如何在Bash中将字符串转换为整数？

string="123"
integer=$(expr "$string" + 0)
echo $integer

string="123"
integer=$((string + 0))
echo $integer

string="123"
integer=$(($string + 0))
echo $integer

string="123"
integer=$(awk -v str="$string" 'BEGIN {print int(str)}')
echo $integer

string="123"
integer=$(echo "$string + 0" | bc)
echo $integer

string="123abc"
if [[ $string =~ ^[0-9]+$ ]]; then
    integer=$(echo "$string + 0" | bc)
    echo $integer
else
    echo "Error: '$string' is not a valid number."
fi

string=""
if [ -z "$string" ]; then
    echo "Error: String is empty."
else
    integer=$(echo "$string + 0" | bc)
    echo $integer
fi