If you have libxml2-utils installed (package name may vary depending on your distro, but it almost certainly has a package) you can probably do something like "xmllint --format". You can also use "xmllint --shell file.xml" to get an interactive shell, or execute an xpath query and return, eg. "xmllint --xpath //foo file.xml". Use "-" for the file to read from stdin, as you might expect.
