Splitting A File Based On Its Content

March 11, 2014

My previous post reminded me of a similar problem: how do you split a file based on its content? Counting might just be one of the things you want to do with each subset of the file.

$ cat sample.data
cats
cats
cats
dogs
birds
cats
dogs
dogs
birds
dogs
dogs
birds

$ cat sample.data | awk '{ print > $1 ".data" }'

$ ls
birds.data  cats.data  dogs.data  sample.data

$ wc -l *.data
       3 birds.data
       4 cats.data
       5 dogs.data
      12 sample.data
      24 total

For those not comfortable with awk:

IMPORTANT: redirection is slightly different in awk than on the shell. Like in bash, > means overwrite and » means append but in awk the file is only opened once per session (and closed automatically at the end). That’s why the files will contain all the matching lines and not only the last matched line. (details)

Without awk, I managed to make this work by extracting each unique line (k uniques) and by iterating over the whole file (n lines) to extract each subset… it was clumsy and slow: O(k × n).

This is a general technique, but I usually use it to split HTTP server log files by:

to count requests, review in Vim, and, probably, do some calculations on durations.

It’s possible to do the calculations directly in awk in one pass, but I find that splitting the file isn’t wasteful if you’re not sure exactly how you’ll treat the data – each subset file will be smaller and easier to deal with than the whole. It can be painful to run a long job on a file and find your logic faulty. It’s better to do multiple passes, each trivial.

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