`man bash`, search for section _Special Parameters_. For `_` you'll find: > expands to the last argument to the > previous command, _after expansion_. Emphasis is mine because the key difference in a command line context is _expansion_. For example if I have a variable `FOO` set to 'bar' and I execute `echo $FOO` then `$_` on the next line will display `bar` while `!$` will just display `$FOO` ---- Note that `$_` has different meanings in other contexts: > Also set to the full pathname used > to invoke each command executed and placed in the environment exported > to that command. ...and... > When checking mail, this parameter holds the name > of the mail file currently being checked.