KornShell - Test with variable that may be not set

Question

I have the following code in KornShell (ksh):

FAILURE=1
SUCCESS=0

isNumeric(){

    if [ -n "$1" ]; then
        case $1 in
            *[!0-9]* | "")  return $FAILURE;
                       * )  return $SUCCESS;
        esac;
    else
        return $FAILURE;
    fi;
}

#...
FILE_EXT=${FILE#*.}

if [ isNumeric ${FILE_EXT} ]; then
    echo "Numbered file."
fi
#...

In some cases the file name not have an extension, and this causes the FILE_EXT variable to be empty, which causes the following error: ./script[37]: test: 0403-004 Specify a parameter with this command.

How should I be calling this function so that I do not get this error?

Solution 1

You should leave off the square brackets when you are testing the exit code of a function, otherwise, you'll always get "true". Also, you should quote your variable. You could put an additional test for an empty extension as shown:

FILE_EXT=${FILE#*.}

if isNumeric "${FILE_EXT}" &&
    [ "${FILE_EXT}" != "${FILE}" -a "${FILE_EXT}" != "" ]
then
    echo "Numbered file."
fi

Edit: added test to handle filenames that end in "."

Solution 2

you should use ${FILE##*.} with double "#" instead. also what do you mean the variable $FILE_EXT will be empty? if your file don't have extension, then when you do ${FILE#*.} you will get the just file name in FILE_EXT. how is it empty ?

Solution 3

Assuming ksh93, it should be possible to use it's own arithmetic. But we need to be careful: Just ((n)) will fail if n==0, so we test for ((n || !n)) which should always be true for any proper number.

To prevent ksh from exiting, we run the expression in a subshell ( ), adding spaces to prevent conflicts with the arithmetic expression (( ... )).

Finally, we close stderr with '2>& -' to prevent any error messages from non numeric arguments, although you might want to keep them in.

function isNumeric { 
  (
    typeset n=${1:?}
    ((n||!n))
  ) 2>& -
}

Solution 4

I would do this:

if [ isNumeric ${FILE_EXT:="no"} ]; then
    echo "Numbered file."
fi

If all you want to do is determine that the file has a numeric extension

The ${FILE_EXT:="no"} will expand to either the value of FILE_EXT, or 'no' if FILE_EXT is null or empty.

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