As part of the crouton/chroagh project, I wanted to be able parse D-bus configuration file, to figure out what user account the system D-bus runs under.

The configuration file looks like this:

# /etc/dbus-1/system.conf
<!DOCTYPE busconfig PUBLIC "-//freedesktop//DTD D-Bus Bus Configuration 1.0//EN"
 "http://www.freedesktop.org/standards/dbus/1.0/busconfig.dtd">
<busconfig>

  <!-- Our well-known bus type, do not change this -->
  <type>system</type>

  <!-- Run as special user -->
  <user>dbus</user>

  <!-- Fork into daemon mode -->
  <fork/>

  .....

</busconfig>

Yes, that’s XML. And normally, XML and shell scripts are not exactly good friends…

What we want to be able to do here is to fetch the content of the user tag.

Solution 1 - sed

Well, that’s without doubt the easiest:

sed -n 's|.*<user>\(.*\)</user>|\1|p' /etc/dbus-1/system.conf

Problem is that the is no guarantee that there would be no other user tag1, in other parts of the file, and, who knows, the tag might be commented out. We also do not handle “misplaced” newlines in the file…

Solution 2a - xmllint

A more proper and generic solution, making use of the xmllint parser:

echo "cat /busconfig/user/text()" | xmllint --shell /etc/dbus-1/system.conf

Problem is, xmllint is not really script-friendly, and outputs some garbage along with the desired output.

$ echo "cat /busconfig/user/text()" | xmllint --shell /etc/dbus-1/system.conf
/ > cat /busconfig/user/text()
 -------
dbus
/ > $

In my case, I know how a valid username looks like, so I just pipe the output through grep '^[a-z][-a-z0-9_]*$', and that’s the solution that is used in my code. Also, xmllint is installed by default on Chrome OS, so that’s good enough for me.

Solution 2b - xmllint and write

An improved version would be:

echo "cd //busconfig/user/text()
     write tmp" | xmllint --shell /etc/dbus-1/system.conf
cat tmp

This is cleaner, as the full text of the tag is written to a file, but this requires an intermediate file, so, maybe not that nice…

Solution 2c - xmllint, write, and fd/3

Maybe an even nicer one, assuming /dev/fd exists (as it does on recent Linux distributions):

exec 3>/dev/null
echo "cd //busconfig/user/text()
     write /dev/fd/3" | xmllint --shell /etc/dbus-1/system.conf 3>&1 1>/dev/null 2>/dev/null
exec 3<&-

No temporary file!

Solution 3 - xsltproc

Finally, the most powerful version, using XSLT:

xsltproc - /etc/dbus-1/system.conf <<END
<?xml version="1.0" encoding="utf-8"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="text" encoding="utf-8" />
<xsl:template match="/"><xsl:value-of select="/busconfig/user"/></xsl:template>
</xsl:stylesheet>
END

That’s as good as it gets, and you could easily do much more complicated things with XSLT. But, that’s a bit overkill for our purpose (and xsltproc is not installed in Chrome OS).

Imported from “Random ideas of a drinking cat” blog.

  1. Actually, there is no other user tag, according to the dtd file, but not necessarily in a general case… And well, this post is boring if I stop here, right? ↩