Grep is a really powerful tool for finding things in files. I often use it to scan for plugins in the Drupal codebase or to scan through a CSV or log file for data.
For example, to scan for user centric ViewsFilter plugins in the Drupal core directory use this command (assuming you are relative to the core directory).
grep "@ViewsFilter(\"user" -r core
The -r flag here recursively scans the 'core' directory. This command returns the following output.