I was creating a program that calculates the area of circle, but bash doesnt compile and execute due to error message in the title. Here is my code:

    elif [ $num -le 6 ] && [ $num -ge 4 ]
  then
      read -p "Enter radius: " radius
      let areaCirc=("scale=2;3.1416 * ($radius * $radius)"|bc)
      echo "Area of the circle is: " $areaCirc

and the error message is: syntax error near unexpected token '|'

can someone help me?

CodePudding user response：

1. To send a string to a command via stdin, use a here-string command <<< string, not a pipe.
2. Command substitution syntax is $(...), not (...).
3. Don't use let here. Shell arithmetic only supports integers.

areaCirc=$(bc <<< "scale=2;3.1416 * ($radius * $radius)")

CodePudding user response：

let provides arithmetic context, but we have an ambiguity here, because in a let expression, the vertical bar (|) means bitwise or, but in the shell it has also the meaning of a pipe operator. Look at the following examples:

let bc=4
let a=(4 bc)   # Sets a to 8
let b=("5 bc") # Sets b to 9

let c=("(2 4)|bc")

This is syntactically correct and sets c to 6 (because 2 4 equals 6, and the bitwise or of 6 and 4 equals 6).

However if we decided to quote only part of the argument, i.e.

let c=("(2 4)"|bc)

or don't quote at all, i.e.

let c=((2 4)|bc)

we get a syntax error. The reason is that the shell, when parsing a command, first separates it into the different commands, which then are strung together. The pipe is such an operator, and the shell thinks that you want to do a let areaCirc=("scale=2;3.1416 * ($radius * $radius)" and pipe the result into bc. As you can see, the let statement is uncomplete; hence the syntax error.

Aside from this, even if you would fix it, your using of let would not work, because you are using a fractional number (3.1416), and let can do only integer arithmetic. As a workaround, either you do the whole calculation using bc, or some other language (awk, perl,...), or, if this is an option, you switch from bash to zsh, where you can do floating point arithmetic in the shell.