What does the output of the command below mean? (result is positive integer)
echo $$
As the other (accepted) answer indicated, the $ param represents the current process id (PID) of the current shell.
Echoing this param should work in most shells (not just Bash):
Bash:
$ echo $0
bash
$ echo $$
6780
Bourne:
$ echo $0
sh
$ echo $$
6988
C-Shell:
% echo $shell
/bin/csh
% echo $$
7613
It's important to note too, that commands from within the shell will run under the shell's parent process id (PPID).
$ echo $$
6780
$ ps -ef | grep 6780
aploetz 6780 6770 0 11:15 pts/2 00:00:00 bash
aploetz 8572 6780 0 11:34 pts/2 00:00:00 ps -ef
aploetz 8573 6780 0 11:34 pts/2 00:00:00 grep --color=auto 6780