我正在尝试编写一个 shell 程序来一次执行多个命令

Question

My Code

#include<stdio.h> 
#include <unistd.h>
#include <stdlib.h>
#include <sys/wait.h>

int main() 
{ 

char * arg_list[3];
arg_list[0] = "ls";
arg_list[1] = "-l";
arg_list[2] = 0;

char *arg_list2[3];
arg_list2[0] = " ps";
arg_list2[1] = "-ef";
arg_list2[2] = 0;

    for(int i=0;i<5;i++){ // loop will run n times (n=5) 

       if(fork() == 0) { 
    if (i == 0){
    execvp("ls", arg_list);
}else if(i==1){
execvp("ps" , arg_list2);
}else if(i>1){
           printf("[son] pid %d from [parent] pid %d\n",getpid(),getppid()); 
        exit(0); 
}
      } 
    } 
    for(int i=0;i<5;i++) // loop will run n times (n=5) 
           wait(NULL); 

} 

ME trying to modify it

#include<stdio.h> 
#include <unistd.h>
#include <stdlib.h>
#include <sys/wait.h>

int main() 
{ 
    for(int i=0;i<5;i++){ // loop will run n times (n=5) 

       if(fork() == 0) { 
           printf("[son] pid %d from [parent] pid %d\n",getpid(),getppid()); 
        execlp(argv[i],argv[i],argv[i+1],(char*)NULL);
        exit(0); 
      } 
    } 
    for(int i=0;i<5;i++) // loop will run n times (n=5) 
           wait(NULL); 

} 

-- NEED GUIDANCE AND UNDERSTANDING

I am trying to make my own tiny little shell program. When I run my first code works fine, runs all commands on the command line. But I cannot know and define all commands the user might enter. So i am trying to get a base code which could run any commands single or multiple entered by user. I tried using execlp where it does not compile saying argv is not defined which is true as i don't want to specifically define it.

Answer 1

I am trying to make my own tiny little shell program. When I run my first code works fine, runs all commands on the command line. But I cannot know and define all commands the user might enter.

For sure.... A shell program purpose is basically:

1. Read user input
2. Execute user input
3. Return result of execution.

There's nothing in your code that read user input....

So i am trying to get a base code which could run any commands single or multiple entered by user.

So read user input;-)

I tried using execlp where it does not compile saying argv is not defined which is true as i don't want to specifically define it.

For sure... but how would GCC guessed that `argv[]̀ must be automaticallty filled with user input? There's nothing automatic when coding in C language. You have to manage this manually.

Also, note that argc , argv et envp are usually reserved for main() function:

main(int argc, char **argv, char **envp)

So you may use something else to build your command array.

In pseudo code, what you must implement is:

quit=0
while (quit = 0) {
    command_to_run = read_user_input();
    if (command_to_run == "exit") {
       quit = 1;
    } else {
       execute(command_to_run);
    }
}

Some advices:

• Try to use more functions. For example, implement a fork_and_run(char **cmd) function to fork and then execute command provided by the user. Il will make your code more readable and easy to maintain.

• Read carefully manpages: everything you should know (like, for example, the fact that array provided to execvp() must be NULL-terminated) is written in it.

• Your debugging messages should be printed to stderr . The result of the command run must be printed to stdin , so use fprintf() instead of printf() to write to the correct stream.

• I would use a #define debug(x) fprintf(stderr, x) or something similar for debugging output so that you can easily disable later;-)