C Program to Run External Command using System (Synchronous)

in #programming • 4 years ago

We can use the system function to run an external command. The method invokes the command synchronously and return the status code. Given the following C program, we first concatenate all command line paramters into a string/char buffer of size 256. And then we invoke the command string using system function from stdlib header. And return the status code as the main function.

This is just a wrapper of the command. You can echo $? to see the status code:

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

int main(int argc, char* argv[]) {
    if (argc == 0) {
        return 0;
    }
    char cmd[255];
    int s = 0;
    for (int i = 1; i < argc; ++ i) {
        strcpy(cmd + s, argv[i]);        
        s += strlen(argv[i]);
        cmd[s ++] = ' ';
    }
    cmd[s] = '\0';
    printf("Running `%s` ...\n", cmd);
    int ret = system(cmd);
    printf("Return Code: %d\n", ret);
    return ret;
}

Compiling the above C source e.g. run.c

$ gcc run.c -o run

And then run:

$ ./run echo HelloACM.com
Running `echo HelloACM.com ` ...
HelloACM.com
Return Code: 0
$ echo $?
0

--EOF (The Ultimate Computing & Technology Blog) --

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4 years ago in #programming by
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     4 years ago 

    Do not forget to mention that system 1) fork the shell (/bin/sh on nix systems) and 2) it can be a security risk and shouldn't be used lightheartly...

    An interesting fact about sprintf is that it returns the number of bytes actually written. Hence the concatenation loop could look like:

          for (int i = 1; i < argc; ++i) {
                s += sprintf(buf + s, "%s ", argv[i]);
        }

    An extra space will be written at the end of the buffer, and there's no check for buffer overflow (another security risk), as in your version.

    [-]
     4 years ago 

    Nice. thanks!

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