Inverse grep using a pattern file doesn't show non-matches

Question

I have two nearly identical files, "foo" and "bar".

Each file has thousands of lines, one number each line.

If I sort and then diff both files, this is the result:

$ diff foo bar
5984a5985,5986
> 15676
> 15677
7703,7706d7704
< 17486
< 17487
< 17488
< 17489

However, if I grep -Fvf both files, I get zero "non-matches" in both directions:

$ grep -Fvf foo bar
$ grep -Fvf bar foo
$

Why is that? Shouldn’t I be getting results similar to diff? I’d expect the first command to return me the "first half" of the values on diff, and the second grep command to return me the rest.

Why am I not getting the expected results?

bash command line grep linux

John1024 · Answer

For diff, unlike grep -Fvf, the order that lines appear matters. Also, since the -x option was not included in the grep command, lines with mere partial matches are excluded from the output. Reproducible example Consider these two files: $ cat foo 156 174 15676 17486 17487 $ cat bar 15677 15678 156 174 15676 The diff command shows they are different while the grep commands produce no output: $ diff foo bar 0a1,2 > 15677 > 15678 4,5d5 < 17486 < 17487 $ grep -Fvf foo bar; grep -Fvf bar foo If we were to add -x to require full-line matches, then grep produces: $ grep -Fvxf foo bar 15677 15678 $ grep -Fvxf bar foo 17486 17487 Additional suggestion As abligh suggests in the comments, diff -u produces more context which can make the output easier to understand.

Inverse grep using a pattern file doesn't show non-matches

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Reproducible example

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