sending output of a c++ program into a variable bash

Question

I'm trying to write a system that grades a c++ code with pre-written examples that i have ready. It's a very simple c++ code like the following:

#include <iostream>
using namespace std;
int main()
{
    int a;
    cin >> a;
    if (a > 100)
        cout << "Big";
    else
        cout << "Small";
    return 0;
}

So i want to test and grade this program with a bash, declare a score variable and echo it in the end. Here's what i wrote(I've marked where i need help writing with double quotes)

#!/bin/bash
g++ new.cpp -o new
test1=101
test2=78
score=0
if [ "Result of executing ./new with $test1 variable as input"=="Big" ]
then
(( score += 50 ))
fi
if [ "Result of executing ./new with $test2 variable as input"=="Small" ]
then
(( score += 50 ))
fi
echo $score

Also i'm still very new to bash so if you can tell me a simpler way to use bash for the examples (like loops) i'd love to hear it. Thanks!

IMHO, using a temporary file would be easier. Your C++ program should write to a temporary file that your bash script can read. — Thomas Matthews
– Thomas Matthews, Commented Mar 24, 2017 at 20:29
if [ "Result of executing ./new with $test1 variable as input"=="Big" ] is always true, since Result of executing ./new with $test 1 variable as input==Big is a non-empty string. [ is a fickle beast. — William Pursell
– William Pursell, Commented Mar 24, 2017 at 20:31

CrociDB · Accepted Answer · 2017-03-24 20:46:44Z

2

If you want to execute new with the params and get its result, you should try something like this:

#!/bin/bash
g++ new.cpp -o new
test1=101
test2=78
score=0
if [ $(./new $test1) == "Big" ]; then
    (( score += 50 ))
fi
if [ $(./new $test2) == "Small" ]; then
    (( score += 50 ))
fi
echo $score

answered Mar 24, 2017 at 20:46

CrociDB

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2 Comments

Benjamin W. Over a year ago

You should quote the command substitution $() within [ ], or the script breaks if the output of new contains spaces; the output will also be subject to glob expansion if unquoted.

Benjamin W. Over a year ago

Alternatively, you could use [[ ]], which does the quoting for you. == is a Bashism anyway, so why not use other Bash tools as well.

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