Question

I have a script which takes user input, the REFRESH option is optional. I need to test to see if $REFRESH exists and is equal to the string “REFRESH”, if it is then run a specific block of code.

The user would execute

./export_data.sh <user> <type> [REFRESH]

If I was doing this in PHP I would simply use the isset() function, does an equivelent exist in ksh?

I have tried the following but this fails as in the 2nd test $REFRESH is not set:

if [ -n $REFRESH ] && [ $REFRESH == "REFRESH" ]
then
    echo "variable is set and the expected value";
    # do stuff
fi

The only other way I can think to do this is a nested if but this seems messy:

if [ -n $REFRESH ]
then
    if [ $REFRESH == "REFRESH" ]
    then
        echo "variable is set and the expected value";
        # do stuff
fi

Is there a better way to do this?

Answer 1

if [ "${REFRESH:-unset}" = "REFRESH" ]
then ...

This substitutes unset if there is no value in $REFRESH or if the value is the empty string. Either way, it is not the same as “REFRESH”, so it behaves as required.

When testing variables, enclose them in double quotes; it saves angst. In fact, it would mean that you could simply write:

if [ "$REFRESH" = "REFRESH" ]
then ...