number=42
#every bash variable is a string already
s1="$number"
#s1 is "42"
#printf -v writes the formatted text into a variable
printf -v s2 '%03d' "$number"
#s2 is "042"
#the result of $(( )) is a string, too
value=-12345
s3="$((value))"
#s3 is "-12345"
len=${#s3}
#len is 6
echo "s1 is $s1"
echo "s2 is $s2"
echo "s3 is $s3"
echo "len is $len"