Select attribute of xsl:sort

      <xsl:stylesheet
                  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                  version="2.0">
            <xsl:output  method="xml"
                        indent="yes"
                        omit-xml-declaration="yes"/>

            <xsl:template  match="/">
                  <xsl:apply-templates  select="/aaa/bbb">
                        <xsl:sort  select="@x"/>
                  </xsl:apply-templates>
            </xsl:template>

            <xsl:template  match="bbb">
                  <bbb>
                        <xsl:value-of  select="."/>
                  </bbb>
            </xsl:template>

      </xsl:stylesheet>

      <aaa>
            <bbb  x="d">1</bbb>
            <bbb  x="b">2</bbb>
            <bbb  x="c">3</bbb>
            <bbb  x="a">4</bbb>
      </aaa>

      <bbb>4</bbb>
      <bbb>2</bbb>
      <bbb>3</bbb>
      <bbb>1</bbb>