How to capture C++ output to a Bash variable?

Question

I'd like to compile and run a C++ program via Bash and then capture the output produced by cout (which will be a int) to a variable in the Bash script.

I'm currently able to direct the output to a text file like so:

./prog >> output.txt

My research has led me to this, which appears to be creating an empty string:

output=$(./prog) | bc -l

In case it's relevant, I'm trying to capture this value so that I can average the output of my program over multiple executions. Here's my entire script as it currently stands:

count=1
while [ $count -le 8192 ]
do
    sum=0
    arraysize=$(( $count * 1024 ))
    g++-7 prog.cpp -DGLOBAL=${arraysize} -DLOCAL=32 -o prog -lm -fopenmp -framework OpenCL
    for try in {0..9}
    do
        output=$(./prog) | bc -l
        echo ${output} # this line appears to print an empty string
        sum=$(( (sum + output) ))
    done
    echo ${sum / 10}
    count=$(( $count * 2 ))
done

I know that there are likely other errors in the script above, but the one that's currently hanging me up is just capturing the value produced by the program.

Why are you piping to bc? Maybe try to move that into the parenthesis, if you actually want to capture the output bc gives for the output of your program. — sticky bit
– sticky bit, Commented Jun 1, 2018 at 1:05
@AlexJohnson Anything that looks like a number will be treated as a number. You can also use typeset -i output to declare the variable to hold an integer. — Barmar
– Barmar, Commented Jun 1, 2018 at 1:44

user5550963 · Accepted Answer · 2018-06-01 01:03:31Z

4

Here is what could help:

var=$(./prog)

answered Jun 1, 2018 at 1:03

user5550963

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Comments

Sean F · Accepted Answer · 2018-06-01 01:04:19Z

3

There's more than one way, but one is to use the backtick character:

VAR=`./prog`

answered Jun 1, 2018 at 1:04

Sean F

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2 Comments

l'L'l Over a year ago

Why is $(...) preferred over `...` (backticks)?

Alex Johnson Over a year ago

This (and $(...), for the record) appear to have solved my problem. Thank you.

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