Is there an easy way to determine if user input is an integer in bash?

Question

I am a new student to bash scripting, and I am stumped on an assignment question. I was wondering if there is an easy way to determine whether a users' input is an integer or not. More specifically, if a user is prompted to input an integer, is there a quick check to validate?

Answer 1

One way is to check whether it contains non-number characters. You replace all digit characters with nothing and check for length -- if there's length there's non-digit characters.

if [[ -n ${input//[0-9]/} ]]; then     echo "Contains letters!" fi 

Another approach is to check whether the variable, evaluated in arithmetic context, is equal to itself. This is bash-specific

if [[ $((foo)) != $foo ]]; then     echo "Not just a number!" fi 

Answer 2

This is kind of a kludge, it's using -eq for something other then what it was intended, but it checks for an integer, if it doesn't find an int it returns both an error which you can toss to /dev/null and a value of false.

read input   if [[ $input ]] && [ $input -eq $input 2>/dev/null ]   then      echo "$input is an integer"   else      echo "$input is not an integer or not defined"   fi