#include <stdio.h>
#include <sys/types.h>
#include <pthread.h>
#include <string.h>

void  parse(char *line, char **argv)
{
    while (*line != '')    // If not the end of line...
    {
        while (*line == ' ' || *line == 't' || *line == 'n')
        {
            // Replace white spaces with 0.
            *line++ = '';
        }

        // Save the argument position.
        *argv++ = line;

        while ((*line != '') && (*line != ' ') &&
               (*line != 't') && (*line != 'n'))
        {
            // Skip the argument until...
            line++;
        }
    }

    // Mark the end of argument list.
    *argv = '';
}

void  execute(char **argv)
{
    pid_t  pid;
    int    status;

    // Fork a child process.
    if ((pid = fork()) < 0)
    {
        printf("*** ERROR: forking child process failedn");
        exit(1);
    }
    else if (pid == 0)    // For the child process:
    {
        // Execute the command.
        if (execvp(*argv, argv) < 0)
        {
            printf("*** ERROR: exec failedn");
          exit(1);
        }
    }
    else                 // For the parent:
    {
        // Wait for completion.
        while (wait(&status) != pid);

    }
}

int  main()
{
    char line[1024];             /* the input line                 */
    char *argv[64];              /* the command line argument      */

    // Repeat until done...
    while (1)
    {
        printf("Shell -> ");     /*   display a prompt             */
        gets(line);              /*   read in the command line     */
        printf("n");

        parse(line, argv);       /*   parse the line               */

        // Is it an "exit"?
        if (strcmp(argv[0], "exit") == 0)
          exit(0);               /*   exit if it is                */

        execute(argv);           /* otherwise, execute the command */
    }
}

g++ -pthread test.cpp
test.cpp:16:11: warning: expression which evaluates to zero treated as a null pointer constant of type 'char *' [-Wnon-literal-null-conversion]
  *argv = '';                 /* mark the end of argument list  */
          ^~~~
test.cpp:24:14: error: use of undeclared identifier 'fork'
  if ((pid = fork()) < 0) {     /* fork a child process           */
             ^
test.cpp:26:5: error: use of undeclared identifier 'exit'
    exit(1);
    ^
test.cpp:29:9: error: use of undeclared identifier 'execvp'
    if (execvp(*argv, argv) < 0) {     /* execute the command  */
        ^
test.cpp:31:7: error: use of undeclared identifier 'exit'
      exit(1);
      ^
test.cpp:35:12: error: use of undeclared identifier 'wait'
    while (wait(&status) != pid)       /* wait for completion  */
           ^
test.cpp:51:7: error: use of undeclared identifier 'exit'
      exit(0);            /*   exit if it is                */
      ^
1 warning and 6 errors generated.
Tylers-MacBook-Pro:BlastUniversal tylerpfaff$ g++ -lpthread test.cpp
test.cpp:16:11: warning: expression which evaluates to zero treated as a null pointer constant of type 'char *' [-Wnon-literal-null-conversion]
  *argv = '';                 /* mark the end of argument list  */
          ^~~~
test.cpp:24:14: error: use of undeclared identifier 'fork'
  if ((pid = fork()) < 0) {     /* fork a child process           */
             ^
test.cpp:26:5: error: use of undeclared identifier 'exit'
    exit(1);
    ^
test.cpp:29:9: error: use of undeclared identifier 'execvp'
    if (execvp(*argv, argv) < 0) {     /* execute the command  */
        ^
test.cpp:31:7: error: use of undeclared identifier 'exit'
      exit(1);
      ^
test.cpp:35:12: error: use of undeclared identifier 'wait'
    while (wait(&status) != pid)       /* wait for completion  */
           ^
test.cpp:51:7: error: use of undeclared identifier 'exit'
      exit(0);            /*   exit if it is                */
      ^
1 warning and 6 errors generated.
Tylers-MacBook-Pro:BlastUniversal tylerpfaff$ clang -pthread -c test.cpp
test.cpp:16:11: warning: expression which evaluates to zero treated as a null pointer constant of type 'char *' [-Wnon-literal-null-conversion]
  *argv = '';                 /* mark the end of argument list  */
          ^~~~
test.cpp:24:14: error: use of undeclared identifier 'fork'
  if ((pid = fork()) < 0) {     /* fork a child process           */
             ^
test.cpp:26:5: error: use of undeclared identifier 'exit'
    exit(1);
    ^
test.cpp:29:9: error: use of undeclared identifier 'execvp'
    if (execvp(*argv, argv) < 0) {     /* execute the command  */
        ^
test.cpp:31:7: error: use of undeclared identifier 'exit'
      exit(1);
      ^
test.cpp:35:12: error: use of undeclared identifier 'wait'
    while (wait(&status) != pid)       /* wait for completion  */
           ^
test.cpp:51:7: error: use of undeclared identifier 'exit'
      exit(0);            /*   exit if it is                */