How do I delete all lines from a file after (and including) a line that contains a defined string in a Bash script?

Question

I'm hacking about a text file in the middle of a Bash script (on an RPI3B+ with OSMC installed) and trying to crop a file at the first line that contains the text "BLAH DE BLAH" (deleting everything in the same file after and including the first line it finds that text on).

For example (in the file filename.text):

This is the first line

This is the second line

This is the third line containing "BLAH DE BLAH"

This is the fourth line

This is the fifth line

Required output (in the file filename.text):

This is the first line

This is the second line

I've tried to investigate awk and sed related posts, but I'm finding it all so confusing as I can't find anything that does exactly what I need (some split at certain line numbers, some from the command line not a bash script, some before and after certain strings)... and I'm stuck. As you can see, I can't even work out how to format this post properly (my head hurts so much)!

Any help appreciated - thanks!

Answer 1

Looks like

sed '/BLAH DE BLAH/Q'

would do the job in GNU sed.