How to stop the first child process from being executed? - c++

Question

Aim: To design a linux shell, which shows a prompt to take input from user, creates a new process to execute that command then terminates/exits the process. Here is my code
#include <iostream>
#include <unistd.h>
#include <sys/wait.h>
#include <string.h>
using namespace std;
string cmd; //global string so cmd copied to child to execute
void HandleAsParent(){
cout<<"Linux Shell 1.0\n";
string s;
while (!exitflag) {
cout<<"myShell>";
getline(cin,cmd); //Take user input
fork();
wait(NULL);
}
}
void HandleAsChild(){
cout<<"Executing";
system(cmd.c_str());
}
int main() {
pid_t p = fork();
if(p != 0){
HandleAsParent(); //This is parent process
}
else {
HandleAsChild(); //This is child process
}
}
The problem is that, because of the first fork() call in the main,
myShell>Executing
is displayed on the first line when the program runs instead of just
myShell>
.
I am able to understand why this is happening but cannot figure out how do I stop that first child process from being executed.
Please suggest me workarounds/solutions to my problem.
Edit 1: This is one of my Assignment(for learning UNIX Processes)
questions, and It is clearly stated that the program " prompts the
user for a command, parses the command, and then executes it with a
child process "

Answer 1

As I already guessed, system() probably uses a combination of fork(), exec() and wait(). Out of curiosity, I googled for source code and found one on woboq.org: glibc/sysdeps/posix/system.c.
This in mind, using system(), the required child process "comes for free". So, I got this minimal sample:
#include <iostream>
void callCmd(const std::string &cmd)
{
system(cmd.c_str());
}
int main()
{
std::cout << "My Linux Shell 1.0\n"
<< "Type exit[Enter] to exit.\n";
for (;;) {
std::cout << "> ";
std::string input; std::getline(std::cin, input);
if (input == "exit") return 0;
callCmd(input);
}
}
Compiled and tested on cygwin on Windows 10:
$ g++ -std=c++11 -o mycroShell mycroShell.cc
$ ./mycroShell
My Linux Shell 1.0
Type exit[Enter] to exit.
> echo "Hello"
Hello
> exit
$
After getting this running, the system() call in callCmd() can be replaced by fork()/exec()/wait() without the necessity to change anything else.
A simplified version could look like this:
#include <iostream>
#include <string>
#include <vector>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
void callCmd(const std::string &input)
{
// the pre-processing: split the input into command and arguments
std::string cmdArgs = input;
std::vector<char*> args;
char *cmd = &cmdArgs[0];
args.push_back(cmd);
for (char *c = cmd; *c; ++c) {
if (*c == ' ') {
*c = '\0'; args.push_back(c + 1);
}
}
args.push_back(nullptr); // append terminator
// simple replacement of system() (not that sophisticated)
int ret = fork();
if (ret < 0) { // failure
std::cerr << "Failed to execute '" << cmd << "'!\n";
} else if (ret == 0) { // child
execvp(cmd, args.data());
} else { // parent
waitpid(ret, nullptr, 0);
}
}
int main()
{
std::cout << "My Linux Shell 1.1\n"
<< "Type exit[Enter] to exit.\n";
for (;;) {
std::cout << "> ";
std::string input; std::getline(std::cin, input);
if (input == "exit") return 0;
callCmd(input);
}
}
Compiled and tested on cygwin on Windows 10 again:
$ g++ -std=c++11 -o mycroShell mycroShell.cc
$ ./mycroShell
My Linux Shell 1.1
Type exit[Enter] to exit.
> /usr/bin/echo "Hello"
"Hello"
> exit
$
Notes:
IMHO, the most tricky part of this is to prepare a proper argument vector for execvp.
I tried with echo "Hello" as well and it worked. This surprised me a bit as echo is a bash built-in command. I assume that it found /usr/bin/echo and used it as well as in my above output.
The error handling is rather poor – something which should be extended for serious applications.