5

I have a large file (50 GB) and I could like to count the number of occurrences of different lines in it. Normally I'd use

sort bigfile | uniq -c

but the file is large enough that sorting takes a prohibitive amount of time and memory. I could do

grep -cfx 'one possible line'

for each unique line in the file, but this would mean n passes over the file for each possible line, which (although much more memory friendly) takes even longer than the original.

Any ideas?

A related question asks about a way to find unique lines in a big file, but I'm looking for a way to count the number of instances of each -- I already know what the possible lines are.

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Charles
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  • Arguably this is a degenerate case of http://stackoverflow.com/questions/3502177/how-to-count-number-of-unique-values-of-a-field-in-a-tab-delimited-text-file; the http://stackoverflow.com/a/22250460/14122 answer is pretty much exactly what we already have, only picking out a column rather than using the whole line. – Charles Duffy Sep 02 '15 at 22:35

3 Answers3

10

Use awk

awk '{c[$0]++} END {for (line in c) print c[line], line}' bigfile.txt

This is O(n) in time, and O(unique lines) in space.

Barmar
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3

Here is a solution using jq 1.5. It is essentially the same as the awk solution, both in approach and performance characteristics, but the output is a JSON object representing the hash. (The program can be trivially modified to produce output in an alternative format.)

Invocation:

$ jq -nR 'reduce inputs as $line ({}; .[$line] += 1)' bigfile.txt

If bigfile.txt consisted of these lines:

a
a
b
a
c

then the output would be:

{
  "a": 3,
  "b": 1,
  "c": 1
}
peak
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1
#!/bin/bash
# port this logic to awk or ksh93 to make it fast

declare -A counts=( )
while IFS= read -r line; do
  counts[$line]=$(( counts[$line] + 1 )) # increment counter
done

# print results
for key in "${!counts[@]}"; do
  count=${counts[$key]}
  echo "Found $count instances of $key"
done
Charles Duffy
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