Shell Programming and Scripting

Your function depends on eval executing an empty result if positional parameter 1 has anything but lower case, numbers, or ".". Not sure if that's well received everywhere...?
Try this with only bash buitlins:

Code:

f1()    { FN=${1%.*}
         XT=${1#$FN}
         CR=${FN#${FN//[[:punct:]]}}
         echo "$1 is ${CR:+not }a ${CR:+correct }file${CR:+name}${XT:+ with extension $XT}."
         return  ${CR:+1}
         }
 f1 A
A is a file.
f1 wei
wei is a file.
f1 wei.rd
wei.rd is a file with extension .rd.
f1 wei:rd
wei:rd is not a correct filename.

You don't understand, what I gave was a beginner's example. Some completely foolish function that no sane person on the planet would expect to work! I can't believe you're actually offering alternatives. Everybody knows a filename can be made of ANY character except /. The whole purpose was to explain that one cannot sanely rely on ${var:+set} to tell if a variable is set because a variable can very well be set with null string. I can very well run a function calling :

Code:

f1 arg1 "" arg3

The second argument is SET but it's null string.

set -u is meant to enable errors on referencing an unset variable. That and set -e + trap are the standard, intended solutions for detecting unset variables. I do not see how to implement what you want without calling getenv() in C: and then modifying bash by adding a builtin. See: Using and Writing Bash Dynamically Loadable Built-In Commands by Chris F.A. Johnson