How do I run grep multiple time parsing arguments from an input stream?

Question

Imagine that input pipe is a list of words like "foo", "bar" in CSV format.

And then there's this file or something and I'd like to test whether it contains "foo" (1) or "bar" (2) so something like:

echo "foo\nbar" | grep input.txt $0

and I'd like to get merged result of these 2 grep operations in the output:

grep "foo" input.txt
grep "bar" input.txt

Example:

input.txt = "foo1\nfoo2n\bar\n123"
output = "foo1\nfoo2n\bar\n"

I don't care how many times it runs if it does search for all the arguments.

Answer 1

You can use the -f flag that reads from a file, giving /dev/stdin as the filename, which makes it point to standard input.

printf '%s\n' "foo" "bar" | grep -f /dev/stdin input.txt 

Some implementations of Grep also allow for a hyphen - to represent standard input.

I have also used Printf (which prints "foo" and "bar" separated by newlines) because it is more portable than Echo. Bash Echo would require the -e flag to interpret \n as a newline character.