There are cases that need not wait until background process finish
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In this case, zsh and bash wait 3 seconds to finish the assignment operation.
because sleep command's STDOUT is the pipe of command substitution.
< zsh >
$ AA=$( sleep 3 & date ) # wait 3 seconds to finish
< bash >
$ AA=$( sleep 3 & date ) # same as zsh
----------------------------------------------------------------------------
In this case, sleep command's STDOUT is redirected to FD 3
so there is no needs to wait 3 seconds
zsh return immediately with the value of background process PID
but bash still waits 3 seconds to finish the assignment operation.
< zsh >
$ exec 3>&1
$ PID=$(
date > >( sleep 3 >&3 ) & echo $!
)
< bash >
$ exec 3>&1
$ PID=$(
date > >( sleep 3 >&3 ) & echo $!
)
