bin/tarsnap-backup: ShellCheck

[jlduran]

Apply ShellCheck recommendations:

• Double quote variables to prevent globbing and word splitting
• Use $(..) instead of legacy ..
• expr is antiquated. Rewrite using $((..))
• Fix if [ -n something_wrong ];

[AnotherKamila]

LGTM, but currently I cannot test this (AFK for about a week). Have you tried running the fixed script, just in case?

Thanks!

[jlduran]

Yes, it runs on a server everyday (installed as a package)... thank you!

[AnotherKamila]

Awesome, thanks.