Does $! mean something in shell scripting

问题:

Does $! exists in bash/shell script, if yes, please inform for what it is used for. And why it gives blank when echo $! is run on the command line?

回答1:

In addition to other answer, this echo

echo $!

Will print blank if you haven't yet run any process in background in current shell. If you now run:

date &
echo $!

Then it will print something like (i.e. process id of last executed background process):

47833

回答2:

$! is the PID of the last program your shell ran in the background

回答3:

$! - shows last process ID which has started in background.