Re: gawk cannot control the use of interval expressions from within a program

[Stephane Chazelas]

On Sun, Nov 07, 2004 at 11:49:18AM -0700, Bob Proulx wrote:
> Hello Stéphane,
> (offlist)
> 
> Stephane Chazelas wrote:
> > You can do it as:
> > 
> > #! /bin/sh -
> > ":" +0 && "exec" "/usr/bin/gawk" "-Wre-interval" "-f" "$0"
> > 
> > <Your gawk script goes here>
> 
> I am not familiar with some of the constructs so I did not respond to
> the list.  But that needs to be "$@" or ${1+"$@"} in the above use of
> "$0", right?

You're right, I forgot about the arguments, it should be:

":" +0 && "eval" "exec /usr/bin/gawk -Wre-interval -f \"\$0\" 
\${1+\"address@hidden"}" 

> I am curious about two of the constructs you used.
> 
> 1. #!/bin/sh -
> 
>    Why the "-" in the option list? 

Same thing as in rm -- "$file", if $file starts with a "-", you
get unexpected behavior if you forget the "--". Here, if the name
of the script (as given as the first argument to execve(2))
starts with "-", you will also get an error.

>    That does not seem portable to
>    me.  What effect is desired there.

It is supposed to be portable, it worked already with the first
release of the Bourne shell

> 
> 2. : +0
> 
>   What effect is desired there?  This would seem to me to always be
>   true.  Of course we are feeding this to awk as stdin.  But what is
>   it going to be doing there?

":" +0
expands to 0 (hence "false") in awk and to true (the ":" noop
command returns a 0 exit status) in shell.

That line is supposed to be interpreted by the shell, and not by
awk.

> 3. I am assuming that you are quoting everything so that awk will
>    parse it correctly?

That's right.

regards,
Stéphane