这是操作系统原理课程的第一个编程作业，补全shell的功能。
主要实现了基础的三类命令
- 可执行的程序命令
- 重定向命令
- 管道命令
实现的"基类" (原谅我用了这个词)就是struct cmd这个结构体就一个成员，用于记录命令的类型．
三类, ' ' 表示可执行程序 '|' 表示管道命令, '<' 和'>' 表示重定向类型．
每一个类型分别继承基类，派生出对应的三类结构体
struct execcmd
struct redircmd
struct pipecmd
void runcmd(struct cmd * cmd);
这个函数是真正驱动调用实现shell的核心．负责调用系统接口函数 execv(), open(), close(), dup(), pipe()等等一系列函数，来完成我们既定的目标．
作业也就是补全这个函数．
这是个递归的函数！
下面是来自github上的代码，仅摘录runcmd函数
1 void
2 runcmd(struct cmd *cmd)
3 {
4 int p[2]; // used for pipe line in shell
5 int r; // return value
6 struct execcmd *ecmd;
7 struct pipecmd *pcmd;
8 struct redircmd *rcmd;
9
10 if(cmd == 0)
11 exit(0);
12
13 switch(cmd->type){
14 default:
15 fprintf(stderr, "unknown runcmd
");
16 exit(-1);
17
18 case ' ':
19 ecmd = (struct execcmd*)cmd;
20 if(ecmd->argv[0] == 0)
21 exit(0);
22 //fprintf(stderr, "exec not implemented
");
23 // Your code here ...
24 if (access(ecmd->argv[0], S_IXUSR | S_IRUSR) == 0)
25 {
26 execv(ecmd->argv[0], ecmd->argv);
27 }
28 else
29 {
30 if(chdir("/bin/") < 0)
31 {
32 printf("change directory failed in line %d
", __LINE__);
33 exit(0);
34 }
35 execv(ecmd->argv[0], ecmd->argv);
36 }
37 fprintf(stderr, "execv() %s failed in line %d
", ecmd->argv[0], __LINE__);
38 break;
39
40 case '>':
41 case '<':
42 rcmd = (struct redircmd*)cmd;
43 //fprintf(stderr, "redir not implemented
");
44 // Your code here ...
45 close(rcmd->fd);
46 if(open(rcmd->file, rcmd->mode) < 0)
47 {
48 fprintf(stderr, "Try to open :%s failed
", rcmd->file);
49 exit(0);
50 }
51 runcmd(rcmd->cmd);
52 break;
53
54 case '|':
55 pcmd = (struct pipecmd*)cmd;
56 //fprintf(stderr, "pipe not implemented
");
57 // Your code here ...
58 if(pipe(p) < 0)
59 {
60 fprintf(stderr, "call syscall pipe() failed in line %d
", __LINE__);
61 exit(0);
62 }
63
64 if(fork1() == 0)
65 {
66 close(1);
67 dup(p[1]);
68 close(p[0]);
69 close(p[1]);
70 runcmd(pcmd->left);
71 }
72
73 if(fork1() == 0)
74 {
75 close(0);
76 dup(p[0]);
77 close(p[0]);
78 close(p[1]);
79 runcmd(pcmd->right);
80 }
81
82 close(p[0]);
83 close(p[1]);
84 wait();
85 wait();
86 break;
87 }
88
89 exit(0);
90 }