From the bash manpage:<p><pre><code>    If the histappend shell option  is  enabled (see  the description of shopt
    under SHELL BUILTIN COMMANDS below), the lines are appended to the history
    file, otherwise the history  file  is overwritten.


    histappend
        If  set,  the history list is appended to the file named
        by the value of the HISTFILE  variable  when  the  shell
        exits, rather than overwriting the file.</code></pre>