Getting "numeric argument required" when running a script with arithmetic operations

Question

I'm trying to write a function that is supposed to convert a time stamp of the form hr:min:sec,ms (i.e 15:41:47,757) to milliseconds. The function is the following:

#!/bin/sh
mili () {

    hr=$(echo "$1" | cut -c1-2)
    echo "hr is: " $hr
    min=$(echo "$1" | cut -c4-5)
    echo "min is: " $min
    sec=$(echo "$1" | cut -c7-8)
    echo "sec is: " $sec
    ms=$(echo "$1" | cut -c10-12)
    echo "ms is: " $ms
    total=$(($hr \* 3600 + $min \* 60 + $sec) \* 1000 + $ms)

    return "$total"
    #echo "Result is: "$total" "
}

mili $1

However, when I run it:

./mili.sh "15:41:47,757"

I get the following output message:

./mili.sh: command substitution: line 15: syntax error near unexpected token 
`\*'
./mili.sh: command substitution: line 15: `($hr \* 3600 + $min \* 60 + $sec) 
\* 1000 + $ms'
./mili.sh: line 17: return: : numeric argument required

I've tried variations of expr with and without single quotes, double quotes, and backticks but can never seem to get it to compute the arithmetic. I can confirm a simple command like this works: expr 2 * 3 but when I try to use something similar in my script it fails.

How can I get it to simply compute my expression?

Answer 1

Inside arithmetic, * does not need to be escaped. Also, some parentheses were missing. Thus, replace:

total=$(($hr \* 3600 + $min \* 60 + $sec) \* 1000 + $ms)

With:

total=$((($hr * 3600 + $min * 60 + $sec) * 1000 + $ms))

Alternative

The code can be simplified avoiding the need for multiple calls to cut:

mili() {
    IFS=':,' read hr min sec ms <<<"$1"
    echo "hr is: " $hr
    echo "min is: " $min
    echo "sec is: " $sec
    echo "ms is: " $ms
    total=$((($hr * 3600 + $min * 60 + $sec) * 1000 + $ms))
    echo "Total=$total"
    return "$total"
}

Aside: Bash arithmetic and dollar signs

In a Bash arithmetic context, the dollar sign before a variable is optional. For example:

$ a=1; echo "$((1 + a)) and $((1+ $a))"
2 and 2

While some style guides recommend omitting $ in an arithmetic context, there is a key difference. As Chepner points out in the comments, the treatment of undefined variables is very different:

$ unset a
$ echo $((1 + $a))
bash: 1 + : syntax error: operand expected (error token is "+ ")
$ echo $((1 + a))
1

In summary:

1. If you want an undefined variable to default to zero, then omit the $.

2. If you want an undefined variable to be replace with nothing, possibly causing an invalid expression, then include the $.

In the shell function mili, an undefined variable hr, min, etc., would indicate a code error and we might want an error message to warn us about it and we would want to include the $. In other circumstances where a default value of zero is reasonable, we would not and omitting the $ would be correct.