Testing for Zero or Positive Whole Number

Question

How can I test whether a number in a bash script is 0 (zero) or a positive whole number?

Answer 1

The common idiom for that is to check if the variable's value consists of nothing but numbers. With newer bash shells, you can use =~ and a regular expression:

zero_or_positive_integer(){
    for i in "$@"; do
        if [[ "$i" =~ ^[0-9]+$ ]]; then
            echo "YES $i is either 0 or a positive integer"
        else
            echo "NO $i is neither 0 nor a positive integer"
        fi
    done
}

Which results in:

$ zero_or_positive_integer foo -2 1.5 0 12
NO foo is neither 0 nor a positive integer
NO -2 is neither 0 nor a positive integer
NO 1.5 is neither 0 nor a positive integer
YES 0 is either 0 or a positive integer
YES 12 is either 0 or a positive integer

Answer 2

To do tests, you typically use the [ or [[ operator. [ is a synonym for the command test (not part of Bash specifically)

$ help [
[: [ arg... ]
    Evaluate conditional expression.
    
    This is a synonym for the "test" builtin, but the last argument must
    be a literal `]', to match the opening `['.

$ man test
[...]
       INTEGER1 -eq INTEGER2
              INTEGER1 is equal to INTEGER2
       INTEGER1 -ge INTEGER2
              INTEGER1 is greater than or equal to INTEGER2
[...]

So it looks like you can write (don't forget spaces -- [ is just a built-in program, so keep its arguments separate)

if [ "$var" -eq 0 ]; then
  echo "Zero"
elif [ "$var" -gt 0 ]; then
  echo "Positive"
fi

Note that if $var is not an integer, this will result in an error. We're assuming $var is some integer already.

If you're not sure that $var is an integer, I'm not aware of a great test, so here's some code using regular expressions:

if [[ "$var" =~ ^[1-9][0-9]+$ ]]; then
  echo "Positive"
elif [[ "$var" = 0 ]]; then
  echo "Zero"
else
  echo "Other"
fi

Answer 3

The printf Bash built-in will read a string and verify that it can be successfully converted as specified by the format string. You can check the status "${?}", use -v to put the value into a temporary variable, and discard the error message with 2>/dev/null.

$ a=-56
$ printf -v q '%d' "${a}"; echo $? "${q}"
0 -56
$ a=-56.3
$ printf -v q '%d' "${a}"; echo $? "${q}"
bash: printf: -56.3: invalid number
1 -56
$ a=315.8e4
$ printf -v q '%d' "${a}"; echo $? "${q}"
bash: printf: 315.8e4: invalid number
1 315
$ printf -v q '%f' "${a}"; echo $? "${q}"
0 3158000.000000