Re: Bash-specific performance by avoiding sed

[Eric Blake]

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.
>>
>> 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
      IFS=$save_IFS post=${post%.}
      quoted=${post#.} ;;
    *) quoted=$1 ;;
  esac
}

(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 "$1"
_G_unquoted_arg=$quoted

for the same result, without any forking.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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