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Re: Bash-specific performance by avoiding sed

From: Eric Blake
Subject: Re: Bash-specific performance by avoiding sed
Date: Mon, 09 Mar 2015 14:48:39 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.5.0

On 03/09/2015 01:50 PM, Bob Friesenhahn wrote:
> On Mon, 9 Mar 2015, Mike Gran wrote:
>> Hello libtool,
>> 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.
> 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:

_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 $pre; do
      IFS=$save_IFS post=${post%.}
      quoted=${post#.} ;;
    *) quoted=$1 ;;

(of course, with proper munging of internal variable names [$save_IFS,
$pre, $post, $char, $quoted] so as to be less likely to collide with
other variables in use)

So, where we were previously doing:

_G_unquoted_arg=`printf '%s\n' "$1" | $SED "$sed_quote_subst"`

we would now do:

func_quote "$1"

for the same result, without any forking.

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library

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