[clue] arithmetic in bash

[Will]

Korn shell interprets the value in side of $[ ] to be a variable name.  It
was added so that you can surround a variable name with other text and be
sure that your variable will be expanded properly.

echo some$[variable_name]text

The POSIX shell standard was based on ksh88 so the function of $[ ] was
already there.  I would guess that when they decided to make
bash compliant with the POSIX shell standard or add the feature to the
POSIX spec they had to add $(( experssion )) and left $[ expression ] in
bash for backwards compatibility.

In short always use $(( expression )) as it will work in more shells.


On Thu, Apr 12, 2012 at 12:11 PM, David L. Willson <DLWillson at thegeek.nu>wrote:

> Aaron,
>
> Thanks for the well-thought-out answer. I'll return to using '$((
> EXPRESSION ))', even though I can depend on bash for all my work.
>
> I guess that '$[ EXPRESSION ]' has some capabilities over the old style,
> but I never do anything terribly complex.
>
> In my specific application, getting some random numbers, '$(( EXPRESSION
> ))' is working perfectly.
>
> Case in point:
>
> # Generate a bunch of random looking numbers, in the range 1 to 10,
> inclusive.
> while true ; do echo -n "$(( RANDOM % 10 + 1 )) " ; done
>
> --
> David L. Willson
> Trainer, Engineer, Enthusiast
> RHCE Network+ A+ Linux+ LPIC-1 Ubuntu
> Mobile 720-333-LANS(5267)
>
> This is a good time for a r3volution.
>
> ----- Original Message -----
> > David L. Willson writes:
> > > Why are the square brackets preferred (see below)? Why are there
> > > two
> > > forms?
> >
> > I probably don't have this completely correct, but...  I'd say the
> > recommendation is incorrect.
> >
> > It is my understanding that the $(( ... )) syntax is a POSIX shell
> > feature, while the $[ ... ] is something that bash added in the
> > early-mid 1990s.  Given that, I try to use the $(( ... )) syntax
> > exclusively.  The $[ .. ] syntax generally doesn't work on non-bash
> > Bourne shells.  This may or may not be a concern for you.
> >
> > Please stand by while I fire up the HP-UX box...
> >
> > Here's what happens with $[ .. ] on a non-bash Bourne style shell:
> >
> >     $ SHELL=/usr/bin/sh; exec $SHELL -o vi
> >     $ uname -a
> >     HP-UX billandd B.11.23 U 9000/800 1822910606 unlimited-user
> >     license
> >     $ echo $((2 * 5))
> >     10
> >     $ echo $[2 * 5]
> >     $[2 fileset-cleanup 5]
> >     $ exit
> >
> > Compare with the results on a machine using bash:
> >
> >     adj at sacredchao:~$ echo $SHELL
> >     /bin/bash
> >     adj at sacredchao:~$ uname -a
> >     Linux sacredchao 3.2.0-0.bpo.1-amd64 #1 SMP Sat Feb 11 08:41:32
> >     UTC 2012 x86_64 GNU/Linux
> >     adj at sacredchao:~$ echo $((2 * 5))
> >     10
> >     adj at sacredchao:~$ echo $[2 * 5]
> >     10
> >     adj at sacredchao:~$
> >
> > Googling for "bashism" will turn up many similar instances where Chet
> > and crew added non-standard stuff to bash, and the efforts needed to
> > weed them out when they're used in non-bash environments.
> >
> > To sum up: I disagree with the recommendation in the Bash Guide for
> > Beginners.  Use the POSIX syntax for arithmetic.  But if you can
> > guarantee your scripts will run on nothing but bash, do whatever
> > tickles your fancy.
> >
> > - Aaron
> > _______________________________________________
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