Match two strings in one line with grep

You can use

grep 'string1' filename | grep 'string2'

Or

grep 'string1.*string2\|string2.*string1' filename

I think this is what you were looking for:

grep -E "string1|string2" filename

I think that answers like this:

grep 'string1.*string2\|string2.*string1' filename

only match the case where both are present, not one or the other or both.

To search for files containing all the words in any order anywhere:

grep -ril \'action\' | xargs grep -il \'model\' | xargs grep -il \'view_type\'

The first grep kicks off a recursive search (r), ignoring case (i) and listing (printing out) the name of the files that are matching (l) for one term ('action' with the single quotes) occurring anywhere in the file.

The subsequent greps search for the other terms, retaining case insensitivity and listing out the matching files.

The final list of files that you will get will the ones that contain these terms, in any order anywhere in the file.