shell script $$ value

Question

When I use these shell commands:

[root@linux /tmp]# a=$$
[root@linux /tmp]# echo $a
3985

where does the value 3985 come from? And why?

Answer 1

man bash

explains it.

Expands to the process ID of the shell.  In a () subshell, 
it expands to the  process ID of the current shell, not the subshell.

Answer 2

That's why some people use it to construct a filename that's only used temporarily and then destroyed, as in this script fragment.

SCRATCHFILE=/tmp/emphemeral.$$ ;
# Now have the script write to and read from the temp file
rm $SCRATCHFILE ;

As mentioned above, the $$ in the filename will be the PID of the main script.