SHELL

bash if is number

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

check if variable is a number in bash

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

Bash test if variable is a number

[[ $1 == ?(-)+([0-9]) ]] && echo "$1 is an integer"