Short version:
Just visit the project’s website and follow the 5 simple instructions (sounds like clickbait, doesn’t it? ;-)) in the file README.md.
Motivation
When I accidentally messed up my installation of Sailfish OS on my Gemini phone I had to use Android as a backup until I found the time to fix the issue. In the meantime several SMS arrived, which I obviously also wanted to see in Sailfish OS. So I started to investigate the possible options.
Development
My naive attempts to just export the database from the Android partition failed miserably due to Android’s security mechanisms. In the end I settled by using SMS Backup & Restore to get a backup of my messages. Not only does this app seem to be the de-facto standard for SMS backups, but it also provides good documentation on the format of the generated XML file.
I already used commhistory-tool to import my SMS history from Firefox OS, so commhistory-tool’s JSON format was set as the target format. And with an XML file as the source: What would be suited better for conversion than an XSL Transformations (XSLT)?
Already preinstalled on my (Linux) system is one of the most commonly used XSLT processors - xsltproc, so I decided to use that one. Unfortunately I had to find out this processor only supports XSLT 1.0, so is missing a lot of convenience functions. On the positive side it supports several EXSLT commands instead, e.g. for dates, strings and functions, which are used extensively.
I later found out that using xsltproc also has another advantage: It is also preinstalled on Sailfish OS! This completely eliminates the need for a separate PC to convert the files - you can do that directly on your Jolla / Sailfish OS phone. In the case of the Gemini I was able to just export the file to the memory card in Android, reboot into Sailfish, generate the JSON file and import it. Could this be any easier?
The XSLT file can be found on https://gitlab.com/laenion/android-sms-backup-restore-2-jolla-commhistory-tool/blob/master/android-sms-backuprestore-2-jolla-commhistory-tool.xsl, the project’s README.md file contains detailed instructions on how to use this file.