Append data to the end of a specific line in text file - bash

Question

I admit to being a novice in bash script, but can't quite seem to figure out how to accomplish a key step in a script and couldn't quite find what I was looking for in other threads.
I am trying to extract some specific data (numerical values) from multiple .xml files and add those to a space or tab delimited text file. The files will be generated over time so I need a way to append a new dataset to the pre-existing text file.
For instance, I would like to extract values for 3 different categories, 1 per row or column, and the value for each category from multiple xml files. Basically, I want to build a continuous graph of the data from each of 3 categories over time.
I have the following code which will successfully extract the 3 numbers from the xml file and trim the unnecessary text:
#!/bin/sh
grep "<observation name=\"meanGhost\" type=\"float\">" "/Users/Erik/MRI/PHANTOM/2/phantom_qa/summaryQA.xml" \
| sed 's/<observation name=\"meanGhost\" type=\"float\">//g' \
| sed 's/<\/observation>//g' >> $HOME/Desktop/testxml.txt
grep "<observation name=\"meanBrightGhost\" type=\"float\">" "/Users/Erik/MRI/PHANTOM/2/phantom_qa/summaryQA.xml" \
| sed 's/<observation name=\"meanBrightGhost\" type=\"float\">//g' \
| sed 's/<\/observation>//g' >> $HOME/Desktop/testxml.txt
grep "<observation name=\"std\" type=\"float\">" "/Users/Erik/MRI/PHANTOM/2/phantom_qa/summaryQA.xml" \
| sed 's/<observation name=\"std\" type=\"float\">//g' \
| sed 's/<\/observation>//g' >> $HOME/Desktop/testxml.txt
This gives the output:
1.12
0.33
134.1
I would like to then read in another xml file to get:
1.12 1.45
0.33 0.54
134.1 144.1
I would be grateful for any help with doing this! Thanks in advance.
Erik

Answer 1

It's much safer to use proper XML handling tools. For example, in xsh, you can write something like
$f1 := open /Users/Erik/MRI/PHANTOM/2/phantom_qa/summaryQA.xml ;
$f2 := open /path/to/the/second/file.xml ;
echo ($f1 | $f2)//observation[#name="meanGhost"] ;
echo ($f1 | $f2)//observation[#name="meanBrightGhost"] ;
echo ($f1 | $f2)//observation[#name="std"] ;