How to do the opposite of diff?

Question

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how to show lines in common (reverse diff)?

Is there a command to do the opposite of diff? I want to compare two files if the same thing exists in both create a list of them. i am trying to figure out what entry's exist in both files.

Solution 1

Here is a solution that WILL NOT change the order of the lines:

fgrep -x -f file1 file2

Solution 2

Use the join command:

join a.txt b.txt

assuming the files are sorted; if not:

sort a.txt > sorted_a.txt; sort b.txt > sorted_b.txt; join sorted_a.txt sorted_b.txt

Comments

[Arthur]

This is not a reverse diff, but may be useful nonetheless. It prints the lines in file2 that exactly match some line in file1. fgrep searches for fixed string matches -f uses file1 as a list of grep search patterns -x print only lines matched entirely