How do I test if a variable is a number in Bash?

Question

I just can't figure out how do I make sure an argument passed to my script is a number or not.

All I want to do is something like this:

test *isnumber* $1 && VAR=$1 || echo "need a number" 

Any help?

Answer 1

One approach is to use a regular expression, like so:

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

If the value is not necessarily an integer, consider amending the regex appropriately; for instance:

^[0-9]+([.][0-9]+)?$ 

...or, to handle numbers with a sign:

^[+-]?[0-9]+([.][0-9]+)?$ 

Answer 2

Without bashisms (works even in the System V sh),

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

This rejects empty strings and strings containing non-digits, accepting everything else.

Negative or floating-point numbers need some additional work. An idea is to exclude - / . in the first "bad" pattern and add more "bad" patterns containing the inappropriate uses of them (?*-* / *.*.*)