Question

I am trying to learn shell scripting and I am kind of confused with the idea of := or default value

#!/bin/sh                                                                                                                             

echo "Please enter a number \c"
read input
input=$((input % 2))

if [ $input -eq 0 ]
then
    echo "The number is even"
else
    echo "The number is odd"
fi

echo "Beginning of second part"
a="BLA" 
a="Dennis"
echo $a
unset a
echo "a after unsetting"
echo $a
${a:=HI}
echo "unsetting a again"
unset a
echo $a

And I get this

Dennis
a after unsetting

./ifstatement.sh: line 21: HI: command not found
unsetting a again

Answer 1

There isn’t a way to set a value that a variable will always “fall back” to when you un-set it. When you use the unset command, you are removing the variable (not just clearing the value associated with it) so it can’t have any value, default or otherwise.

Instead, try a combination of two things. First, make sure the variable gets initialized. Second, create a function that sets the variable to the desired default value. Call this variable instead of unset. With this combination, you can simulate a variable having a “default” value.