Curious ksh hack

[M.BRILLIANT]

In article <340 at manta.UUCP>, brant at manta.UUCP (Brant Cheikes) writes:
> In a memo by David Korn describing ksh, there is a paragraph that
> states:
> 
> "The ENV file can have an undesirable effect on performance....
> ....  If you export the startup file name in the
> variable START, then setting
> 
>    ENV='${START[(_$-=1)+(_=0)-(_$-!=_{-%%*i*})]}'
> 
> will only invoke the startup file for interactive shells since the
> subscript evaluates to 0 only if the shell is interactive."
> 
> Now, I've done this and it works, but I can't quite figure out how.

I've figured out how, but I can't make it work.

The manual entry for ksh says that the subscript in an array parameter,
that is, x in $START[x], is evaluated as an arithmetic expression.
In arithmetic evaluation, as described under the "let" command, you can
put subexpressions in parentheses and use = for arithmetic replacement.

In an interactive shell, $- usually evaluates to "is".  So the first
term in parentheses, (_$-=1), evaluates to 1 but also sets the
identifier _is equal to 1.  The second term, (_=0), evaluates to zero
and sets the identifier _ to zero.  The third term is apparently a
misprint for (_$-!=_${-%%*i*}).  The part before the != evaluates, as
previously defined, to 1.  The part after the != uses ${..%..} to
lop off any trailing substring in $- that contains an i, so that part
evaluates to _ (zero) in an interactive shell, but to _$- (=1) in a
noninteractive shell.  The != is true in an interactive shell, equals
1, but is false in an noninteractive shell, equals zero.  The sum of
the three terms is zero in an interactive shell but is 1 in a
noninteractive shell.

The desired effect is achieved if, as it says in the manual entry,
"command and parameter substitution is performed on the value [of $ENV]
to generate the pathname of the script ...."  On my system, substitution
doesn't seem to happen, so it doesn't work.

M. B. Brilliant					Marty
AT&T-BL HO 3D-520	(201)-949-1858
Holmdel, NJ 07733	ihnp4!houdi!marty1

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