powerful command line tool ?

I am looking for a command line tool that:

1) can easily merge XML files of the same format
2) can modify/remore/add attributes or tags easily
3) does all that quickly (including save)

I am mostly using the TMX format (a format used in the localization industry to keep translation information) and the dedicated GUI tools I use are just painfully slow even on a Core 2 Duo 3ghz with 8gm ram recent iMac.

Is there anything like this around ?

Jean-Christophe Helary
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