Implementation of exec function and shell

exec function

problem ： Why? bash The child process created is different from its own ？

execl、execlp、execle/

function ：

Replace （replace） The impression of the current process （process image）

The point is replace： It's understandable , Because the cells of the body are always aging and changing , Seven years later , Your body cells are not the original cells ," Replace " Into new cells ;

execl Will replace the buffer , So the front needs to brush new flow ;

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main()
{
    puts("Begin...");
    fflush(NULL);      stay execl Need to brush the new flow before 
    
    execl("/bin/date", "date", "+%s", NULL);
    perror("execl()");
    exit(1);

    puts("End...");

    exit(0);
}

shell The working process of the command line ：few Trilogy

• fork（）、exec（）、wait（） Three important functions of a process ;
• Blood spurts ： This is it. shell The working process of the command line ：

1、shell The main program fork Subprocesses shell

2、excel take shell Replace bit command line （ Such as date） Binary program ;

3、 Subroutines date work ;

4、shell Parent program wait Collect the body of the subroutine ;

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

int main(int argc, char **argv)
{
    //1、 The parent process fork();
    printf("Beginning\n");

    fflush(NULL);
    pid_t pid = fork();
    if (pid < 0)
    {
        perror("fork");
        exit(1);
    }
    else if (pid == 0)//2、 Subprocesses work 
    {
        execl("/bin/date","date",NULL);//3、 The child process is replaced as a whole ;
        perror("execl");
        exit(1);
    }
    //1、 Parent process corpse collection ;
    wait(NULL);
    printf("End\n");

    exit(0);
}

Why does the parent-child process print on a terminal ： Because the file descriptors of the parent and child processes point to the same terminal

• Hacker virus means 1： adopt execl(argv0) Hide the virus process as a regular process name ;

shell programmatic   Simple implementation ：

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

#define DELIMS " \t\n"

struct cmd_st
{
    glob_t globres;
};


static void prompt(void)
{
    printf("mysh-0.1$ ");
}

static void parse(char *line, struct cmd_st *res)
{
    char *tok;
    int i = 0;

    while(1)
    {
        tok = strsep(&line, DELIMS);
        if(tok == NULL)
            break;
        
        if(tok[0] == '\0')
            continue;

        glob(tok, GLOB_NOCHECK | GLOB_APPEND * i, NULL, &res->globres);
        i = 1;
    }
}

int main()
{
    char *linebuf = NULL;
    size_t linebuf_size = 0;
    struct cmd_st cmd;
    
    while(1)
    {
        pid_t pid;
        prompt();

        if(getline(&linebuf, &linebuf_size, stdin) < 0)
            break;

        parse(linebuf, &cmd);

        if(0)
        {
            //  Execute internal commands 
            printf("Internal command is not executed\n");
        }
        else
        {
            //  Execute external command 
            pid = fork();

            if( pid < 0)
            {
                perror("fork()");
                exit(1);
            }
            if(pid == 0)
            {
                execvp(cmd.globres.gl_pathv[0], cmd.globres.gl_pathv);
                perror("execvp()");
                exit(1);
            }
            else
            {
                wait(NULL);
            }
        }
    }
    exit(0);
}