How to find a specific tag section in an XML file?

Question

The last few lines of my file /usr/share/glib-2.0/schemas/org.gnome.Vino.gschema.xml:

<schemalist>
  <schema>
   <!-- some other tags -->

    <key name='notify-on-connect' type='b'>
      <summary>Notify on connect</summary>
      <description>
        If true, show a notification when a user connects to the system.
      </description>
      <default>true</default>
    </key>

    <key name='enabled' type='b'>
      <summary>Enable remote access to the desktop</summary>
      <description>
      If true, allows remote access to the desktop via the RFB
      protocol. Users on remote machines may then connect to the
      desktop using a VNC viewer.
      </description>
      <default>false</default>
    </key>
  </schema>
</schemalist>

If I want to grep this paragraph:

<key name='enabled' type='b'>
  <summary>Enable remote access to the desktop</summary>
  <description>
  If true, allows remote access to the desktop via the RFB
  protocol. Users on remote machines may then connect to the
  desktop using a VNC viewer.
  </description>
  <default>false</default>
</key>

How should I use the grep command to achieve this?

Answer 1

Since your given example is a valid XML file, so I would use xq XML parser tool for that which is part of the yq installation package.

xq -x --xml-root key '
    .schemalist.schema.key[] | select(."@name" == "enabled")
' infile.xml

select the "key" tag if its "@name" attribute was equal to 'enabled'.

from the xq -h:

--xml-output, -x
Transcode jq JSON output back into XML and emit it
--xml-root XML_ROOT
When transcoding back to XML, envelope the output in an element with this name

Answer 2

Since you are dealing with valid XML, you can use xmlstarlet:

xmlstarlet sel -t -c "/schemalist/schema/key[@name='enabled']" infile.xml

This will query (sel) the XML document and print a copy -c of the XPATH element /schemalist/schema/key, where that XML node with attribute name set to enabled is selected.

Output for your example:

<key name="enabled" type="b">
      <summary>Enable remote access to the desktop</summary>
      <description>
      If true, allows remote access to the desktop via the RFB
      protocol. Users on remote machines may then connect to the
      desktop using a VNC viewer.
      </description>
      <default>false</default>
    </key>

Answer 3

To be clear:

This is most likely a job for some tool that understands XML documents, not grep or the like. There are good answers on this.

---

Perhaps:

grep

Using perl-regexp, and other additions.

grep -Pzo "(?s)\N*<key name='enabled'.*<\/key>\n" the_file.xml

Or a bit more limited (Is not capturing leading blanks ):

grep -zo "<key name='enabled'.*<\/key>."

awk

awk '
/<key name='\''enabled'\''/ { p=1 }
p { print $0 }
p && /<\/key>/ { exit }
' the_file.xml

Or

awk "/<key name='enabled'/,/<\/key>/" the_file.xml

sed

sed -n '/<key name='\''enabled'\''/,/<\/key>/p' the_file.xml

Answer 4

If you assume the file contains valid XML, you can use xmllint from the package libxml2-utils and find the desired XML element using XPath expression:

    $ xmllint --xpath "/schemalist/schema/key[@name='enabled' and @type='b']" /usr/share/glib-2.0/schemas/org.gnome.Vino.gschema.xml 
    <key name="enabled" type="b">
          <summary>Enable remote access to the desktop</summary>
          <description>
          If true, allows remote access to the desktop via the RFB
          protocol. Users on remote machines may then connect to the
          desktop using a VNC viewer.
          </description>
          <default>false</default>
        </key>

Answer 5

Another way to obtain the desired output is to use XSL (eXtensible Stylesheet Language) and xsltproc - which is installed by default if you are using the GNOME desktop - to transform the input document into the desired document (the output.)

Here is the stylesheet:

$ cat transform.xsl
<?xml version="1.0" encoding="UTF-8"?>

<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method='xml' omit-xml-declaration='yes' />

<xsl:template match="/schemalist/schema/key[@name='enabled']">
    <xsl:copy-of select="." />
</xsl:template>

<xsl:template match='text()|@*'/>

</xsl:stylesheet>

Here is the output:

$ xsltproc transform.xsl input.xml
<key name="enabled" type="b">
      <summary>Enable remote access to the desktop</summary>
      <description>
      If true, allows remote access to the desktop via the RFB
      protocol. Users on remote machines may then connect to the
      desktop using a VNC viewer.
      </description>
      <default>false</default>
    </key>
$

Answer 6

Using Perl:

#!/usr/bin/perl

use 5.010;
use strict;
use warnings;
use XML::LibXML;

my $filename='/home/emmmcl01/gschema.xml';
my $dom = XML::LibXML->load_xml(location => $filename);
foreach my $key ($dom->findnodes('/schemalist/schema/key')) {
  my $desc = $key->findvalue('./description');
  my $nm = $key->findvalue('./@name');

  if ('enabled' eq $nm ) {
   print $key;
  }
}

This solution iterates through each key element and when it finds a key having a description whose "named" attribute is equal to "enabled" it prints out the element. The solution leverages a Perl XML parser: XML::LibXML which might have to be installed.