The shell language as defined by POSIX is considerably larger than the original V7 Bourne shell. However, it is considerably smaller than the languages implemented by ksh93 and bash, the two most commonly used extended versions of the Bourne shell.
It is likely that if you'll be doing heavy-duty scripting that takes advantage of shell-language extensions, you'll be using one or the other or both of these two shells. Thus, it's worthwhile to be familiar with features that the shells have in common, as well as their differences.
Over time, bash has acquired many of the extensions in ksh93, but not all of them. Thus, there is considerable functional overlap, but there are also many differences. This chapter outlines areas where bash and ksh93 differ, as well as where they have common extensions above and beyond the features of the POSIX shell.
Many of the features described here are available only in recent
versions of ksh93. Some commercial
Unix systems have older versions of ksh93, particularly as a program called
dtksh (the desktop Korn shell,
/usr/dt/bin/dtksh), which won't have
the newer features. Your best bet is to download the source for the
current ksh93 and build it from
scratch. For more information, see Section 14.4.
Here is a "laundry list" of things to watch out for:
Example 14-1 shows how to save the shell's state into a file. An apparent oversight in the POSIX standard is that there's no ...