Diff head of files

diff()

I have two files. One file, I suspect, is a subset of the other. Is there a way to diff the files to identify (in a succinct manner) where in the first file the second file fits?

Best Answer

diff -e bigger smaller will do the trick, but requires some interpretation, as the output is a "valid ed script".

I made two files, "bigger" and "smaller", where the contents of "smaller" is identical to lines 5 through 9 of "bigger" doing `diff -e bigger smaller" got me:

% diff -e bigger smaller
10,15d
1,4d

Which means "delete lines 10 through 15 of 'bigger', and then delete lines 1 through 4, to get 'smaller'". That means "smaller" is lines 5 through 9 of "bigger".

Reversing the file names got me something more complicated. If "smaller" truly constitutes a subset of "bigger", only 'd' (for delete) commands will show up in the output.

Related Solutions

Getting diff (or git diff) to show inserted hunks properly

Git 2.9 was released earlier this year which included the experimental flag --compaction-heuristic on the git diff command:

In 2.9, Git's diff engine learned a new heuristic: it tries to keep hunk boundaries at blank lines, shifting the hunk "up" whenever the bottom of the hunk matches the bottom of the preceding context, until we hit a blank line.

I don't think GitHub has it enabled for diffs on the web UI for Pull Requests and comparisons, but you can do it locally. I'd recommend using it in conjunction with --word-diff if you need that level of granularity.

More details available on the GitHub blog: https://github.com/blog/2188-git-2-9-has-been-released

Diff reports the same line as different in 2 files

My guess is you simply haven't sorted the files. That's one of the behaviors you can get on unsorted input:

$ cat file1 
foo
bar
$ cat file2
bar
foo
$ $ diff file1 file2
1d0
< foo
2a2
> foo

But, if you sort:

$ diff <(sort file1) <(sort file2)
$

The diff program's job is to tell you whether two files are identical and, if not, where they differ. It is not designed to find similarities between different lines. If line X of the one file is not the same as line X of the other, then the files are not the same. It doesn't matter if they contain exactly the same information, if that information is organized in a different way, the files are reported as different.

Best Answer

Related Solutions

Getting diff (or git diff) to show inserted hunks properly

Diff reports the same line as different in 2 files

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