Trying to calculate radius and area of circle in B

问题:

I'm trying to write a basic script to calculate the radius and area of a circle, where PI=3.14, and the circumference is given. I am very very new to scripting, and I can't seem to figure this out.

#!/bin/bash
PI=3.14
CIRC=5
RAD=echo "((CIRC/2*PI))" | bc-l
printf "Radius: %.2f" $RAD
AREA=echo "((PI*RAD**2))" | bc-l
printf "Area: %.2f" $AREA

The sum of both equations are not being stored in those variables, and I have no idea why. I hope someone can help explain.

回答1:

Below script would do it :

#!/bin/bash
pi=3.14
circ=5
rad=$( echo "scale=2;$circ / (2 * $pi)" | bc )
printf "Radius: %.2f\n" $rad
area=$( echo "scale=2;$pi * $rad * $rad" | bc )
printf "Area: %.2f\n" $area

Notes

1. See [ command substitution ].
2. Never use full uppercase variables in your script as they are usually reserved for the system, check [ this ].
3. scale with bc controls the precision, check [ this ].

回答2:

1. Since bc can print strings, there's no need for printf. Nor backticks or $(), or even some of the variables. With bash, the echo can be replaced with <<<:

   #!/bin/bash
   PI=3.14
   CIRC=5
   bc <<< "scale=2; r=$CIRC/(2*$PI)
           print "'"Radius: ", r, "\nArea: ", '"$PI"'*(r^2), "\n"'
   

2. POSIX shell code version, using a here document:

   #!/bin/sh
   PI=3.14
   CIRC=5
   bc << snip
         scale=2; r=$CIRC/(2*$PI)
         print "Radius: ", r, "\nArea: ", $PI * (r^2), "\n"
   snip
   

3. Pure bc:

   #!/usr/bin/bc -q
   pi=3.14; circ=5; scale=2; r=circ/(2*pi)
   print "Radius: ", r, "\nArea: ", pi*(r^2), "\n"
   quit