Question

I’m trying to call a shell script from C++ with custom input. What I could do is:

void dostuff(string s) {
    system("echo " + s + " | myscript.sh");
    ...
}

Of course, escaping s is quite difficult. Is there a way that I can use s as stdin for myscript.sh? Ie, something like this:

void dostuff(string s) {
    FILE *out = stringToFile(s);
    system("myscript.sh", out);
}

Answer 1

A simple test to reassign stdin and restore it after the system call:

#include <cstdlib>     // system
#include <cstdio>      // perror
#include <unistd.h>    // dup2
#include <sys/types.h> // rest for open/close
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>

#include <iostream>

int redirect_input(const char* fname)
{
    int save_stdin = dup(0);

    int input = open(fname, O_RDONLY);

    if (!errno) dup2(input, 0);
    if (!errno) close(input);

    return save_stdin;
}

void restore_input(int saved_fd)
{
    close(0);
    if (!errno) dup2(saved_fd, 0);
    if (!errno) close(saved_fd);
}

int main()
{
    int save_stdin = redirect_input("test.cpp");

    if (errno)
    {
        perror("redirect_input");
    } else
    {
        system("./dummy.sh");
        restore_input(save_stdin);

        if (errno) perror("system/restore_input");
    }

    // proof that we can still copy original stdin to stdout now
    std::cout << std::cin.rdbuf() << std::flush;
}

Works out nicely. I tested it with a simple dummy.sh script like this:

#!/bin/sh
/usr/bin/tail -n 3 | /usr/bin/rev

Note the last line dumps standard input to standard output, so you could test it like

./test <<< "hello world"

and expect the following output:

won tuodts ot nidts lanigiro ypoc llits nac ew taht foorp //    
;hsulf::dts << )(fubdr.nic::dts << tuoc::dts    
}
hello world