Add Get Matching Filenames As Output From Grep as a unix til

unix/get-matching-filenames-as-output-from-grep.md

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+# Get Matching Filenames As Output From Grep
+
+Standard use of the [`grep`
+command](http://man7.org/linux/man-pages/man1/grep.1.html) outputs the lines
+that match the specified pattern. You can instead output just the names of
+the files where those matches occur. To do this, include the `-l` flag.
+
+```bash
+$ grep -Rl hashrocket .
+./elixir/run-exunit-tests-in-a-deterministic-order.md
+./git/show-file-diffs-when-viewing-git-log.md
+./git/single-key-presses-in-interactive-mode.md
+./internet/enable-keyboard-shortcuts-in-gmail.md
+...
+```
+
+This recursive grep finds all the files where `hashrocket` appears. It only
+looks for the first match in a file, so each file will only be listed once
+even if there may have been multiple matches.
+
+See `man grep` for more details.