Suppose I have the following code
for n in {50..300};
do
(( a = 0.3*$n))
#do something
echo $n
echo $a
done
when I run the code, I received an error, it says
((: a = 0.3*50: syntax error: invalid arithmetic operator (error token is ".3*50")
I know it must be because 0.3 or any decimal number isn't recognizable or perhaps due to some formatting issues, as I previously tried ((a = $n / 2)) which worked fine, much appreciated if anyone can give me a hint.
While this is quite a trivial syntax issue, using shellcheck.net to debug such errors would have been much efficient. Your error line needs to be something like
a=$(echo "0.3*$n" | bc ) # 'echo' to print an arithmetic expression
# feeding it to 'bc' for the actual computation.
expr does.
a: command not found does that mean I need to declare the variable a? but I thought in bash script we do not need to?
expr is the wrong command here; if bash could do floating-point arithmetic, you wouldn't need bc in the first place. It would seem to work since you didn't quote the argument; the expansion of 0.3*$n didn't match any files (* makes it a glob pattern), so was passed literally to expr, which treated it as a single non-numeric argument and simply spit it back out. I've edited the answer to use echo to simply feed a valid expression to bc.(( a = 5*$n )) would work fine as long as $n expands to an integer expression.