The !$ trick works on bash, csh, tcsh and zsh, but does not work on ksh.
The !$ trick is very usefull in the Linux and Unix shells.
In the Bash shell, the !$ inserts (and expands) the last used argument (from the previous command) in the current command:
$ ls -l /etc/passwd
-rw-r--r-- 1 root root 2579 2012-07-24 23:52 /etc/passwd
$ ls -l !$
ls -l /etc/passwd
-rw-r--r-- 1 root root 2579 2012-07-24 23:52 /etc/passwd
$ ls /etc/passwd
/etc/passwd
$ ls !$
ls /etc/passwd
/etc/passwd
In Bash, to insert the last used argument in the command line, you can also use $_ :
$ ls -l /etc/shadow
-rw-r----- 1 root shadow 2559 2012-07-24 23:52 /etc/shadow
$ ls -l $_
-rw-r----- 1 root shadow 2559 2012-07-24 23:52 /etc/shadow
The bash has the Alt-. key combination binded to insert the last argument of the previous command in the current one:
$ ls /etc/shadow
/etc/shadow
$ ls [Alt-.] # in this case, Alt-. expanded is /etc/shadow
Every bash user should masterize the !$ and the !! bash tricks. To read the other bash tricks, click here.