grep solution

search

grep '^=.*rst.*$' *.adoc 

where

• ^ begin of line
• = equal
• .* anything any number of time
• rst string rst
• .* anything any number of time
• $ end of line

this will list files and found line

first.adoc:= The title of the first file 

print result

grep will highlight what part of line triggered the match, so

grep '^=.*rst' *.adoc 

will highlight

first.adoc:= The title of the first file

because search to end-of-line wasn't requested.

grep '^=.*rst.*$' *.adoc 

will highlight

first.adoc:= The title of the first file

because search to end-of-line was requested.

to only files use -l ( or --file-with-matches)

grep -l '^=.*rst' *.adoc 

to only list lines and not the filename use -h (or --no-filename)

see man grep