The question is about special variables. Documentation says:
designates the last argument of the preceding command. This may be shortened to
$_, an underscore.) At shell startup, set to the absolute pathname used to invoke the shell or shell script being executed as passed in the environment or argument list. Subsequently, expands to the last argument to the previous command after expansion. Also set to the full pathname used to invoke each command executed and placed in the environment exported to that command.
There must be some difference I cannot catch, because:
$ echo "hello" > /tmp/a.txt $ echo "!$" echo "/tmp/a.txt" /tmp/a.txt $ echo "hello" > /tmp/a.txt $ echo $_ hello
What is the difference?