Run help50 COMMAND as though COMMAND were typed directly

Students used to run `help50 make foo`. Help now arrives automatically after
any failed command, so run COMMAND with the same exit status and let the prompt
hook handle the rest: help50 make foo behaves exactly like make foo. Builtins go
through bash -c with the shell's error wording preserved; an unknown command
fails with the shell's own "command not found" message so helpers match it.
Bare help50 prints usage (exit 0) explaining the new behaviour. The subcommands
(start/stop/...) are unchanged; only they require non-root.

Author: rongxin-liu

opt/cs50/bin/help50

@@ -1,10 +1,5 @@
 #!/bin/bash
 
-# If root
-if [[ `id -u` -eq 0 ]]; then
-    exit 1
-fi
-
 function _disable() {
     touch /tmp/help50.lock
 }
@@ -80,28 +75,43 @@ function _stop() {
     kill -SIGTERM $ggppid
 }
 
+# Run a command as though typed directly. Students used to run `help50 make foo`; help now
+# arrives automatically after any failed command, so just run the command, with the same
+# exit status, and let the shell's prompt hook handle the rest.
+function _run() {
+    if [[ -n "$(type -P "$1")" ]]; then
+        exec "$@"
+    elif [[ -n "$(type -t "$1")" ]]; then # Builtin or keyword, e.g., cd
+        exec bash -c "$(printf '%q ' "$@")" 2> >(sed 's/^bash: line [0-9]*: /bash: /' >&2)
+    else # Fail the way the shell would, so that helpers recognize it
+        echo "bash: $1: command not found" >&2
+        exit 127
+    fi
+}
+
+function _usage() {
+    echo "Usage: help50 COMMAND [ARGS...]"
+    echo
+    echo "Runs COMMAND. If it fails, help appears automatically, as it now does for any"
+    echo "command you run, so you can also just run COMMAND directly."
+    echo
+    echo "Other usage: help50 [disable|enable|is-enabled|start|status|stop]"
+}
+
 # Parse argument
 case "$1" in
-    disable)
-        _disable
-        ;;
-    enable)
-        _enable
-        ;;
-    is-enabled)
-        _is-enabled
-        ;;
-    start)
-        _start
-        ;;
-    status)
-        _status
+    disable|enable|is-enabled|start|status|stop)
+
+        # Not for root
+        if [[ `id -u` -eq 0 ]]; then
+            exit 1
+        fi
+        _${1}
         ;;
-    stop)
-        _stop
+    ""|-h|--help)
+        _usage
         ;;
     *)
-        echo "Usage: $0 [disable|enable|is-enabled|start|status|stop]"
-        exit 1
+        _run "$@"
         ;;
 esac