編寫(xiě)一個(gè)簡(jiǎn)易shell命令行

用fork()創(chuàng)建子進(jìn)程后執(zhí)行的是和父進(jìn)程相同的程序（但有可能執(zhí)行不同的代碼分支），子進(jìn)程往往要調(diào)用一種exec函數(shù)以執(zhí)行另一個(gè)程序。

進(jìn)程程序替換：替換數(shù)據(jù)和代碼，執(zhí)行不同邏輯，一旦替換之后，父，子進(jìn)程代碼，數(shù)據(jù)獨(dú)立。

主要是創(chuàng)建進(jìn)程，讓子進(jìn)程去執(zhí)行命令。

主要有6個(gè)調(diào)度函數(shù)，函數(shù)名中p表示傳文件名（靠環(huán)境變量來(lái)看是否存在PATH），沒(méi)p的需把文件路徑傳過(guò)來(lái)，要執(zhí)行一個(gè)程序必須先找到它.

帶e表示用自定義環(huán)境變量，不帶e表示從父進(jìn)程繼承而來(lái)的環(huán)境變量。

#include<stdio.h>
#include<unistd.h>
#include<stdlib.h>
#include<string.h>
#include<sys/types.h>
int main(int argc,char* argv[])
{
        pid_t id;
        while(1)
        {
                printf("[lxj/home]$");
                fflush(stdout);
                char cmd_str[1024]={0};
                ssize_t size=read(0,cmd_str,sizeof(cmd_str)-1);
                char cmd[32]={0};
                char* my_argv[32];
                memset(my_argv,0,sizeof(char*)*32);
                int length=strlen(cmd_str)-2;
        //      printf("%s",cmd_str);
                if(size!=-1)
                {
                        int index=0;
                        while (length >= 0)
                       {
                                while (length >= 0&&cmd_str[length]==' ')
                                {
                                        cmd_str[length--] = 0;
                                }
                                while (length >= 0 && cmd_str[length] != ' ')
                                        length--;
                                my_argv[index++]=cmd_str+length+1;
                        }
                        my_argv[index]=NULL;
                        int i=0;
                        index--;
                        for(;i<index;++i,--index)
                        {
                                char*  tmp=my_argv[i];
                                my_argv[i]=my_argv[index];
                                my_argv[index]=tmp;
                        }
                        if(strcmp(argv[0],"cd")==0)
                        {
                                chdir(my_argv[1]);
                                continue;
                        }
                }
                id=fork();
                if(id<0)
                        perror("fork");
                else if(id==0){
                        execvp(my_argv[0],my_argv);
                        exit(1);
                }
                else if(id>0){
                        pid_t waitPid=waitpid(id,NULL,0);

                }
        }
        return 0;
}