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Re: Recomended tutoial(s) on doing arithmetic in Bash scripts

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Another external tool at least as good as bc is wcalc and once that 
package gets installed just run wcalc at the command prompt and you'll 
have quite a load of examples show up.

On Sun, 5 Mar 2017, tomas@tuxteam.de wrote:

> Date: Sun, 5 Mar 2017 11:25:49
> From: tomas@tuxteam.de
> To: debian-user@lists.debian.org
> Subject: Re: Recomended tutoial(s) on doing arithmetic in Bash scripts
> Resent-Date: Sun,  5 Mar 2017 16:26:04 +0000 (UTC)
> Resent-From: debian-user@lists.debian.org
> 
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> On Sun, Mar 05, 2017 at 09:42:16AM -0600, Richard Owlett wrote:
>> I'm interested in "expr" and "bc".
>> The man pages lack reasonable examples.
>> The tutorial/HOWTO pages confuse the issue with fancy page layouts
>> and/or code samples showing how impressive the author can make the
>> script's output.
>
> Hm. Neither expr nor bc are bash, they are "external" binaries. If you
> want to do arithmetic in bash, there's $((...)):
>
>  tomas@rasputin:~$ echo $(( (3+4)/3 ))
>  2
>
> As you see, this does integer arithmetic. There's not much to it, the
> usuall stuff (more or less known from C, like pre/post increment, negation,
> bitwise and/or/xor... see the section "ARITHMETIC EVALUATION" in the
> bash manual.
>
> Expr and bc can do more -- but you've to take into account that they
> are external programs and thus give their response on stdout: you
> have to "catch" it somehow to make use of it. The `...` or the more
> modern and highly recommended $(...) come in:
>
>  tomas@rasputin:~$ foo=$(expr 3 + 4)
>  tomas@rasputin:~$ echo $foo
>  7
>
> Note that expr can also only do integers. I don't know whether expr
> brings anything to the table beyond portability. If you stick to bash
> (or dash, or...) it seems better to keep to the builtin $((...))
>
> If you need floating point numbers, bc (or dc) are your next stops.
>
> Perhaps you start with a couple of small examples on what you want
> to achieve, post them here and we take them as riddles :-)
>
> Here's a little example with dc, to get your appetite going. As you
> might know, I'm a Luddite and have no desktop environment. As a laptop
> user, I'm still interested on my battery's status: my laptop just
> quits pretty abruptly when empty, putting the excellent ext4 file
> system to test. For that, I've a small xterm which displays the
> battery fill status (which I want as a fraction of 1, to five
> significant digits, so I can see it's moving). The repeating is
> done outside the program, with "watch" -- invoked from my WM.
>
> Here's the shell snippet. The dc magic (dividing the battery's
> current status, called NOW, by the FULL status) is done at
> the last line:
>
>  #!/bin/bash
>  # Notes:
>  # for colors:
>  # tput setaf 1 ; tput bold ; echo -n 123 ; tput setaf 7 ; tput sgr0 ; echo 456
>  # cf tput(1) terminfo(5)
>  # do continuous mode with watch -c, possibly t
>
>  BAT='/sys/class/power_supply/BAT0'
>  AC='/sys/class/power_supply/ADP1'
>  FULL=$(cat $BAT/energy_full)
>  NOW=$(cat $BAT/energy_now)
>  case $(cat $AC/online) in
>    0) online="BAT" ;;
>    1) online="AC " ;;
>    *) online="???" ;;
>  esac
>  echo -ne "$online " ; dc -e "5k $NOW $FULL / p"
>
> The $online variable tells me whether I'm on AC or BATtery, for the dc line it's
> "5k" to set the five significant digits, then $NOW $FULL and the division "/"
> (note that dc is a stack calculator [1]), note that the expansion of $NOW, etc.
> is done by the shell.
>
> If you wanted to catch the value instead of printing it right away, you would
> do $fill=$(dc -e "5k $NOW $FULL / p")
>
> With bc it'd be similar; I never wrapped my head around bc's syntax, since
> dc has served me so well, but given enough motivation I don't think it's
> too hard.
>
> The script's comments hint at the next steps: COLORZ ;-)
>
> Enjoy
>
> [1] Like "Foot in yourself shoot.": look under Forth here:
>    http://www.thealmightyguru.com/Humor/Docs/ShootYourselfInTheFoot.html
>
> - -- tom?s
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Thread

Recomended tutoial(s) on doing arithmetic in Bash scripts Richard Owlett <rowlett@cloud85.net> - 2017-03-05 16:50 +0100
  Re: Recomended tutoial(s) on doing arithmetic in Bash scripts Reco <recoverym4n@gmail.com> - 2017-03-05 17:30 +0100
  Re: Recomended tutoial(s) on doing arithmetic in Bash scripts <tomas@tuxteam.de> - 2017-03-05 17:30 +0100
    Re: Recomended tutoial(s) on doing arithmetic in Bash scripts GiaThnYgeia <GiaThnYgeia@openmailbox.org> - 2017-03-05 19:20 +0100
      Re: Recomended tutoial(s) on doing arithmetic in Bash scripts <tomas@tuxteam.de> - 2017-03-05 21:40 +0100
      Re: Recomended tutoial(s) on doing arithmetic in Bash scripts Greg Wooledge <wooledg@eeg.ccf.org> - 2017-03-06 14:40 +0100
        Re: Recomended tutoial(s) on doing arithmetic in Bash scripts <tomas@tuxteam.de> - 2017-03-06 15:00 +0100
        Re: Recomended tutoial(s) on doing arithmetic in Bash scripts Jörg-Volker Peetz <jvpeetz@web.de> - 2017-03-06 16:00 +0100
          Re: Recomended tutoial(s) on doing arithmetic in Bash scripts <tomas@tuxteam.de> - 2017-03-06 16:40 +0100
    Re: Recomended tutoial(s) on doing arithmetic in Bash scripts Richard Owlett <rowlett@cloud85.net> - 2017-03-05 19:20 +0100
      Re: Recomended tutoial(s) on doing arithmetic in Bash scripts Richard Owlett <rowlett@cloud85.net> - 2017-03-05 19:50 +0100
      Re: Recomended tutoial(s) on doing arithmetic in Bash scripts <tomas@tuxteam.de> - 2017-03-05 21:30 +0100
        Re: Recomended tutoial(s) on doing arithmetic in Bash scripts Richard Owlett <rowlett@cloud85.net> - 2017-03-05 22:00 +0100
    Re: Recomended tutoial(s) on doing arithmetic in Bash scripts Jude DaShiell <jdashiel@panix.com> - 2017-03-05 23:20 +0100
      Re: Recomended tutoial(s) on doing arithmetic in Bash scripts Richard Owlett <rowlett@cloud85.net> - 2017-03-06 12:20 +0100
        Re: Recomended tutoial(s) on doing arithmetic in Bash scripts <tomas@tuxteam.de> - 2017-03-06 12:40 +0100
  Re: Recomended tutoial(s) on doing arithmetic in Bash scripts David Christensen <dpchrist@holgerdanske.com> - 2017-03-05 20:50 +0100
    Re: Recomended tutoial(s) on doing arithmetic in Bash scripts Richard Owlett <rowlett@cloud85.net> - 2017-03-05 21:50 +0100
      Re: Recomended tutoial(s) on doing arithmetic in Bash scripts David Christensen <dpchrist@holgerdanske.com> - 2017-03-06 04:40 +0100
    Re: Recomended tutoial(s) on doing arithmetic in Bash scripts <tomas@tuxteam.de> - 2017-03-05 22:30 +0100
  Re: Recomended tutoial(s) on doing arithmetic in Bash scripts Jude DaShiell <jdashiel@panix.com> - 2017-03-05 23:10 +0100

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