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KornShell?
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The ksh93 is an updated version of the ksh88. The ksh88 is often shipped with Unices like AIX, but the differences are neglegible. The ksh93 fully supports the ksh88 syntax.
ref.:
Beware that sometimes ksh equals mksh (depending on your operating system). Both implement the KornShell language; however, since the "original" ksh was a commercial software throughout the 20th century, a couple of other shells (namely, the pdksh which was used as the base for OpenBSD's default ksh) were created to adapt the language under a free license. The mksh is a fork of the pdksh, it covers most of the ksh88 and some of the ksh93 standard. All of those cover POSIX though.
If you don't care too much about licensing terms, I'd advise you to stick with the ksh93 shell which is still updated regularly.