Awk Tutorial, part 3

June 16, 2016

I already mentioned why you should learn AWK.
In part 1, we laid a solid foundation.
In part 2, we covered most of what you would ever need.
Let’s cover what’s left.

Input Separators

How does AWK decide what a “column” is and isn’t?

AWK “trims” the line and separates on adjacent whitespace: \s+ as a regex. That’s usually what you want, but you can specify what you need with the -F option:

$ cat netflix.csv | awk -F, '{print $5}'     # split columns on commas
Close
98.360001
99.589996
99.839996
101.059998
101.120003
# snip

(source file: netflix.csv)

In reality, the -F option takes a regular expression:

$ cat netflix.csv | awk -F'[,-]' '{print $3, "--", $0}'
High -- Date,Open,High,Low,Close,Volume,Adj Close
24 -- 2016-03-24,98.639999,98.849998,97.07,98.360001,10646900,98.360001
23 -- 2016-03-23,99.75,100.389999,98.809998,99.589996,8292300,99.589996
22 -- 2016-03-22,100.480003,101.519997,99.199997,99.839996,9039500,99.839996
21 -- 2016-03-21,101.150002,102.099998,99.50,101.059998,9562900,101.059998
18 -- 2016-03-18,100.50,102.410004,100.010002,101.120003,15437300,101.120003
# snip

It separated on commas or hyphens, and picked the 3rd column (the day). This can be a useful approach to extract subfields.

Output Separators

When you use a comma (,) in a print statement, that means “space”, right?
By default, it does. But that’s configurable:

$ cat netflix.tsv | awk '{print $1, $6}'                    # default: space
Date Volume
2016-03-24 10646900
2016-03-23 8292300
2016-03-22 9039500
2016-03-21 9562900
2016-03-18 15437300
# snip

$ cat netflix.tsv | awk 'BEGIN {OFS=","} {print $1, $6}'    # custom: comma
Date,Volume
2016-03-24,10646900
2016-03-23,8292300
2016-03-22,9039500
2016-03-21,9562900
2016-03-18,15437300

The OFS (Output Field Separator) variable controls what goes between each field. I don’t use it very much; I usually format the output explicitly. But it’s sometimes useful and it’s good to know.

Passing in Variables

As in the previous example, you could decide to initialize variables in the BEGIN block. But that’s not always possible – the BEGIN block lives inside the single-quotes which severely limits what you can do.

What if you want to pass in variables, maybe from a shell script?

$ cat netflix.tsv | awk -v col=6 '{print $col}'    # instead of hardcoding $6
Volume
10646900
8292300
9039500
9562900
# snip

The -v (mnemonic “var”) option allows you to set variable from outside the script, a convenient place where shell variables and substitutions are available. We revisit the OFS variable from above, in the way I would prefer to set it:

$ cat netflix.tsv | awk -v OFS=, '{print $1, $6}'
Date,Volume
2016-03-24,10646900
2016-03-23,8292300
2016-03-22,9039500
2016-03-21,9562900
2016-03-18,15437300
# snip

Arrays

Imagine a programming language without arrays or dictionaries. That’s how we’ve been using AWK up until now. But everything truly clever you can do with AWK (or any programming language) probably requires arrays.

AWK arrays are very similar to JavaScript arrays: they can serve both as “regular” arrays (with number keys) or as dictionaries (with string keys).

How would we SUM(volume) GROUP BY year?

# warning -- scroll right...
$ cat netflix.csv | awk -F'[,-]' '{volume[$1] += $8} END { for(year in volume) print year, volume[year]}'
2009 2904000400
Date 0
2010 7126840000
2002 782530000
2011 11185849500
2003 4256021000
# snip

When working with array, this pattern of “accumulation” and “reporting” at the END is commonplace. But there are some problems with the output, they highlight interesting points:

Longer Scripts

I’ve been selling AWK as a language optimized for one-liners, but it’s possible to reach unpleasant extremes. The last example wasn’t too complicated, but it was long. It could be made more readable.

It’s possible to package multiple lines of AWK in a bash script:

#!/bin/bash

cat "$@" | awk -F'[,-]' '

{volume[$1] += $8}

END {
  for(year in volume) {
    print year, volume[year]
  }
}

'

Now you can combine the best of bash scripting, with the power of AWK – all in a very portable package.

Taking inventory: what can you do?

Everything I can.

Specifically, you have:

Exercises

Try to:

Answers are here.

What’s next?

As a conclusion: my best AWK tricks.

At this point, I hope they won’t be a list of opaque incantations.
You will be able to see what and how it’s done.

Discussion, links, and tweets

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