$_ - Does some things at shell startup, but most useful because it expands to
the last argument to the previous simple command.
i[akraker@localhost wiki]$ ls -lah ~/bash
total 4.0K
drwxrwxr-x. 1 akraker akraker 36 Sep 18 06:44 .
drwx------. 1 akraker akraker 1.4K Sep 18 06:40 ..
-rwxrwxr-x. 1 akraker akraker 169 Sep 18 06:44 countparameters.sh
i[akraker@localhost wiki]$ echo $_
/home/akraker/bash
Note ~ expansion occurred before _ variable was set.