Adding new attribute to existing xml file using sed or awk

Try this way:

sed -i -r 's/(.*HostName.*)/\1\n<attribute name="Port">1414<\/attribute>/g' filename

Result in:

$ cat filename
<server>
  <mbean code="WSMQConnectionFactory" name="service=MQQueueConnectionFactory">
  <attribute name="JndiName">WSMQQueueConnectionFactory</attribute>
  <attribute name="QueueManagerName">QMPMP</attribute>
  <attribute name="HostName">10.10.20.21</attribute>
<attribute name="Port">1414</attribute>
  <attribute name="Channel">CHANNEL01</attribute>
  <attribute name="TransportType">MQJMS_TP_CLIENT_MQ_TCPIP</attribute>
  <depends>jboss:service=Naming</depends>
 </mbean>
</server>

Please don't do this. XML is a structured data type, and it's one that doesn't 'fit' a regular expression. Whilst you can pretend your XML is plain text, and use e.g. 'sed' to adjust it, this is a very good way of creating brittle code - because different XML structures that are semantically identical won't work in the same way.

To do this, you really need a parser. I would suggest Perl (which is ubiquitous) and XML::Twig which is quite common and easily installed.

This code will do it (it's a little longer than it actually needs to be, but this is in the interests of clarity).

#!/usr/bin/env perl

use strict;
use warnings;

use XML::Twig;

sub paste_port {
    my ( $twig, $attribute ) = @_;
    my $port_attr =
        XML::Twig::Elt->new( 'attribute', { 'name' => 'Port' }, 1414 );
    print "Inserting:\n", $port_attr->sprint, "\n";
    $port_attr->paste_after($attribute);
}

my $twig = XML::Twig->new(
    'pretty_print'  => 'indented',
    'twig_handlers' => { 'attribute[@name="HostName"]', \&paste_port }
);
$twig->parsefile('your_xml.xml');
$twig->print;

#save to file 'new_xml.xml'
open( my $output_file, ">", "new_xml.xml" ) or warn $!;
print {$output_file} $twig->sprint;
close($output_file);

This will produce the output:

<server>
  <mbean code="WSMQConnectionFactory" name="service=MQQueueConnectionFactory">
    <attribute name="JndiName">WSMQQueueConnectionFactory</attribute>
    <attribute name="QueueManagerName">QMPMP</attribute>
    <attribute name="HostName">10.10.20.21</attribute>
    <attribute name="Port">1414</attribute>
    <attribute name="Channel">CHANNEL01</attribute>
    <attribute name="TransportType">MQJMS_TP_CLIENT_MQ_TCPIP</attribute>
    <depends>jboss:service=Naming</depends>
  </mbean>
</server>