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Approximate String Matching

Submitted by on August 18, 2021 – 11:15 am

On occasion I find myself searching for something in log files or in my Bash history, but I can’t quite remember what it is that I am looking for. Come to think of it, this happens a lot.

I usually get some of the keywords wrong and those I do get right may be out of order. In other words, grep just doesn’t cut it. There are a couple of excellent command-line utilities available for just this sort of task – fzf and agrep – that specialize in approximate string matching.

The fzf – or “fuzzy find” – is geared more toward interactive use. The most common way sysadmins use this utility is by running history|fzf. This allows for very flexible searching of your shell history for commands long forgotten.

But fzf also offers some non-interactive functionality that can come in handy in scripts. In the example below, we will download Cervantes’ “Don Quixote” and then will try to find the oft-quoted line about battle wounds and honor. I can’t quite remember how it went, so this would be a good test for fzf.

First, we will download the book to a temp file, while doing some basic text formatting, such as converting text to Unix format and joining some of the sentences to reduce the number of line breaks.

curl -s0 -k > "${f}"
cat -v "${f}" | \
sed 's/\^M$//g' | \
sed -r '/[[:alnum:]]$/N;s/\n/ /' | \
sponge "${f}"

And now we can try to find the relevant line:

cat $f | fzf --filter 'battle wounds honor' | \
grep -i 'wounds' | head -1 | grep --color -Ei "wounds|$"

# To which Don Quixote replied, "Wounds received in battle confer honour 
# instead of taking it away; and so, friend Panza, say no more, but, as I

This is awesome. And in case the first hit found by fzf is not what you were looking form just change head -1 to, say, head -10 and look through the matching lines.

Now, agrep is quite a bit less flexible (not to mention slower), but it can also work:

agrep -i -k -E 5 'battle wounds honour' $f

# To which Don Quixote replied, "Wounds received in battle confer honour 
# instead of taking it away; and so, friend Panza, say no more, but, as I

Note that I cheated a little by using the British English spelling of “honor”. Unlike fzf, agrep is not good with spelling discrepancies.

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