Arithmetic operations with expr and variables

Question

Why won't this statement work?

total=`expr $mPercent / 100 "*" .482 + $fPercent / 100 "*" .518`

mPercent is a number as a result of an awk statement, as well as fPercent. I also get "expr: non-numeric argument" as an error message.

NOTE: For the purposes of his question, let's say that the 2 variables have the value 3.27.

Answer 1

bash cannot do floating point operations, only integers. I suggest to use bc -l instead:

$ mPercent='2.72'
$ fPercent='3.14'
$ bc -l <<< $(echo "$mPercent / 100 "*" .482 + $fPercent / 100 "*" .518")
.02937560000000000000

Answer 2

The expr command can only do integer or string manipulations. Look at the man page for this guidance:

Operands are either integers or strings. Integers consist of one or more decimal digits, with an optional leading '-'. 'expr' converts anything appearing in an operand position to an integer or a string depending on the operation being applied to it.

So to perform this type of operation you'll need to enlist a command line caluclator such as bc.

Example

$ mPercent='.123'
$ fPercent='.345'
$ echo "$mPercent / 100 * .482 + $fPercent / 100 * .518" | bc -l
.00237996000000000000

NOTE: expr is not part of Bash, it's a standalone executable that's part of the package coreutils. On systems using RPMs you can see this like so:

$ rpm -qf $(type -p /usr/bin/expr)
coreutils-8.21-13.fc19.x86_64

Using your data that you provided in the comments of 3.27 for the 2 variables yields the following:

$ mPercent='3.27'
$ fPercent='3.27'
$ echo "$mPercent / 100 * .482 + $fPercent / 100 * .518" | bc -l
.03270000000000000000

Answer 3

My suggestion is that use bc for arithmetic operation and <<< (Here String) for inputting the values to bc.

$ mPercent=3.27
$ fPercent=3.27

$ bc -l <<< "$mPercent / 100 * .482 + $fPercent / 100 * .518"
.03270000000000000000

Here -l option to bc will allow you to use floating point arithmetic with more mathematical functions.