Replace dynamic content in XML file

Quick Summary: I need to create a Bash script to change the text within a node automatically every week. The script will match the node and replace the text inside them (if this is possible)? How would I do this?

Long Summary: I host a Minecraft server which has shops, each of which have their own .xml file in the /ShowcaseStandalone/ffs-storage/ directory. Every Sunday my server restarts and executes several commands into the terminal to reset several things. One thing that I am trying to make change is one of the shops. I am wanting to change the text in the node <itemstack> and the text in the node <price>. I am simply wanting to take text from a .txt file in a different folder, and insert it into that node. The problem is, that the text in the node will change every week. Is there any way to replace a specific line or text within two nodes using bash?

XML file:

<?xml version="1.0" encoding="UTF-8"?>
<scs-shop usid="cac8480951254352116d5255e795006252d404d9" version="2" type="storage">
    <enchantments type="string"/>
    <owner type="string">Chadward27</owner>
    <world type="string">Frisnuk</world>
    <itemStack type="string">329:0</itemStack>
    <activity type="string">BUY</activity>
    <price type="double">55.0</price>
    <locX type="double">487.5</locX>
    <locY type="double">179.0</locY>
    <locZ type="double">-1084.5</locZ>
    <amount type="integer">0</amount>
    <maxAmount type="integer">0</maxAmount>
    <isUnlimited type="boolean">true</isUnlimited>
    <nbt-storage usid="23dffac5fb2ea7cfdcf0740159e881026fde4fa4" version="2" type="storage"/>
</scs-shop>

Operating System: Linux Ubuntu 12.04

Solution

You can use xmlstarlet to edit a XML file in a shell like this :

xmlstarlet edit -L -u "/scs-shop/price[@type='double']" -v '99.66' file.xml

NOTE

"/scs-shop/price[@type='double']" is a Xpath expression
see xmlstarlet ed --help