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done，故意写成=，表示先赋值，再判断，分割之后，strtok会返回NULL，刚好让gArgv最后一个元素是NULL，并且while判断结束

Makefile

  1 myshell:myshell.c                                                                                                                                                      2   gcc -o $@ $^3 .PHONY:clean4 clean:5   rm -f myshell

myshell.c

  1 #include<stdio.h>2 #include<unistd.h>3 #include<sys/types.h>4 #include<sys/wait.h>5 #include<stdlib.h>6 #include<string.h>7 #include<errno.h>8 9 #define SIZE 51210 #define ZERO '\0'11 #define SEP " "12 #define NUM 3213 #define SkipPath(p) do{p += (strlen(p) - 1);while(*p != '/') p--;}while(0)14 15 char cwd[SIZE*2];16 char *gArgv[NUM];17 int lastcode = 0;18 void Die()19 {20     exit(1);21 }22 23 const char *GetHome()24 {25     const char *home = getenv("HOME");                                                                                                                                 26     if(home == NULL) return "/";27     return home;28 }29 const char *Getusername()30 {31     const char *name = getenv("USER");32     if(name == NULL) return "None";33     return name;34 }35 const char *Gethostname()36 {                                                                                                                                                                      37     const char *hostname = getenv("HOSTNAME");38     if(hostname == NULL) return "None";39     return hostname;40 }41 const char *Getcwd()42 {43     const char *cwd = getenv("PWD");44     if(cwd == NULL) return "None";45     return cwd;46 }47 48 void makecommandline()49 {50     char line[SIZE];51     const char *username = Getusername();52     const char *hostname = Gethostname();53     const char *cwd = Getcwd();54                                                                                                                                                                        55     SkipPath(cwd);56     snprintf(line, sizeof(line), "[%s@%s %s] love:", username, hostname, strlen(cwd) == 1 ? "/" : cwd + 1);57     printf("%s", line);58     fflush(stdout);59 }60 61 int Getusercommand(char command[], size_t n)62 {63     char *s = fgets(command, n, stdin);64     if(s == NULL) return -1;65 66     command[strlen(command) - 1] = ZERO;67     return strlen(command);68 }69 70 void SplitCommand(char command[], size_t n)71 {72     (void)n;73     gArgv[0] = strtok(command, SEP);74     int index = 1;75     while((gArgv[index++] = strtok(NULL, SEP)));                                                                                                                       76 }77 void ExecuteCommand()78 {79     pid_t id = fork();80     if(id < 0) Die();81     else if(id == 0)82     {83         //child84         execvp(gArgv[0], gArgv);85         exit(errno);86     }87     else 88     {89         //father90         int status = 0;91         pid_t rid = waitpid(id, &status, 0);92         if(rid > 0)93         {94             lastcode = WEXITSTATUS(status);95             if(lastcode != 0) printf("%s:%s:%d\n", gArgv[0], strerror(lastcode), lastcode);96         }97     }98 }99 
100 void Cd()
101 {
102     const char *path = gArgv[1];
103     if(path == NULL) path =GetHome();
104 
105     chdir(path);                                                                                                                                                       
106     //刷新环境变量
107     char temp[SIZE*2];
108     getcwd(temp, sizeof(temp));
109     snprintf(cwd, sizeof(cwd), "PWD=%s", temp);
110     putenv(cwd);
111 }
112 int CheckBuildin()
113 {
114     int yes = 0;
115     const char *enter_cmd = gArgv[0];
116     if(strcmp(enter_cmd, "cd") == 0)
117     {
118         yes = 1;
119         Cd();
120     }
121     else if(strcmp(enter_cmd, "echo") == 0 && strcmp(gArgv[1], "$?") == 0)
122     {
123         yes = 1;
124         printf("%d\n", lastcode);
125         lastcode = 0;
126     }
127 
128     return yes;
129 }
130 int main()
131 {
132     int quit = 0;                                                                                                                                                      
133     while(!quit)
134     {
135         makecommandline();
136 
137         char usercommand[SIZE];
138         int n = Getusercommand(usercommand, sizeof(usercommand));
139         if(n <= 0) return 1;
140 
141         SplitCommand(usercommand, sizeof(usercommand));
142         
143         n = CheckBuildin();
144         if(n) continue;
145         ExecuteCommand();
146     }
147     
148     return 0;
149 }