumulate in an array, report at the END. In both cases, pay attention to the columns you use.

Set operations: union, intersection, difference

If you have multiple files, and you consider their content as sets, you can generate a bunch of interesting subsets.

Union

cat all the files and use the “uniq without sort” recipe from above :-)

Intersection

For both intersection and difference, you need to accumulate from one file and process the other file.

$ awk 'NR == FNR {lut[$0] = 1; next} $0 in lut {print}' FILE1 FILE2

The main trick is to realize that NR and FNR will, by definition, only be equal during the processing of the first file. The next statement ensures the rest of the one-liner is skipped. We load the lut (LookUp Table) array with the relevant parts from the first file.

Why use $0 in lut instead of lut[$0] for the condition? That’s an optimization I learned the hard way: even a miss lookup in lut[$0] will instantiate the array location to an empty string – and over the processing of HUGE files, you’ll eventually consume a lot of memory.

It takes a LOT for this problem to be a problem with the amount of memory that computers have nowadays … that’s why I didn’t cover the in operator in the tutorial.

Difference

$ awk 'NR == FNR {lut[$0] = 1; next} !($0 in lut) {print}' FILE1 FILE2

This operation isn’t symmetrical: you’re removing the entries from FILE1 from FILE2. Switch the files around to get the other set difference.

Easy performance

If your AWK script isn’t fast enough, it might be time to consider whether AWK is the right tool for the job. How many GB of data are you piping through it?!

That being said, I know 2 tricks to speed up AWK:

Drop unicode support

$ LC_ALL=C awk 'YOUR SCRIPT'

The LC_ALL variable forced to C will drop unicode support and, sometimes, greatly speed up processing.

Use mawk

There are many variants of AWK, and the one you’re using is probably GNU AWK.
There are others: mawk is one of the FAST one.

These are all good questions. In all likehood:

If you’re hitting the performance wall, giving mawk a chance might be worth it.

Discuss on Twitter