<code>!</code>
     <dd>Start a history substitution, except when followed by a space, tab,
the end of the line, or <code>=</code>.

     <br><dt><code>!</code><var>n</var><code></code>
     <dd>Refer to command line <var>n</var>.

     <br><dt><code>!-</code><var>n</var><code></code>
     <dd>Refer to the command <var>n</var> lines back.

     <br><dt><code>!!</code>
     <dd>Refer to the previous command.  This is a synonym for <code>!-1</code>.

     <br><dt><code>!</code><var>string</var><code></code>
 