Re: for loop 'bad math expression'

["Mark J. Reed" <markjreed@gmail.com>]

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> I know and expect that $((()) has its
internal grammar.  But 'let' seems ... naked ... for lack of a better word
and it's a strain on grammatical rigor that it gets special, invisible
favors.

Well, it doesn't, really. Grammar-wise, let is treated as a normal command,
which is why you need quoting to use special characters or white space with
it. The ((...)) construct was added to the shell (and described in the
KornShell book) as effectively a version of let that is quoted for you -
that's where the special parsing comes in. let itself doesn't have that.

It does evaluate its arguments in an arithmetic context, and is one of the
main ways to achieve that effect, but it's not the only one, or even the
only one outside ((...)).  You've seen it kick in for variables declared as
integers; it also shows up in array subscripts:

zsh% ary=(a b c d e); i=2; j=1; k="i + j"
zsh% echo $ary[$k]
c


--
Mark J. Reed <markjreed@gmail.com>

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