Testing if a Variable is a Number in Bash

In Bash, it’s often necessary to verify whether a variable contains a numeric value. This can be useful when validating user input, processing data, or performing arithmetic operations. In this tutorial, we’ll explore different methods for testing if a variable is a number in Bash.

Using Regular Expressions

One approach is to use regular expressions (regex) to match the variable’s content against a pattern that represents a number. The regex ^[0-9]+$ matches any string consisting only of digits from start (^) to end ($). You can use this regex with the [[ ]] conditional expression, like so:

re='^[0-9]+$'
if ! [[ $yournumber =~ $re ]]; then
   echo "error: Not a number" >&2; exit 1
fi

To match numbers with decimal points or signs, you can modify the regex accordingly. For example, ^[+-]?[0-9]+([.][0-9]+)?$ matches numbers with optional signs and decimal points.

Using Pattern Matching

Another approach is to use Bash’s pattern matching feature, which allows you to match a string against a pattern using the case statement:

case $string in
    ''|*[!0-9]*) echo bad ;;
    *) echo good ;;
esac

This method rejects empty strings and strings containing non-digits. However, it doesn’t handle negative or floating-point numbers out of the box.

Using Arithmetic Expansion

A more straightforward approach is to use Bash’s arithmetic expansion feature, which allows you to perform arithmetic operations on a variable. If the variable contains a non-numeric value, the operation will fail, and you can catch the error:

if [ -n "$var" ] && [ "$var" -eq "$var" ] 2>/dev/null; then
  echo number
else
  echo not a number
fi

This method uses the [ -eq ] operator to compare the variable with itself. If the comparison succeeds, it means the variable contains a numeric value.

Tips and Caveats

When testing if a variable is a number in Bash, keep in mind:

• Regular expressions can be powerful but may not cover all edge cases.
• Pattern matching can be simpler but less flexible than regex.
• Arithmetic expansion can be concise but may not work as expected for non-Bash shells or certain input values.

In general, it’s essential to choose the method that best fits your use case and handle potential errors and edge cases accordingly.