Add a demo of background status.

tests/background-status.sh

@@ -0,0 +1,60 @@
+#!/bin/bash
+#
+# Usage:
+#   ./background-status.sh <function name>
+
+set -o nounset
+set -o pipefail
+#set -o errexit  # wait waill fail with this
+
+do-some-work() {
+  sleep 0.2
+  exit $1
+}
+
+wait-pids() {
+  { sleep 0.1; exit 5; } &
+  pid1=$!
+
+  do-some-work 6 &
+  pid2=$!
+
+  { sleep 0.3; exit 7; } &
+  pid3=$!
+
+  do-some-work 8 &
+  pid4=$!
+
+  echo "Waiting for PIDs $pid1 $pid2 $pid3"
+
+  wait $pid1; echo $?
+  wait $pid2; echo $?
+  wait $pid3; echo $?
+  wait $pid4; echo $?
+
+  echo 'Done'
+}
+
+wait-next() {
+  { sleep 0.1; exit 8; } &
+  pid1=$!
+
+  { sleep 0.2; exit 9; } &
+  pid2=$!
+
+  { sleep 0.3; exit 10; } &
+  pid3=$!
+
+  wait -n; echo $?
+  wait -n; echo $?
+  wait -n; echo $?
+
+  echo 'Done'
+}
+
+both() {
+  wait-pids
+  wait-next
+}
+
+"$@"