13.5. grepping for a List of Patterns
egrep (Section 13.4) lets you look for multiple patterns using its grouping and alternation operators (big words for parentheses and a vertical bar). But sometimes, even that isn't enough.
Both egrep and fgrep support a -f option, which allows you to save a list of patterns (fixed strings in the case of fgrep) in a file, one pattern per line, and search for all the items in the list with a single invocation of the program. For example, in writing this book, we've used this feature to check for consistent usage in a list of terms across all articles:
% egrep -f terms *
(To be more accurate, we used rcsegrep (Section 13.7), since the articles are all kept under RCS (Section 39.5), but you get the idea.)
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