Saturday·27·November·2010
Useful but Unknown Unix Tools: units //at 14:56 //by abe
Ever wondered how to easily convert e.g. 10 seamiles into kilometres? Use units:
$ units
2411 units, 71 prefixes, 33 nonlinear units
You have: 10 seamiles
You want: km
* 18.288
/ 0.054680665
You have: ^C
$
Of course this is interactive. There’s also a non-interactive mode:
$ units '10 seamiles' 'km'
* 18.288
/ 0.054680665
$
The line with the asterisk means that 10 seamiles are 18.288 kilometres or 1 kilometre is the 0.054680665th part of 10 seamiles.
This quite non-intuitive output is caused by the fact that unit is designed to be used with units only:
$ units seamiles km
* 1.8288
/ 0.54680665
$
Now this makes more sense: You have to multiply seamiles with 1.8288 to get kilometres and you have to multiply (not divide) kilometres with 0.54something to get seamiles.
But this output is still a little bit cumbersome, and annoying if you want to use it in shell scripts. But for luck, units knows some nice options, especially “-v” (“–verbose”) and “-t” (“–terse”):
$ units -v '10 seamiles' km
10 seamiles = 18.288 km
10 seamiles = (1 / 0.054680665) km
$ units -t '10 seamiles' km
18.288
Now that’s way easier to read and script!
You can also script more complex things like
$ units -v '100 attoparsec/microfortnight' m/s
100 attoparsec/microfortnight = 2.5509901 m/s
100 attoparsec/microfortnight = (1 / 0.39200466) m/s
Unfortunately not all common units are unambiguous for units:
units -v '100 km/h' m/s
conformability error
100 km/h = 1.5091905e+38 s / kg m
m/s = 1 m / s
Well, “h” seems not to be units unit for “hours”, so lets tell it explicity that we want km per hour:
units -v '100 km/hour' m/s
100 km/hour = 27.777778 m/s
100 km/hour = (1 / 0.036) m/s
Looks more like what I expected.
“units” behaves though a little bit strange when I try to convert litres per 100 kilometres into miles per gallon:
$ units -t '6L/100km' 'mpg' conformability error 6e-08 m^2 425143.71 / m^2 $
Interestingly changing the verbosity helps already in this case:
$ units -v '6L/100km' 'mpg'
reciprocal conversion
1 / (6L/100km) = 39.202431 mpg
1 / (6L/100km) = (1 / 0.025508622) mpg
Greetings from the Debian booth at LinuxDay.at and thanks to Y_Plentyn, rhalina and bzed
for example ideas and the suggestion to write a blog posting about
units. :-)
Tagged as: bzed, CLI, conversion, LinuxDay.at, rhalina, units, UUUT, Y_Plentyn
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