What does wait $! mean

Question

context of the command is

su postgres sh -c "$POSTGRES -D $DATADIR -c config_file=$CONF" &
wait $!

not recorded in the manual

man wait

NAME
   wait, waitpid, waitid - wait for process to change state
......

documentation

and http://www.gnu.org/software/bash/manual/bashref.html#index-wait which is refered from wikipedia

Answer 1

From man bash:

($!) Expands to the process ID of the job most recently placed into the background, whether executed as an asynchronous command or using the bg builtin