A while back, I had extracted some code out of a large file into a separate file and made some mod­i­fi­ca­tions. I wanted to check that the dif­fer­ences were minimal. Let’s say that the extracted code had been between lines 123 and 456 of large_old_­file.

diff -u <(sed -n '123,456p;457q' large_old_file) new_file

What’s happening here?

• sed -n '123,456p' is printing lines 123–456 of large_old_­file.
• The 457q tells sed to abandon the file at line 457. Otherwise, it will keep reading all the way to the end.
• The <(sed ...) is an example of process sub­sti­tu­tion. The output of the sed invocation becomes the first input of the diff command.

A similar example: Diff a Trans­formed File.

BTW, these days, I usually use delta for diffing at the command line, especially with Git.