Testing for unset variables in a for loop

Question

I've written a script which parses a file and sets variables according to a fixed width within the file.

I then want to check each variable and, if it's a blank field, set to "0". A simple variation of this would be:

one=1  
two=""  
three=3  
four=""  

for num in $one $two $three $four  
do  
   echo num is $num  
done

gives me an output of:

num is 1   
num is 3

What I want to do is code for if $two is null then two=0.

How can I do that?

Answer 1

You want the -z comparison operator http://tldp.org/LDP/abs/html/comparison-ops.html

if [ -z $two ]; then   ## or [ -z $num ] to test them all 
    two=0
fi
echo "num is $num"

Answer 2

Your question doesn’t make a lot of sense.  First of all, if you say

one=1
two=""
three=3
four=""

for num in $one $two $three $four
do
    ︙

then the for statement is interpreted as

for num in 1  3

and so there are only two iterations, with num equal to 1 and 3.  If you want to have four iterations, with num equal to 1, (null), 3 and (null), then you need to say

for num in "$one" "$two" "$three" "$four"

You should quote shell variable references unless you have a good reason not to, and you’re sure you know what you’re doing.

So,

for num in "$one" "$two" "$three" "$four"
do
    echo "num is $num"
done

will result in

num is 1
num is 
num is 3
num is 

Now, if you want the results to say 1, 0, 3, 0, it’s a simple matter of saying

for num in "$one" "$two" "$three" "$four"
do
    if [ "$num" = "" ]
    then
        num=0
    fi
    echo "num is $num"
done

Note that

• You must have whitespace before and after the [,
• You must have whitespace before and after the =,
• You must have whitespace before the ] (and after it, if there’s anything else after it), and
• fi is if spelled backwards.

But your question title mentions “unset variables”.  two and four are set to a blank string; five and six (and orange, pumpkin, tiger, and a quasi-infinite list of other potential variable names) are unset.  There’s a difference.  If that is what you’re interested in, you need to ask a different question, because, if you do

for num in "$one" "$two" "$three" "$four" "$five" "$six" "$orange" "$pumpkin" "$tiger"
do
    ︙

then it will be impossible to distinguish the five, six, orange, pumpkin, and tiger, iterations from the two and four iterations — num will be set to a blank string in all seven cases.  You would need to test the $four and $five (etc.) variables directly.

P.S. Don’t ask a new question; do a search.  This has been answered before.