Question

I have two text files, A and B:

A:

a start
b stop
c start
e start

B:

b
c

How can I find which lines in A are not started by lines from B using shell(bash…) command. In this case, I want to get this answer:

a start
e start

Can I implement this using a single line of command?

Answer 1

This should do:

sed '/^$/d;s/^/^/' B | grep -vf - A

The sed command will take all non-empty lines (observe the /^$/d command) from the file B and prepend a caret ^ in front of each line (so as to obtain an anchor for grep‘s regexp), and spits all this to stdout. Then grep, with the -f option (which means take all patterns from a file, which happens to be stdin here, thanks to the - symbol) and does an invert matching (thanks to the -v option) on file A. Done.