Thanks -- good to know that there is a fast and POSIX compliant method of
doing this. I should have included my optimized counting loop -- it's what
we switched to when we realized that $(expr) was a problem. Here it is now:
# This code performs quite well
function BashCount() {
i=$1
while [ $i -gt 0 ]; do
(( i-- ))
done
echo Just did $1 iterations using bash math
}
time BashCount 150000
It's a *lot* faster, of course. BTW, I've poked around in the 'time' source
code enough to know that it is just displaying the results of wait3(), so
the misleading CPU consumption information is ultimately a wait3()/kernel
issue. However showing this in the documentation would be great.
-----Original Message-----
From: Chris F.A. Johnson [mailto:ch...@cfajohnson.com]
Sent: Saturday, March 16, 2013 7:16 PM
To: bug-bash@gnu.org
Cc: Chris Down; Bruce Dawson
Subject: Re: Bug/limitation in 'time'
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)
