[Linux] system(3)

system(3) is a C library function and also can be used in perl.

SYNOPSIS in Perl

system LIST
system PROGRAM LIST

Description

• system will do a fork first and then the parent will wait fro the child process, where the command is running on, to exit.

• The return code from the command is got by wait(&ret) in parent process. An example of fork can be found by $man 2 wait.

  /**
   * wait(2)
   */
  #include <sys/wait.h>
  #include <stdlib.h>
  #include <unistd.h>
  #include <stdio.h>
  
  int main(int argc, char *argv[])
  {
      pid_t cpid, w;
      int status;
  
      cpid = fork();
      if (cpid == -1) { perror("fork"); exit(EXIT_FAILURE); }
  
      if (cpid == 0) {
          /* Code executed by child */
          printf("Child PID is %ld\n", (long) getpid());
          if (argc == 1)
              pause();                    /* Wait for signals */
          _exit(atoi(argv[1]));           /* exit code from child process */
      } else {
          /* Code executed by parent */
          do {    
              /* get exit code of child process in status */
              w = waitpid(cpid, &status, WUNTRACED | WCONTINUED); 
              if (w == -1) { perror("waitpid"); exit(EXIT_FAILURE); }
  
              if (WIFEXITED(status)) {
                  printf("exited, status=%d\n", WEXITSTATUS(status));
              } else if (WIFSIGNALED(status)) {
                  printf("killed by signal %d\n", WTERMSIG(status));
              } else if (WIFSTOPPED(status)) {
                  printf("stopped by signal %d\n", WSTOPSIG(status));
              } else if (WIFCONTINUED(status)) {
                  printf("continued\n");
              }
          } while (!WIFEXITED(status) && !WIFSIGNALED(status));
          exit(EXIT_SUCCESS);
      }
  }
      

• The return code of system is exactly the same as status in above example. It's a 16-bit value with high 8-bit for exit code and low 8-bit for signal id. WEXITSTATUS just extracts exit code from status and like status >> 8.

• use $man 3 system to get its reference in C and $perldoc -f system to get in perl.

• See this link for more information

  http://perldoc.perl.org/functions/system.html