[TUHS] Arithmetic expansion in Unix shells

[Chet Ramey]

On 6/21/21 5:57 AM, arnold at skeeve.com wrote:
> Arithmetic expansion dates back at least as far as ksh88.

ksh had the `let' builtin from at least 1983. The ((...)) compound command
was there by ksh-86.

> Bash likely picked it up from there.

Sort of, see below.

> The original was only integer math and Bash remains that way (IIRC,
> Chet can correct me if I'm wrong). ksh93 added floating point math.

Yes, bash only has integer arithmetic, since it's all POSIX requires.

> POSIX would have picked it up from ksh88.

The $((...)) form of arithmetic expansion is something POSIX picked up
from ksh-88, eventually. The early drafts of the standard (through 1003.2
d9, at least), used $[...], but they eventually adopted $((...)) because
ksh-88 had already implemented it, though it's not documented in Bolsky
and Korn.

I put $[...] into bash first (it's still there, though deprecated), then
`let', then $((...)) after POSIX added it, and finally  `((' for
compatibility.

Chet

-- 
``The lyf so short, the craft so long to lerne.'' - Chaucer
		 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, UTech, CWRU    chet at case.edu    http://tiswww.cwru.edu/~chet/