While going through:info coreutils 'dd invocation'I came across:dd if=/dev/zero of=/dev/null count=10MB & pid=$!What is $! used for?
$! is used for?
shell
If you're talking about Bash they're in the Special Parameters section of the Bash man page.!      Expands to the process ID of the most recently executed background        (asynchronous) command.Example$ sleep 10 &[1] 22257$ echo $!22257Your commandSo with this command:$ dd if=/dev/zero of=/dev/null count=10MB & pid=$!The dd command is backgrounded, and the resulting process ID ($!) is stored in a variable pid for use there after.ReferencesBash Beginners Guide - 3.2.5. Special parameters