Shell Programming and Scripting

It's nonstandard in many ways, the 'read' command doesn't work right in some situations. Besides, it's not a ksh variant. pdksh seems to be a ksh88 variant, but this guy's shell, whatever it is, might be even older yet..!

It is not necesssary for glob-style matching. A case statement works in all shells.

---------- Post updated at 01:13 PM ---------- Previous update was at 01:09 PM ----------

The dash shell is probably the most POSIX-compliant (i.e., the most standard) shell there is, with the fewest extensions.

In what situations does the read command not work correctly?

The [[ builtin is a kornism and bashism with better-defined semantics than [ (a.k.a. test). If you want your script to be portable as in work with all POSIX-conformant shell script interpreters, you should use the more limited [.

As regards dash and POSIX conformance, according to the current manpage for dash:
Unlike a number of other shells, I do not believe that dash has passed the VSC-PCTS2003 POSIX Certification Test Suite.
It is easy to claim POSIX conformance but a testsuite is the only valid indicator of conformance.

In ksh the bracket type is significant and important. It distinguishes between a "Test" and a "Conditional Expression". There is some common syntax but as soon as you get into boolean expressions they are quite different.

Code:

[[ conditional expression ]]
[ test ]

Please tell me what can be done with [[ ... ]] that cannot be done with [ ... ] or case.

ERE matching in modern bash/ksh93:

Code:

if [[ "a.3.578.yyy" =~ .\.[0-9]\..*y+ ]]; then
  echo hello
fi

@cfajohnson I sort of agree. If you only work in modern "Posix" so called "standard" Shells there is really no need for "[[ condition ]]" syntax because of the enhancements to Shell. I work with new, old and ancient Shell scripts and it is imperative that I can read, understand and change scripts in the context of the "Shell of the day".