Pipe output to use as the search specification for grep on Linux

Try

grep ... | fgrep -f - file1 file2 ...

You need to use xargs's -i switch:

grep ... | xargs -ifoo grep foo file_in_which_to_search

This takes the option after -i (foo in this case) and replaces every occurrence of it in the command with the output of the first grep.

This is the same as:

grep `grep ...` file_in_which_to_search