How can the last command's wall time be put in the Bash prompt?

Question

Is there a way to embed the last command\'s elapsed wall time in a Bash prompt? I\'m hoping for something that would look like this:

[last: 0s][/my/dir]$ sl         


        

Answer 1

I took the answer from Ville Laurikari and improved it using the time command to show sub-second accuracy:

function timer_now {
  date +%s%N
}

function timer_start {
  timer_start=${timer_start:-$(timer_now)}
}

function timer_stop {
  local delta_us=$((($(timer_now) - $timer_start) / 1000))
  local us=$((delta_us % 1000))
  local ms=$(((delta_us / 1000) % 1000))
  local s=$(((delta_us / 1000000) % 60))
  local m=$(((delta_us / 60000000) % 60))
  local h=$((delta_us / 3600000000))
  # Goal: always show around 3 digits of accuracy
  if ((h > 0)); then timer_show=${h}h${m}m
  elif ((m > 0)); then timer_show=${m}m${s}s
  elif ((s >= 10)); then timer_show=${s}.$((ms / 100))s
  elif ((s > 0)); then timer_show=${s}.$(printf %03d $ms)s
  elif ((ms >= 100)); then timer_show=${ms}ms
  elif ((ms > 0)); then timer_show=${ms}.$((us / 100))ms
  else timer_show=${us}us
  fi
  unset timer_start
}

trap 'timer_start' DEBUG
PROMPT_COMMAND=timer_stop

PS1='[last: ${timer_show}][\w]$ '

Of course this requires a process to be started, so it's less efficient, but still fast enough that you wouldn't notice.