- `.!` executes the filter command on the current line.
- `.!` executes the filter command on the current line.
- `!tr '[:lower:]' '[:upper:]'` calls the `tr` command to replace all lowercase characters with uppercase ones.
- `tr '[:lower:]' '[:upper:]'` calls the `tr` command to replace all lowercase characters with uppercase ones.
It is imperative to pass a range to run the external command as a filter. If you try running the command above without the `.` (`:!tr '[:lower:]' '[:upper:]'`), you will see an error.
It is imperative to pass a range to run the external command as a filter. If you try running the command above without the `.` (`:!tr '[:lower:]' '[:upper:]'`), you will see an error.