Re: Bash-specific performance by avoiding sed

Mike Frysinger <[email protected]> Mon, 9 Mar 2015 18:04:34 -0400

Newsgroups	gmane.comp.gnu.libtool.general
Message-ID	<20150309220434.GA9455@vapier>

On 09 Mar 2015 14:48, Eric Blake wrote:
> On 03/09/2015 01:50 PM, Bob Friesenhahn wrote:
> > On Mon, 9 Mar 2015, Mike Gran wrote:
> >> I don't know if y'all saw this blogpost where a guy pushed
> >> the sed regular expression handling into bash-specific
> >> regular expressions when bash was available.  He claims
> >> there's a significant performance improvement because of
> >> reduced forking.
> >>
> >> http://harald.hoyer.xyz/2015/03/05/libtool-getting-rid-of-180000-sed-forks/
> > 
> > There is an issue in the libtool bug tracker regarding this.
> > 
> > This solution only works with GNU bash.  It would be good if volunteers
> > could research to see if there are similar solutions which can work with
> > other common shells (e.g. dash, ksh, zsh).
> 
> For context, we're trying to speed up:
> 
> sed_quote_subst='s|\([`"$\\]\)|\\\1|g'
> _G_unquoted_arg=`printf '%s\n' "$1" |$SED "$sed_quote_subst"`
> 
> How about this, which should be completely portable to XSI shells (alas,
> it still uses ${a#b} and ${a%b} at the end, so it is not portable to
> ancient Solaris /bin/sh):
> 
> # func_quote STRING
> # Escapes all \`"$ in STRING with another \, and stores that in $quoted
> func_quote () {
>   case $1 in
>     *[\\\`\"\$]*)
>       save_IFS=$IFS pre=.$1.
>       for char in '\' '`' '"' '$'; do
>         post= IFS=$char
>         for part in $pre; do
>           post=${post:+$post\\$char}$part
>         done
>         pre=$post
>       done

should we test the size of the string first ?  i've written such raw shell 
string parsing functions before, and once you hit a certain size (like 1k+ 
iirc), forking out to sed is way faster, especially when running in multibyte 
locales (like UTF8) which most people are doing nowadays.
-mike

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