Edit value inside PLIST in Linux

I have an XML Plist, and I need a bash script to edit the integer value of a key in the case that that key exists, or else create the key and integer value if it does not. This is a Linux machine, so Mac OS X' defaults command is not available.

That is, the final Pl开发者_高级运维ist would look like:

<dict>
    …
    <key>myKey</key>
    <integer>54</integer>
    …
</dict>

whether the key myKey already existed in the original Plist or not, and where the integer value 54 is provided by a variable.

In Mac OS X I would do it like this:

defaults write ./myPlist myKey -int $(echo $myKey_value)

but like I said, no defaults command in Linux. I tried with sed, but apparently it can only match single lines. Maybe awk or perl would do, but I know almost nothing of either.

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Data::Plist::XMLWriter