I was integrating rrr with claws-mail and use it to open external files, so I could centralize file opening configuration there. I created a mail profile in ~/.config/rrr.conf with a catch-all rule * echo "'%s'" >> /tmp/mail-rrr.log to debug which files and extensions were being passed. The plan was to set rrr -p mail -F '%s' as the default external application, but the command kept failing.

I wrote a small test.sh script to display each argument individually as passed by claws-mail. That’s when I discovered the issue: claws-mail treats each space in the command as a separate argument delimiter. So rrr -p  mail -F '%s' (note the double space between -p and mail) resulted in the argument list: ["rrr", "-p", "", "mail", "-F", "the-actual-file"]. The empty string broke everything.

So the fix was simple, just had to make sure there were no extra space everywhere.