The -A -B and -Cs of Grep
Given the text file things.txt:
foo
bar
baz
When I type cat things.txt | grep 'bar'
Then I see just the one line with bar and nothing else, in fact, I have no idea what comes before or after bar.
To know what comes 1 line before bar I can type cat things.txt | grep 'bar' -B 1 then I see:
foo
bar
To know what comes 1 line after bar I can type cat things.txt | grep 'bar' -A 1 then I see:
bar
baz
And to see 1 line either side of the grep result I can type cat things.txt | grep 'bar' -C 1.