check if parameter is integer with numeric expression - regex

Question

The following program checks if the first parameter is a number, or a not a number. The best I could come up with so far is:
#/bin/bash
if (( $# )) && (( $1 != 0 )) ; then
echo "number"
else
echo "not number"
fi
This works for numbers like 1, -1, 100, etc, for strings a, abc, a431, or if no parameter is given.
It works with a warning for strings like 1a1, 1.3, -1.2
But FAILS (obviously) if the parameter is 0
How could this be fixed? (using arithmetic expressions)
Related: Bash integer comparison

Answer 1

You can press the printf built-in command to do the job:
if printf "%d" ${1:-foo} > /dev/null; then
echo "number"
else
echo "NOT a number"
fi
printf exits with non-zero status if the conversion fails. The ${1-...} notation
provides a known not-number if $1 isn't set.

Answer 2

You could use a regex:
#/bin/bash
if [[ "$1" =~ ^-?[0-9]+$ ]] ; then
echo "number"
else
echo "not number"
fi
If you wanted it to handle floating point, you could tweak it easily enough:
#/bin/bash
if [[ "$1" =~ ^-?[0-9]+$ || "$1" =~ ^-?[0-9]*\.[0-9]+ ]] ; then
echo "number"
else
echo "not number"
fi

Answer 3

I would rather use something like
if [ $1 = "$(echo $1 | awk '{print strtonum($0)}')" ]; then
echo "number"
else
echo "not number"
fi

Answer 4

Solution using extended globs
#/bin/bash
shopt -s extglob
if [[ $1 = *[0-9]* && $1 = ?([+-])*([0-9])?(.*([0-9])) ]]; then
echo "Is a number"
else
echo "Not a number"
fi
Output
$ ./test -1
Is a number
$ ./test 0
Is a number
$ ./test 1
Is a number
$ ./test a
Not a number
$ ./test -1.2
Is a number
$ ./test 0.0
Is a number
$ ./test 1.2
Is a number
$ ./test a.b
Not a number

Answer 5

Might be easier to test for what is not a number:
if (( $# )) ; then
if [[ $1 == *[^0-9]* ]] ; then
echo "Not a number"
else
echo "Is a number"
fi
fi
Note that the above is actually testing only for positive integers. Add +-. to the character class and it will work, sort of, for positive/negative integers and decimal fractions. I say sort of because it would pass 123.456.789 as a number, but it's not valid. You'd need to add additional logic to weed out misplaced or multiple signs and decimal points (and strings like "+", ".", etc.).