Double dollar sign ($$) in bash
It substitutes to PID of the current running shell. For example running
ps -Alf|grep $$
generates interesting results, as shown below
$ ps -Alf|grep $$
0 S juozas 11125 11120 0 80 0 - 3391 do_wai 22:43 pts/0 00:00:00 bash
4 R juozas 11197 11125 0 80 0 - 3506 - 22:46 pts/0 00:00:00 ps -Alf
0 S juozas 11198 11125 0 80 0 - 2859 pipe_w 22:46 pts/0 00:00:00 grep --color=auto 11125
Another example is to use it in the file names where it gets substituted with the PID of the current running shell.
$ cat >f$$.txt <<EOF
This is a test. The pid of running shell is $$
EOF
$ cat f$$.txt
This is a test. The pid of running shell is 11125
$ ls f$$.txt
f11125.txt
$ LC_ALL=C rm -v f$$.txt
removed 'f11125.txt'