man diff says:

diff - compare files line by line

So diff will compare the files. However, eventually, it will show the difference, if there is any. How can I compare the files and show the matching parts?


Output format of diff can be controlled , so for you case , you must 3 options :

diff --old-group-format='' --new-group-format=''  \
     --unchanged-group-format='%=' a.txt  b.txt 

the man diff is not useful better read info diff

or use comm

  • Aaah, I always forget about info for GNU tools – finefoot Nov 5 '19 at 23:47

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