Yep, we've changed our loops to use roughly that syntax. Unfortunately a lot of online resources recommend the $(expr) technique. My understanding is that using $(expr) is more portable, because i-- is bash specific, but I don't really know.
-----Original Message----- From: Chris Down [mailto:ch...@chrisdown.name] Sent: Saturday, March 16, 2013 6:58 PM To: Bruce Dawson Cc: bug-bash@gnu.org; b...@packages.debian.org Subject: Re: Bug/limitation in 'time' 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" } Best, Chris
