Answer by JoL for Show all lines before a match

Current solutions except schrodigerscatcuriosity's print the file contents even when there's no match. schrodigerscatcuriosity's involves using tac and so requires reading the whole input before looking for matches.

Here's another way to do it with just sed and printing only when there's a match:

sed -n '1h;1!H;/foo/{g;p;q}'

• 1h -- copy pattern space to hold space when on the first line
• 1!H -- append pattern space to hold space when not on the first line
• /foo/{...} -- on matching /foo/,
  • g -- copy hold space to pattern space
  • p -- print pattern space
  • q -- quit