`sudo` to append to file with root permissions (in

Question

I am attempting to use sudo to write to a file in /etc that is writable only by root, from inside an unprivileged C program.

system("sudo /bin/sh -c 'echo 'iface wlan0 inet dhcp' >> /etc/network/interfaces'");

This doesn't appear to fail, but the file is unmodified. What is wrong?

Answer 1

For the sake of completeness, here is how I'd do that "the C way":

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

int main(void) {
    const char string_to_append[] = "iface wlan0 inet dhcp\n";
    size_t string_size;
    FILE *fd;

    /* Check for permissions */
    if (getuid()) {
        fprintf(stderr, "Run as root!\n");
        return EXIT_FAILURE;
    }

    /* Now try to open the file for appending
     * NOTE: fopen() will fail anyway if you dont have enough
     * permissions to open this file with the specified mode,
     * so the above check calling getuid() is somewhat redundant
     */
    if ((fd = fopen("/etc/network/interfaces", "a")) == NULL) {
        perror("Error opening file");
        return EXIT_FAILURE;
    }

    /* Actual writing happens here */
    string_size = strlen(string_to_append);
    if (fwrite(string_to_append, sizeof(char), string_size, fd) != string_size) {
        fprintf(stderr, "Error writing to file\n");
        return EXIT_FAILURE;
    }

    return EXIT_SUCCESS;
}

Answer 2

Structured as you have it, the redirection is evaluated outside the sudo operation, and therefore does not have root privileges and cannot open /etc/network/interfaces for writing. You need to do it like this instead:

system("sudo /bin/sh -c \"echo 'iface wlan0 inet dhcp' >> /etc/network/interfaces\"");

so that the redirection is evaluated by the shell inside the sudo.

(Also, you can't nest single quotes inside single quotes.)