Help about comparison

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# 8  
Old 01-27-2014
Originally Posted by learnbash
I have two files, which have usernames ...
Originally Posted by Akshay Hegde
Try :

$ grep -v -f <(sort file2) <(sort file1)

It is a mistake to use regular expression matching when the "patterns" to be matched are not patterns but literal strings. Consider what would happen if one of the literal strings contains a regular expression metacharacter. Perhaps that's extremely unlikely with usernames, but it should be considered.

Even if regular expression matching is disabled, there is still the problem of substring matches, since there is no anchoring in effect.

To disable both regular expression matching (in favor of fixed string matching) and (assuming one username per line) to require whole-line matching to prevent substring matches:
grep -vxFf ...

There is no point in sorting when using grep. Whatever is gained (when a pattern near the head of the pattern list matches one of the first lines of data) is surrendered (when later matches consistently nearly-exhaust the pattern list) and sorting overhead is never recovered.

If you are going to sort the files, comm would be a more efficient alternative.

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