program stops after execvp( command.argv[0], comma

问题:

I am writing a small shell program that takes a command and executes it. If the user enters a not valid command the if statement returns a -1. If the command is correct it executes the command, however once it executes the command the program ends. What am I doing wrong that is does not execute the lines of code after it? I have tested execvp( command.argv[0], command.argv) with ls and cat commands so I am pretty sure it works. Here is my code.

  int shell(char *cmd_str ){
  int commandLength=0;
  cmd_t command;
  commandLength=make_cmd(cmd_str,  command);
  cout<< commandLength<<endl;
  cout << command.argv[0]<< endl;
  if( execvp( command.argv[0], command.argv)==-1)
//if the command it executed nothing runs after this line
  {
    commandLength=-1;

}else
{
  cout<<"work"<<endl;
}

  cout<< commandLength<<endl;
   return commandLength;


}

回答1:

From man page of execvp(3)

The exec() family of functions replaces the current process image with a new process image

So your current process image is overwritten with the image of your command! Hence you need to use a fork+exec combination always so that your command executes in the child process and your current process continues safely as a parent!

On a lighter note I want to illustrate the problem with a picture as a picture speaks a thousand words. No offence intended :) :)

回答2:

From the documentation on exec

The exec() family of functions replaces the current process image with a new process image. The functions described in this manual page are front-ends for execve(2). (See the manual page for > execve(2) for further details about the replacement of the current process image.)

If you want your process to continue, this is not the function you want to use.

回答3:

@Pavan - Just for nit-pickers like myself, technically the statement "current process is gone" is not true. It's still the same process, with the same pid, just overwritten with a different image (code, data etc).