grep egrep multiple-strings

Question

Suppose I have several strings: str1 and str2 and str3.

• How to find lines that have all the strings?
• How to find lines that can have any of them?
• And how to find lines that have str1 and either of str2 and str3 [but not both?]?

Answer 1

Personally, I do this in perl rather than trying to cobble together something with grep.

For instance, for the first one:

while (<FILE>)
{
   next if ! m/pattern1/;
   next if ! m/pattern2/;
   next if ! m/pattern3/;

   print $_;
}

Answer 2

You can't reasonably do the "all" or "this plus either of those" cases because grep doesn't support lookahead. Use Perl. For the "any" case, it's egrep '(str1|str2|str3)' file.

The unreasonable way to do the "all" case is:

egrep '(str1.*str2.*str3|str3.*str1.*str2|str2.*str1.*str3|str1.*str3.*str2)' file

i.e. you build out the permutations. This is, of course, a ridiculous thing to do.

For the "this plus either of those", similarly:

egrep '(str1.*(str2|str3)|(str2|str3).*str1)' file

Answer 3

This looks like three questions. The easiest way to put these sorts of expressions together is with multiple pipes. There's no shame in that, particularly because a regular expression (using egrep) would be ungainly since you seem to imply you want order independence.

So, in order,

1. grep str1 | grep str2 | grep str3

2. egrep '(str1|str2|str3)'

3. grep str1 | egrep '(str2|str3)'

you can do the "and" form in an order independent way using egrep, but I think you'll find it easier to remember to do order independent ands using piped greps and order independent or's using regular expressions.