Re: Bug/limitation in 'time'

[Chris F.A. Johnson]

On Sun, 17 Mar 2013, Chris Down wrote:

> Hi Bruce,
>
> On 2013-03-16 17:41, Bruce Dawson wrote:
> > I think it's important because when I hit this problem (using $(expr) for
> > looping in shell scripts is slow) I initially assumed that my task was not
> > CPU bound, because that is what 'time' told me. This then led me down the
> > wrong path in my investigation.
>
> No comment on the issue itself, but this is just not a good way of writing bash
> arithmetic loops:
>
> > #!/bin/bash
> > # Warning: this code is excessively slow
> > function ExprCount() {
> >                 i=$1
> >                 while [ $i -gt 0 ]; do
> >                                 i=$(expr $i - 1)
> >                                 #sleep 0.001
> >                 done
> >                 echo Just did $1 iterations using expr math
> > }
> > time ExprCount 1000
>
> You're forking 1000 subshells for `expr' when you can quite easily do it on your
> current shell. A better way would be to write it like this:
>
>    ExprCount() {
>        for (( i = $1 ; i > 0 ; i-- )); do
>            :
>        done
>        echo "$1 iterations"
>    }

   Or, in a POSIX-compliant manner:

ExprCount() {
  i=$1
  while [ $(( i -= 1 )) -ge 0 ]; do
    :
  done
  echo Just did $1 iterations using expr math
}

--
   Chris F.A. Johnson, <http://cfajohnson.com/>
   Author:
   Pro Bash Programming: Scripting the GNU/Linux Shell (2009, Apress)
   Shell Scripting Recipes: A Problem-Solution Approach (2005, Apress)