assign output of a c program to a shell variable: token error

Question

I have a shell script to run several times a .c program ('switch') which admit 3 input paramenters. I want to run the program 4 times passing values {4,16,32,64}, and for parameters {0.1,0.2,.3,.4,.5,.55,.575,.6,.625,.65,.7,.75,.8,.9}, building a table of average and confidence interval. The script is the following

---

#!/bin/bash  

if [ -e salida.txt ]; then  
  # echo "File exists"  
  rm salida.txt  
fi  
touch salida.txt  

touch sal1.txt  
touch sal2.txt  
touch sal3.txt  
touch sal4.txt  
touch salida.txt  

num=1000000  
stud=3.182  
    for j in {0.1,0.2,.3,.4,.5,.55,.575,.6,.625,.65,.7,.75,.8,.9}  
    do  
    rm sal1.txt sal2.txt sal3.txt sal4.txt  
    for i in {4,16,32,64}  
    do  
        margen=0  
        sum=0  
        avg=0  
        for k in {1..4}  
        do  
            a$k=$((`./switch -N$i -r$j -n$num`))  
            sum=$((sum + a$k))  
        done  
        avg=$((sum/4))  
        dvt=0  
        for k in {1..4}  
        do  
            dvt=$((dvt + (a$k - $avg)*(a$k - $avg)))  
        done  
        dvt=$((dvt /3))   
        dvt=$((echo `sqrt($dvt) | bc -l`))  
        margen=$((dvt*$stud/2))  
        echo $avg $margen >> sal$i.txt   
    done  
    join sal1.txt sal2.txt sal3.txt sal4.txt >> salida.txt  
done  

---

Nevertheless is giving me error in line 27

a$k=$((`./switch -N$i -r$j -n$num`))

and lines

dvt=$((echo `sqrt($dvt) | bc -l`))  
margen=$((dvt*$stud/2))  

What am I doing wrong?

Answer 1

The first two lines you identified are using $((command)) to execute a command and capture its output.  That should be $(command).  Your uses of $((expression)) to evaluate a mathematical expression are correct.

Also, the square root line should be

dvt=$(echo "sqrt($dvt)" | bc -l)

rather than

dvt=$((echo `sqrt($dvt) | bc -l`))

or even

dvt=$(echo `sqrt($dvt) | bc -l`)

The problem with the margen= command may be that you’re passing it a float as one of the terms of the expression.

Answer 2

I would restructure the setting of a$k to an associative array like this:

    declare -a a
    for k in {1..4}  
    do  
        a[$k]=$((`./switch -N$i -r$j -n$num`))  
        sum=$((sum + a[$k]))  
    done  

This would allow for the array, a to be accessible after it's populated with values from the above for loop to other loops further down in your code.

This line further down int the last for loop also needs to be adjusted like so:

        dvt=$((dvt + (a[$k] - $avg)*(a[$k] - $avg)))  

Floats with $((..))

I do not believe you can add floats using the $((...)) notation. At least when I tried it, it did not work:

$ echo $((0.10 + .20))
bash: 0.10 + .20: syntax error: invalid arithmetic operator (error token is ".10 + .20")

To perform those operations with floats as tokens you'll need to use bc or a calculator that can perform these.

$ echo "0.10 + .20" | bc
.30