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
}

"$@"