(including non-Latin letters), digits, and underscores.
    </para>

    <para>
    To set a variable, use the <application>psql</application> meta-command
    <command>\set</command>.  For example,
<programlisting>
testdb=&gt; <userinput>\set foo bar</userinput>
</programlisting>
    sets the variable <literal>foo</literal> to the value
    <literal>bar</literal>. To retrieve the content of the variable, precede
    the name with a colon, for example:
<programlisting>
testdb=&gt; <userinput>\echo :foo</userinput>
bar
</programlisting>
    This works in both regular SQL commands and meta-commands; there is
    more detail in <xref linkend="app-psql-interpolation"/>, below.
    </para>

    <para>
    If you call <command>\set</command> without a second argument, the
    variable is set to an empty-string value. To unset (i.e., delete)
    a variable, use the command <command>\unset</command>.  To show the
    values of all variables, call <command>\set</command> without any argument.
    </para>

    <note>
    <para>
    The arguments of <command>\set</command> are subject to the same
    substitution rules as with other commands. Thus you can construct
    interesting references such as <literal>\set :foo
    'something'</literal> and get <quote>soft links</quote> or
    <quote>variable variables</quote> of <productname>Perl</productname>
    or <productname><acronym>PHP</acronym></productname> fame,
    respectively. Unfortunately (or fortunately?), there is no way to do
    anything useful with these constructs. On the other hand,
    <literal>\set bar :foo</literal> is a perfectly valid way to copy a
    variable.
    </para>
    </note>

    <para>
    A number of these variables are treated specially
    by <application>psql</application>. They represent certain option
    settings that can be changed at run time by altering the value of
    the variable, or in some cases represent changeable state of
    <application>psql</application>.
    By convention, all specially treated variables' names
    consist of all upper-case ASCII letters (and possibly digits and
    underscores). To ensure maximum compatibility in the future, avoid
    using such variable names for your own purposes.
   </para>

   <para>
    Variables that control <application>psql</application>'s behavior
    generally cannot be unset or set to invalid values.  An <literal>\unset</literal>
    command is allowed but is interpreted as setting the variable to its
    default value.  A <literal>\set</literal> command without a second argument is
    interpreted as setting the variable to <literal>on</literal>, for control
    variables that accept that value, and is rejected for others.  Also,
    control variables that accept the values <literal>on</literal>
    and <literal>off</literal> will also accept other common spellings of Boolean
    values, such as <literal>true</literal> and <literal>false</literal>.
   </para>

   <para>
    The specially treated variables are:
   </para>

    <variablelist>
      <varlistentry id="app-psql-variables-autocommit">
      <term>
       <varname>AUTOCOMMIT</varname>
       <indexterm>
        <primary>autocommit</primary>
        <secondary>psql</secondary>
       </indexterm>
      </term>
        <listitem>
        <para>
        When <literal>on</literal> (the default), each SQL command is automatically
        committed upon successful completion.  To postpone commit in this
        mode, you must enter a <command>BEGIN</command> or <command>START
        TRANSACTION</command> SQL command.  When <literal>off</literal> or unset, SQL
        commands are not committed until you explicitly issue
        <command>COMMIT</command> or <command>END</command>.  The autocommit-off
        mode works by issuing an implicit <command>BEGIN</command> for you, just
        before any command that is not already in a transaction block and
        is not itself a <command>BEGIN</command> or other transaction-control