bash autocompletions

docs/guide.md

@@ -21,7 +21,8 @@ Run any number of vanilla Terraria and tModLoader dedicated servers on one Linux
 15. [Permissions](#permissions)
 16. [Running by hand for debugging](#running-by-hand-for-debugging)
 17. [Troubleshooting](#troubleshooting)
-18. [Command reference](#command-reference)
+18. [Tab completion](#tab-completion)
+19. [Command reference](#command-reference)
 
 ---
 
@@ -669,6 +670,23 @@ It was generated before the mods were added. Delete the world files in `data/Wor
 
 ---
 
+## Tab completion
+
+`terractl` comes with bash completion. Press Tab to complete:
+
+- command names;
+- instance names;
+- `vanilla`/`tmodloader` for `new`;
+- common server console commands for `cmd` (`save`, `say`, `playing`, `kick`...);
+- the options of `reinstall` and `delete`;
+- `mods` actions. For `mods add` it completes `.tmod` files and folders; for `mods enable` and `mods remove`, the installed mods; for `mods disable`, the enabled ones.
+
+It works with `sudo terractl` too. It needs the `bash-completion` package, which most Debian and Ubuntu systems already have (`sudo apt install bash-completion`). It takes effect in new terminal sessions.
+
+Completion runs as *you*, even when you type `sudo`. Instance and mod names therefore only complete if you can read `/var/lib/terraria`, which means being root or a member of the `terraria` group (see [Permissions](#permissions)). Otherwise, it falls back to the names of servers systemd currently knows about, and mod names don't complete.
+
+---
+
 ## Command reference
 
 ```