How do I deal with empty user input in a Bash script?

Question

When the script asks me for input, I get an error if I just press Return without typing in anything. How do I fix this?

Here's the script:

 #!/bin/bash

   SUM=0
   NUM=0

   while true
   do echo -n "Pruefungspunkte eingeben ('q' zum Beenden): "
      read SCORE
      if test "$SCORE" == "q"
      then echo "Durchschnittspunktzahl: $AVERAGE."
           break
      else SUM=`expr $SUM + $SCORE`
           NUM=`expr $NUM + 1`
           AVERAGE=`expr $SUM / $NUM`
      fi
   done

Answer 1

How about using good bash practices?

#!/bin/bash

sum=0
num=0

while true; do
   read -erp "Pruefungspunkte eingeben ('q' zum Beenden): " score
   if [[ $score = q ]]; then
      echo "Durchschnittspunktzahl: $average."
      break
   elif [[ $score =~ ^-?[[:digit:]]+$ ]]; then
      ((sum+=10#$score))
      ((++num))
      ((average=sum/num))
   else
      echo "Bad number"
   fi
done

Good practice:

• don't use capitalized variable names
• use the [[ builtin instead of the test builtin
• don't use backticks, use (( to invoke shell arithmetic

• to make sure the user inputs a number, check that a number was really entered. The line

  elif [[ $score =~ ^-?[[:digit:]]+$ ]]; then
  

  just does that (see regular expressions). Incidentally it completely solves your original problem, since an empty input will not pass through this test

• to prevent problems if a user enters 09 instead of 9, force bash to interpret the input in radix 10. That's why I'm using (10#$score) instead of just score.
• Use read with the -p (prompt) option, instead of the clumsy combo echo -n / read

This version is much more robust and well-written than yours. Yet, it still has problems:

• will break if user needs large numbers
• as shell arithmetic is used, only integers can be used. Moreover, the average given by this program is rounded: if you want the average of 1 and 2 you'll have 1.

To fix both problems, you'll probably want to use bc or dc. But that will be the purpose of another question. Or not.

Answer 2

Initialise $SCORE beforehand or handle empty input like you do in q case.

Answer 3

[[ -z "$SCORE" ]] && echo "\$SCORE is zero, e.g. \"\"" 

This will test if the variable SCORE is empty string.

You should also set AVERAGE=0 at the beginning.