RogerBW's Blog

The units program as a command-line calculator 28 July 2017

The units(1) program is available on many Unix systems (though rarely installed by default). Most people think of it as a simple interactive converter. It's actually much more powerful than that.

This is how people expect to use it, entering things at an interactive prompt:

$ units
You have: mile
You want: km
        * 1.609344
        / 0.62137119

That's useful but very dull. Functions are slightly more promising:

$ units
You have: tempF(98.4)
You want: tempC
    36.888889

But it also works with pure command-line arguments, potentially of some complexity - which of course allows it to be a handy command-line calculator too. Atmospheric pressure in psi:

$ units atmosphere gravity*lb/inch^2
        * 14.695949
        / 0.068045964

What time will my 3d print finish if I start it now?

$ units $(date +"%Hhour+%Mminute+%Ssecond")+8hour+19minute "hour;minute;second"
        19 hour + 31 minute + 9 second

The blackbody temperature of a 100m-radius sphere radiating at 1MW (yes, lots of physical constants are here too):

$ units "(1MW/stefanboltzmann/((100m^2)*4pi))^(1/4)" tempC
        71.037224

If you're feeding the output into another programme (because this is Unix and you can do that sort of thing), use --compact to strip off the decoration:

$ units --compact "(1MW/stefanboltzmann/((100m^2)*4pi))^(1/4)" tempC
71.037224

Of course it's extensible. As someone who plays a fair bit of GURPS, I find it useful to implement that game's logarithmic range scale, so I add to my local .units file:

gurpsrange(x) units=[1;yard] range=[0,) \
10^((x+2)/6)*yard ; log(gurpsrange/yard)*6-2

and now I can easily calculate ranges for a space game:

$ units 5astronomicalunit gurpsrange
        69.476555
$ units "gurpsrange(40)" mile
        * 5681.8182
        / 0.000176

For any Torg players:

torgdistance(x) units=[1;metre] range=[0,) \
10^(x/5)*metre ; log(torgdistance/metre)*5
torgtime(x) units=[1;second] range=[0,) \
10^(x/5)*second ; log(torgtime/second)*5
torgweight(x) units=[1;kg] range=[0,) \
10^(x/5)*kg ; log(torgweight/kg)*5
torgspeed(x) units=[1;m/s] range=[0,) \
10^((x-5)/5)*m/s ; log(torgspeed*s/m)*5+5

Note that speed is not distance minus time as it should have been in a cleanly-designed system, but based on metres per 10s combat round; thus the fudge factor. What a pity.

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