How to pass integers values to a C program executable file through shell script?

Question

I want to write a simple shell script to automate running 5 test cases for a C program that prints the 3 inputted integers in ascending order.

My shell script looks like:

./a.out << 15 25 97
./a.out < 73 36 12
./a.out < 43 15 99
./a.out < 100 100 100
./a.out < 37 150 37

Clearly, this is wrong because I have been playing around with the syntax and trying different things to see what works but I hope it is clear what I am trying to do here -- Just running the program with 3 different integer inputs every time.

The one method that I tried and worked is putting each of my test cases in a text file but it seems like a bit of a hassle to do that if I want to change the input every time. Looking for a simpler concise solution.

The input reading snippet of the C program is:

int main() {
  int m, n, p;
  //Read m, n, p
  printf("Give values of m, n and p = ");
  scanf("%d%d%d",&m,&n, &p);

The values of m, n and p are then used for calculation.

Please provide a minimal reproducible example of your C program so what we can see which method of geting info it expects. — Yunnosch
– Yunnosch, Commented Sep 6, 2021 at 15:01
On a side note: choose your test cases systematically, not arbitrarily, and make it easy to tell right from wrong. Does testing 15 25 97 provide any information that 1 2 3 doesn't? Does your code depend on the magnitude of the numbers? — molbdnilo
– molbdnilo, Commented Sep 6, 2021 at 15:03
Another note: C and C++ are very different languages. Please don't tag both unless you're asking specifically about their differences. — molbdnilo
– molbdnilo, Commented Sep 6, 2021 at 15:08
@imtryin123 : You can only pass strings as argument to a new process, but C has a standard function atoi, where you can convert the string to an integer. However, you forgot to declare the arguments to your main function. This is discussed here — user1934428
– user1934428, Commented Sep 7, 2021 at 7:47

molbdnilo · Accepted Answer · 2021-09-06 15:04:40Z

2

< and > do redirection from and to files, not from strings.

| creates a pipeline between processes, and echo outputs to standard output.

You want the pattern

echo 1 2 3 | program

answered Sep 6, 2021 at 15:04

molbdnilo

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that other guy · Accepted Answer · 2021-09-06 15:16:34Z

0

If you want to read the numbers with scanf, you can use Bash here-strings:

#!/bin/bash
./a.out <<< "15 25 97"
./a.out <<< "73 36 12"

You can also change your program to read arguments:

int main(int argc, char** argv) {
  int m, n, p;
  if (argc<4) {
    printf("Not enough numbers\n");
    return 1;
  }
  m=atoi(argv[1]);
  n=atoi(argv[2]);
  p=atoi(argv[3]);
}

and then run ./a.out 15 25 97

answered Sep 6, 2021 at 15:16

that other guy

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Comments

user16719198 · Accepted Answer · 2021-09-06 15:41:05Z

inputted integers

These are for most shells and called programs not integers(, yet). You missed the special <<< operator:

$ cat <<<"1 2 3"
1 2 3

Or like commented: echo 1 2 3 | cat

This is reading standard input like a file content. (From cat's perspective). A more useful example:

$ bc <<<"1+2"
3

cat 1 2 3 gives No such file three times. But your a.out could be expecting 3 or more whitespace separated "words" that can be converted to C integers. Just like cat expects filenames there to convert to their contents.

That is passing command line arguments. The program reads an array of strings filled by the shell.

Cf. cpio -o archiving command. With the only non-optional (besides one of -o create or -i extract or -p) argument it is:

cpio -o < name-list

This seems unpractical first. No filenames as arguments, only a compulsory file. But the idea is that:

find ... | cpio -o >bu.cpio

is better than:

cpio -o $(find ...) >bu.cpio

Or: | is simpler than xargs.

Cfcf. tar for the other approach.

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