[alsetmusic]

Wait, now I'm confused. I have:

mkcd() { mkdir -p "$@" && cd "$_"; }

Can someone please detail the differences between $! and $_? (Apologies if I'm missing something obvious)

[tyingq]

  $ echo "test" > /dev/null && echo !$
  /dev/null
  $ echo "test" > /dev/null && echo $_
  test

[karatinversion]

You're thinking of $_ and !$, and I think they are equivalent - $_ is explicitly the last argument of the previous command, whereas !$ is a history expansion command. ! starts a new history expansion command, and $ is a word designator for the last word of the selected history entry. To quote man bash, " If a word designator is supplied without an event specification, the previous command is used as the event."

I would not that !$ is a bash-ism, whereas $_ works at least on zsh and ksh as well as bash.