On the other hand, an equation of the form y = a x + b is a linear equation. If you have Celsius and want Fahrenheit you accomplish that by applying a linear equation (F = 1.8 C + 32), so I certainly can't fault people for saying that the transformation they are doing is linear.

I wonder what people would say for something using an equation of the form y = a x^2 + b x + c to transform something? I can't say that I've heard anyone talk of quadratic transformations. On the other hand, I can't think of ever transforming anything with a quadratic equation, so never had the need t speak of it.

(Also, he called it a linear conversion, not a linear transformation).

I didn’t look too deep into it, my understanding was that the source it uses to update itself has been taken offline. There are workarounds involving data massaging and a cron but honestly that’s a lot more work than typing “1000 chf to usd” into ddg and getting the converted amount. But if you know something I don’t, maybe you could share for everyone’s benefit?

In case anyone else needs this:

  # systemctl edit units-currency-update.service
  
  [Unit]
  Description=Update units(1) currency rates
  
  [Service]
  Type=oneshot
  Nice=19
  ExecStart=/usr/bin/units_cur
  
  # systemctl edit units-currency-update.timer
  
  [Unit]
  Description=Update units(1) currency rates
  
  [Timer]
  OnCalendar=daily
  AccuracySec=3h
  Persistent=true
  
  [Install]
  WantedBy=timers.target
  
  # systemctl daemon-reload
  # systemctl enable units-currency-update.timer

I find the reciprocal thing useless, so I have units='units --compact --one-line', which gives just the number you want.