How to filter out lines of a command output that occur in a text file? Let's say we have a text file of forbidden lines `forbidden.txt`. What is a short way to filter all lines of a command output that exist in the text file? cat input.txt | exclude-forbidden-lines forbidden.txt | sort

Use `grep` like this:


$ grep -v -x -F -f forbidden.txt input.txt


That long list of options to `grep` means

* `-v` Invert the sense of the match, i.e. look for lines _not_ matching.
* `-x` When matching a pattern, require that the pattern matches _the whole line_ , i.e. not just anywhere on the line.
* `-F` When matching a pattern, treat it as a _fixed string_ , i.e. not as a regular expression.
* `-f` Read patterns from the given file (`forbidden.txt`).



Then pipe that to `sort` or whatever you want to do with it.