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As the title says, I have a string which contains a bash command given by input(for example, command="ls -l") and I need to execute it from a C program. I know I could simply use system(command), but it's a school exercise and I can't use system, nor popen. I thought about using exec*, but it would require to parse the string to separate the command and the various parameters. Is there a system call (not system) which allows me to simply execute the command with its parameters without having to separate them? Thank you in advance for your answers :)

user3523375
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  • nope, the point should be to learn using fork(). The exercise asks to execute n bash commands at the same time. But my professor said that system() causes a lot of security issues so we should learn to avoid it... Anyway, if I don't find anything else I have no problem doing the string parse...I just wanted to find a more "elegant" solution :D – user3523375 Apr 11 '14 at 11:04

2 Answers2

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First of all, that's not a "bash" command. bash is a shell, but what you have there is a program command line.

You should look into the syscalls

  • fork – used to create a new process
  • execve – used to replace the process image with the program from a different binary.
  • waitpid – used to wait for termination of the forked process

To give you a head start, here's how you launch a shell from your program without invoking system(…):

pid_t spawnshell(void)
{
    char *argv[]={"/bin/sh", 0};
    char *envp[]={0};

    pid_t shpid = fork();
    if(!shpid) {
        execve(argv[0], argv, envp);
        perror("execve");
        assert(0 && "execve failed");
        _exit(-1);
    }

    return shpid;
}
datenwolf
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5

This is a way to execute a command without parsing the command and its various parameters: 

execl("/bin/sh", "/bin/sh", "-c", "your-command-without-parsing", 0);
  • man, that's perfect! Works like a charm...by the way, how does it work? I can't understand why you passed "/bin/sh" as the second argument. Also, shouldn't the last argument be a zero? – user3523375 Apr 11 '14 at 13:51
  • You are right, the last argument must be a zero. As for the second arg, you can take a look at the description of execl: http://linux.die.net/man/3/execl. Or take a look at http://stackoverflow.com/questions/12596839/how-to-call-execl-in-c-with-the-proper-arguments –  Apr 11 '14 at 14:53