实现了一个支持多个指令的shell
1. 相对路径
- 在内核态中开一个大数组，存储每个进程对应的当前路径
char curpath[NENV][MAXPATHLEN] = {0};
struct Var var[NENV][MAXVARNUM];
- 新增系统调用，读写当前进程的当前路径
void sys_get_curpath(char *s) { strcpy(s, curpath[curenv->env_id >> 11]); }
void sys_set_curpath(char *s) { strcpy(curpath[curenv->env_id >> 11], s); }
- 在进程初始化时，给进程的当前路径设置初值
void env_init(void) {
int i;
/* Step 1: Initialize 'env_free_list' with 'envs[0]' ~ 'envs[NENV-1]'. */
LIST_INIT(&env_free_list);
TAILQ_INIT(&env_sched_list);
for (i = NENV - 1; i >= 0; --i) {
envs[i].env_status = ENV_FREE;
LIST_INSERT_HEAD(&env_free_list, &envs[i], env_link);
strcpy(curpath[i], "/");
}
- 新创建进程时，继承其父进程的相对路径
e->env_id = mkenvid(e);
e->env_parent_id = parent_id;
u_int new_index = e->env_id >> 11;
u_int parent_index = parent_id >> 11;
strcpy(curpath[new_index], curpath[parent_index]);
for (int i = 0; i < MAXVARNUM; i++) {
struct Var temp = var[parent_index][i];
if (temp.valid == 1 && temp.x == 1) {
set_env_var(e->env_id, temp.key, temp.value, temp.r, temp.x);
}
}
- 新增一个从相对路径到绝对路径的转换函数
void tackle_path(const char *in, char *out) {
char temp[MAXPATHLEN] = {0};
char cur[MAXPATHLEN] = {0};
strcpy(temp, in);
int len = strlen(temp);
syscall_get_curpath(cur);
int curlen = strlen(cur);
if (len > 1 && temp[len - 1] == '/') {
temp[len - 1] = '\0';
len--;
}
if (len > 2 && temp[len - 1] == 'b' && temp[len - 2] == '.') {
strcpy(out, temp);
} else if (temp[0] == '/') {
strcpy(out, temp);
} else if (temp[0] == '.' && temp[1] == '.') {
int slash = -1;
for (int i = curlen - 1; i >= 0; i--) {
if (cur[i] == '/') {
slash = i;
break;
}
}
int i = 0, pos = 0;
if (slash <= 0) {
out[pos++] = '/';
} else {
for (; i < slash; i++) {
out[pos++] = cur[i];
}
}
if (out[pos - 1] != '/') {
out[pos++] = '/';
}
for (i = 3; temp[i] == '.' || temp[i] == '/'; i++)
;
for (; temp[i]; i++) {
out[pos++] = temp[i];
}
if (out[pos - 1] == '/' && pos != 1) {
out[pos - 1] = '\0';
} else {
out[pos] = '\0';
}
} else if (temp[0] == '.' && (temp[1] == '/' || temp[1] == '\0')) {
int i = 0, pos = 0;
for (; cur[i]; i++) {
out[pos++] = cur[i];
}
if (out[pos - 1] != '/') {
out[pos++] = '/';
}
for (i = 2; temp[i]; i++) {
out[pos++] = temp[i];
}
if (out[pos - 1] == '/' && pos != 1) {
out[pos - 1] = '\0';
} else {
out[pos] = '\0';
}
} else {
int i = 0, pos = 0;
for (i = 0; cur[i]; i++) {
out[pos++] = cur[i];
}
if (out[pos - 1] != '/') {
out[pos++] = '/';
}
for (i = 0; temp[i]; i++) {
out[pos++] = temp[i];
}
if (out[pos - 1] == '/' && pos != 1) {
out[pos - 1] = '\0';
} else {
out[pos] = '\0';
}
}
}
- 在打开和删除文件的函数中，将传入的路径转换成绝对路径
int open(const char *path, int mode) {
int r;
char out[MAXPATHLEN] = {0};
tackle_path(path, out);
// Step 1: Alloc a new 'Fd' using 'fd_alloc' in fd.c.
// Hint: return the error code if failed.
struct Fd *fd;
if ((r = fd_alloc(&fd)) < 0) {
return r;
}
// Step 2: Prepare the 'fd' using 'fsipc_open' in fsipc.c.
r = fsipc_open((const char *)out, mode, fd);
if (r) {
return r;
}
...
}
int remove(const char *path) {
// Call fsipc_remove.
char out[MAXPATHLEN] = {0};
tackle_path(path, out);
return fsipc_remove((const char *)out);
}
- 实现内建指令的外部指令的分离
void run_outer_cmd(char *s) {
int fork_id = fork();
if (fork_id < 0) {
debugf("fork error: %d\n", fork_id);
return;
}
if (fork_id == 0) {
gettoken(s, 0);
char *argv[MAXARGS];
int rightpipe = 0;
int argc = parsecmd(argv, &rightpipe);
if (argc == 0) {
return;
}
argv[argc] = 0;
int child = spawn(argv[0], argv);
if (child >= 0) {
u_int caller;
int res = ipc_recv(&caller, 0, 0);
if (conditional) {
u_int parent = syscall_get_parent();
ipc_send(parent, res, 0, 0);
}
close_all();
wait(child);
} else {
debugf("spawn %s: %d\n", argv[0], child);
}
if (rightpipe) {
wait(rightpipe);
}
exit();
} else {
wait(fork_id);
}
return;
}
void run_inner_cmd(char *s) {
gettoken(s, 0);
char *argv[MAXARGS];
int rightpipe = 0;
int argc = parsecmd(argv, &rightpipe);
if (argc == 0) {
return;
}
argv[argc] = 0;
char buffer[MAXPATHLEN + 1] = {0};
...
}
void runcmd(char *s) {
int inner = judge_inner(s);
if (inner) {
run_inner_cmd(s);
} else {
run_outer_cmd(s);
}
}
- 通过系统调用，实现内建指令cd和pwd
} else if ((strcmp(argv[0], "cd") == 0) || (strcmp(argv[0], "cd.b") == 0)) {
if (argc == 1) {
strcpy(buffer, "/");
} else if (argc > 2) {
printf("Too many args for cd command\n");
return;
} else {
struct Stat st;
tackle_path((const char *)argv[1], buffer);
if ((fd = open(buffer, O_RDONLY)) < 0) {
printf("cd: The directory '%s' does not exist\n", argv[1]);
return;
}
close(fd);
stat(buffer, &st);
if (!st.st_isdir) {
printf("cd: '%s' is not a directory\n", argv[1]);
return;
}
}
syscall_set_curpath(buffer);
} else if ((strcmp(argv[0], "pwd") == 0) ||
(strcmp(argv[0], "pwd.b") == 0)) {
if (argc > 1) {
printf("pwd: expected 0 arguments; got %d\n", argc - 1);
return;
}
syscall_get_curpath(buffer);
printf("%s\n", buffer);
2. 新增指令
- 创建目录
#include <lib.h>
int mkdir(char *path, int p) {
int fd;
if (p) {
if ((fd = open(path, O_RDONLY)) >= 0) {
close(fd);
return 0;
}
int i = 0;
char str[1024];
for (int i = 0; path[i] != '\0'; i++) {
if (path[i] == '/') {
str[i] = '\0';
if ((fd = open(path, O_RDONLY)) >= 0) {
close(fd);
} else {
break;
}
}
str[i] = path[i];
}
for (; path[i] != '\0'; i++) {
if (path[i] == '/') {
str[i] = '\0';
fd = open(str, O_MKDIR);
if (fd >= 0) {
close(fd);
}
}
str[i] = path[i];
}
str[i] = '\0';
fd = open(str, O_MKDIR);
if (fd >= 0) {
close(fd);
}
} else {
if ((fd = open(path, O_RDONLY)) >= 0) {
close(fd);
printf("mkdir: cannot create directory '%s': File exists\n", path);
return 1;
}
fd = open(path, O_MKDIR);
if (fd == -10) {
printf("mkdir: cannot create directory '%s': No such file or "
"directory\n",
path);
return 1;
} else if (fd >= 0) {
close(fd);
}
}
return 0;
}
int main(int argc, char **argv) {
int p = 0;
ARGBEGIN {
case 'p':
p = 1;
break;
}
ARGEND
int res = 0;
if (argc == 0) {
return 1;
} else {
for (int i = 0; i < argc; i++) {
if (argv[i] == 0) {
continue;
}
res += mkdir(argv[i], p);
}
}
return res;
}
- 创建文件
#include <lib.h>
int touch(char *path) {
int fd;
if ((fd = open(path, O_RDONLY)) >= 0) {
close(fd);
return 0;
}
fd = open(path, O_CREAT);
if (fd == -10) {
printf("touch: cannot touch '%s': No such file or directory\n", path);
return 1;
} else if (fd >= 0) {
close(fd);
}
return 0;
}
int main(int argc, char **argv) {
int res = 0;
if (argc < 2) {
return 1;
} else {
for (int i = 1; i < argc; i++) {
res += touch(argv[i]);
}
}
return res;
}
- 删除文件
#include <lib.h>
int rm(char *path, int r, int f) {
int fd;
struct Stat st;
if ((fd = open(path, O_RDONLY)) < 0) {
if (!f) {
printf("rm: cannot remove '%s': No such file or directory\n", path);
}
return 1;
}
close(fd);
stat(path, &st);
if (st.st_isdir && !r) {
printf("rm: cannot remove '%s': Is a directory\n", path);
return 1;
}
remove(path);
return 0;
}
int main(int argc, char **argv) {
int r = 0, f = 0;
ARGBEGIN {
case 'r':
r = 1;
break;
case 'f':
f = 1;
break;
}
ARGEND
int res = 0;
if (argc == 0) {
return 1;
} else {
for (int i = 0; i < argc; i++) {
if (argv[i] == 0) {
continue;
}
res += rm(argv[i], r, f);
}
}
return res;
}
3. 环境变量
- 在内核态中开一个大数组，存储每个进程对应的环境变量
struct Var {
char key[MAXVARLEN];
char value[MAXVARLEN];
int r;
int x;
int valid;
};
...
extern char curpath[NENV][MAXPATHLEN];
extern struct Var var[NENV][MAXVARNUM];
- 新增系统调用，读写和打印当前进程的环境变量
void set_env_var(u_int envid, char *key, char *value, int r, int x) {
int index = envid >> 11;
for (int i = 0; i < MAXVARNUM; i++) {
if (strcmp(var[index][i].key, key) == 0 && var[index][i].valid == 1) {
if (var[index][i].r == 0) {
strcpy(var[index][i].value, value);
var[index][i].r = r;
var[index][i].x = x;
}
return;
}
}
for (int i = 0; i < MAXVARNUM; i++) {
if (var[index][i].valid == 0) {
strcpy(var[index][i].key, key);
strcpy(var[index][i].value, value);
var[index][i].valid = 1;
var[index][i].r = r;
var[index][i].x = x;
return;
}
}
}
void get_env_var(u_int envid, char *key, char *res) {
int index = envid >> 11;
for (int i = 0; i < MAXVARNUM; i++) {
if (strcmp(var[index][i].key, key) == 0 && var[index][i].valid == 1) {
strcpy(res, var[index][i].value);
return;
}
}
}
void unset_env_var(u_int envid, char *key) {
int index = envid >> 11;
for (int i = 0; i < MAXVARNUM; i++) {
if (strcmp(var[index][i].key, key) == 0 && var[index][i].valid == 1) {
if (var[index][i].r == 0) {
strcpy(var[index][i].key, "\0");
strcpy(var[index][i].value, "\0");
var[index][i].r = 0;
var[index][i].x = 0;
var[index][i].valid = 0;
}
return;
}
}
}
void print_env_var(u_int envid) {
int index = envid >> 11;
for (int i = 0; i < MAXVARNUM; i++) {
if (var[index][i].valid == 1) {
printk("%s=%s\n", var[index][i].key, var[index][i].value);
}
}
}
- 新创建进程时，继承其父进程的全局环境变量‘
int env_alloc(struct Env **new, u_int parent_id) {
...
e->env_id = mkenvid(e);
e->env_parent_id = parent_id;
u_int new_index = e->env_id >> 11;
u_int parent_index = parent_id >> 11;
strcpy(curpath[new_index], curpath[parent_index]);
for (int i = 0; i < MAXVARNUM; i++) {
struct Var temp = var[parent_index][i];
if (temp.valid == 1 && temp.x == 1) {
set_env_var(e->env_id, temp.key, temp.value, temp.r, temp.x);
}
}
...
}
- 在获取当前token的函数中，展开环境变量的引用
if (*s == '$') {
char key[MAXVARLEN];
char value[MAXVARLEN];
char after[1024];
int i;
for (i = 1; s[i] && strchr(CHAR, s[i]); i++) {
key[i - 1] = s[i];
}
key[i - 1] = '\0';
strcpy(after, s + i);
syscall_get_var(key, value);
strcpy(s, value);
strcpy(s + strlen(value), after);
*p1 = s;
s += strlen(value);
while (s[0] && !strchr(WHITESPACE, s[0])) {
s++;
}
*p2 = s;
return 'w';
}
- 通过系统调用，实现内建指令declare和unset
} else if ((strcmp(argv[0], "declare") == 0) ||
(strcmp(argv[0], "declare.b") == 0)) {
int r = 0, x = 0;
char value[MAXVARLEN] = {0};
char key[MAXVARLEN] = {0};
if (argc == 1) {
syscall_print_var();
return;
} else if (argc > 3) {
return;
} else if (argc == 3) {
if (strcmp(argv[1], "-r") == 0) {
r = 1;
} else if (strcmp(argv[1], "-x") == 0) {
x = 1;
} else if ((strcmp(argv[1], "-xr") == 0) ||
(strcmp(argv[1], "-rx") == 0)) {
r = 1, x = 1;
}
}
int i = 0, pos = 0;
for (; argv[argc - 1][i] && argv[argc - 1][i] != '='; i++) {
key[pos++] = argv[argc - 1][i];
}
key[pos] = '\0';
pos = 0;
i++;
for (; argv[argc - 1][i]; i++) {
value[pos++] = argv[argc - 1][i];
}
syscall_set_var(key, value, r, x);
} else if ((strcmp(argv[0], "unset") == 0) ||
(strcmp(argv[0], "unset.b") == 0)) {
if (argc != 2) {
return;
}
syscall_unset_var(argv[1]);
}
4. 注释
- 在指令输入后，将指令井号后部分截断后执行
if (buf[0] == '\0' || buf[0] == '#') {
continue;
}
for (int i = 0; i < strlen(buf); i++) {
if (buf[i] == '#') {
buf[i] = '\0';
}
}
5. 指令输入优化
- 删除读取字符的不必要循环
int sys_cgetc(void) {
int ch = scancharc();
return ch;
}
- 实现指令的重新打印展示
void redisplay_line(char *buf, int len, int cursor) {
printf("\r$ ");
for (int i = 0; i < len; i++) {
printf("%c", buf[i]);
}
printf("\033[K");
if (cursor < len) {
printf("\r$ ");
for (int i = 0; i < cursor; i++) {
printf("%c", buf[i]);
}
}
}
- 修改读取键盘输入的函数
void readline(char *buf, u_int n) {
int r;
int pos = 0;
int len = 0;
memset(buf, 0, n);
history_current = -1;
memset(current_input, 0, sizeof(current_input));
while (1) {
char c;
if ((r = read(0, &c, 1)) != 1) {
if (r < 0) {
debugf("read error: %d\n", r);
}
exit();
}
if (c == '\r' || c == '\n') {
buf[len] = '\0';
return;
} else if (c == '\b' || c == 0x7f) {
if (pos > 0) {
for (int i = pos - 1; i < len - 1; i++) {
buf[i] = buf[i + 1];
}
pos--;
len--;
buf[len] = '\0';
redisplay_line(buf, len, pos);
}
} else if (c == 1) {
pos = 0;
printf("\r$ ");
} else if (c == 5) {
pos = len;
printf("\r$ ");
for (int i = 0; i < len; i++) {
printf("%c", buf[i]);
}
} else if (c == 11) {
len = pos;
buf[len] = '\0';
redisplay_line(buf, len, pos);
} else if (c == 21) {
if (pos > 0) {
for (int i = 0; i < len - pos; i++) {
buf[i] = buf[pos + i];
}
len = len - pos;
pos = 0;
buf[len] = '\0';
redisplay_line(buf, len, pos);
}
} else if (c == 23) {
if (pos > 0) {
int old_pos = pos;
while (pos > 0 &&
(buf[pos - 1] == ' ' || buf[pos - 1] == '\t')) {
pos--;
}
while (pos > 0 && buf[pos - 1] != ' ' && buf[pos - 1] != '\t') {
pos--;
}
int delete = old_pos - pos;
for (int i = pos; i < len - delete; i++) {
buf[i] = buf[i + delete];
}
len -= delete;
buf[len] = '\0';
redisplay_line(buf, len, pos);
}
} else if (c == 27) {
char seq[3];
if (read(0, &seq[0], 1) == 1 && seq[0] == '[') {
if (read(0, &seq[1], 1) == 1) {
if (seq[1] == 'A') {
if (history_count > 0) {
if (history_current == -1) {
strcpy(current_input, buf);
history_current = history_count - 1;
} else if (history_current > 0) {
history_current--;
}
strcpy(buf, history_lines[history_current]);
len = strlen(buf);
pos = len;
printf("\r$ ");
for (int i = 0; i < len; i++) {
printf("%c", buf[i]);
}
printf("\033[K");
}
} else if (seq[1] == 'B') {
if (history_current != -1) {
if (history_current < history_count - 1) {
history_current++;
strcpy(buf, history_lines[history_current]);
} else {
history_current = -1;
strcpy(buf, current_input);
}
len = strlen(buf);
pos = len;
printf("\r$ ");
for (int i = 0; i < len; i++) {
printf("%c", buf[i]);
}
printf("\033[K");
}
} else if (seq[1] == 'C') {
if (pos < len) {
pos++;
printf("\033[C");
}
} else if (seq[1] == 'D') {
if (pos > 0) {
pos--;
printf("\033[D");
}
}
}
}
} else if (c >= 32 && c <= 126) {
if (len < n - 1) {
for (int i = len; i > pos; i--) {
buf[i] = buf[i - 1];
}
buf[pos] = c;
pos++;
len++;
buf[len] = '\0';
redisplay_line(buf, len, pos);
}
}
}
}
6. 历史指令
- 在创建终端时，创建或加载已有历史指令文件
history_fd = open("/.mos_history", O_RDWR | O_CREAT);
load_history();
- 每次输入指令后，将新指令保存到文件中
void save_history(void) {
if (history_fd < 0) {
return;
}
close(history_fd);
history_fd = open("/.mos_history", O_WRONLY | O_CREAT | O_TRUNC);
if (history_fd < 0) {
return;
}
for (int i = 0; i < history_count; i++) {
write(history_fd, history_lines[i], strlen(history_lines[i]));
write(history_fd, "\n", 1);
}
}
void store_history(char *buf) {
if (strlen(buf) == 0) {
return;
}
int len = strlen(buf);
if (len > 0 && buf[len - 1] == '\n') {
buf[len - 1] = '\0';
len--;
}
if (len == 0) {
return;
}
if (history_count > 0 &&
strcmp(buf, history_lines[history_count - 1]) == 0) {
return;
}
if (history_count >= MAXHISTORY) {
for (int i = 0; i < MAXHISTORY - 1; i++) {
strcpy(history_lines[i], history_lines[i + 1]);
}
history_count = MAXHISTORY - 1;
}
strcpy(history_lines[history_count], buf);
history_count++;
save_history();
}
7. 追加重定向
- 解析追加重定向符号
if (*s == '>' && *(s + 1) == '>') {
*p1 = s;
*s++ = 0;
*s++ = 0;
*p2 = s;
return 'a';
}
- 添加追加打开方式
#define O_APPEND 0x0004 /* open for appending */
- 追加模式打开时，修改文件偏移量
ff->f_fd.fd_omode = o->o_mode;
ff->f_fd.fd_dev_id = devfile.dev_id;
if (rq->req_omode & O_APPEND) {
ff->f_fd.fd_offset = ff->f_file.f_size;
}
- 解析命令时，仿照普通重定向实现
case 'a':
if (gettoken(0, &t) != 'w') {
exit();
}
if ((fd = open(t, O_RDONLY)) < 0) {
fd = open(t, O_CREAT);
if (fd < 0) {
exit();
}
}
close(fd);
if ((fd = open(t, O_WRONLY | O_APPEND)) < 0) {
exit();
}
if ((r = dup(fd, 1)) < 0) {
exit();
}
close(fd);
break;
8. 条件执行
- 解析条件执行符号
if (*s == '|' && *(s + 1) == '|') {
*p1 = s;
*s++ = 0;
*s++ = 0;
*p2 = s;
return 'O';
}
if (*s == '&' && *(s + 1) == '&') {
*p1 = s;
*s++ = 0;
*s++ = 0;
*p2 = s;
return 'A';
}
- 将无返回值的外部命令改为有返回值
int cat(int f, char *s) {
long n;
int r;
while ((n = read(f, buf, (long)sizeof buf)) > 0) {
if ((r = write(1, buf, n)) != n) {
printf("write error copying %s: %d", s, r);
return 1;
}
}
if (n < 0) {
printf("error reading %s: %d", s, n);
return 1;
}
return 0;
}
- 捕获main函数的返回值，并传递给父进程
void libmain(int argc, char **argv) {
// set file handles
...
// set env to point at our env structure in envs[].
env = &envs[ENVX(syscall_getenvid())];
int res = main(argc, argv);
u_int parent = syscall_get_parent();
ipc_send(parent, res, 0, 0);
// exit gracefully
exit();
}
- 运行外部指令时，接受返回值，并传递给父进程
int child = spawn(argv[0], argv);
if (child >= 0) {
u_int caller;
int res = ipc_recv(&caller, 0, 0);
if (conditional) {
u_int parent = syscall_get_parent();
ipc_send(parent, res, 0, 0);
}
close_all();
wait(child);
- 解析命令时，右侧指令对应进程等待左侧指令的返回值消息，决定是否执行
case 'O':;
fork_id = fork();
if (fork_id < 0) {
debugf("failed to fork in sh.c\n");
exit();
} else if (fork_id == 0) {
conditional = 1;
return argc;
} else {
u_int caller;
int res = ipc_recv(&caller, 0, 0);
if (res != 0) {
return parsecmd(argv, rightpipe);
} else {
return 0;
}
}
break;
case 'A':;
fork_id = fork();
if (fork_id < 0) {
debugf("failed to fork in sh.c\n");
exit();
} else if (fork_id == 0) {
conditional = 1;
return argc;
} else {
u_int caller;
int res = ipc_recv(&caller, 0, 0);
if (res == 0) {
return parsecmd(argv, rightpipe);
} else {
return 0;
}
}
break;
- 实现内建exit指令
if ((strcmp(argv[0], "exit") == 0) || (strcmp(argv[0], "exit.b") == 0)) {
int parent = syscall_get_parent();
ipc_send(parent, 0, 0, 0);
exit();
9. 反引号
- 对指令进行预处理，执行反引号内的指令，结果通过管道返回，进行字符串替换
int run_quote_cmd(char *cmd, char *output) {
int p[2];
if (pipe(p) < 0) {
return -1;
}
int child = fork();
if (child < 0) {
close(p[0]);
close(p[1]);
return -1;
} else if (child == 0) {
close(p[0]);
dup(p[1], 1);
close(p[1]);
runcmd(cmd);
exit();
} else {
close(p[1]);
int total = 0;
char buffer[1024];
int r;
while ((r = read(p[0], buffer, sizeof(buffer))) > 0) {
if (r > 0) {
memcpy(output + total, buffer, r);
total += r;
}
}
close(p[0]);
wait(child);
output[total] = '\0';
while (total > 0 &&
(output[total - 1] == '\n' || output[total - 1] == '\r')) {
output[--total] = '\0';
}
return total;
}
}
void tackle_quote(char *line) {
char result[1024] = "";
char *dest = result;
char *src = line;
while (*src) {
if (*src == '`') {
src++;
char *start = src;
while (*src && *src != '`') {
src++;
}
if (*src == '`') {
int len = src - start;
char cmd[MAXPATHLEN];
strcpy(cmd, start);
cmd[len] = '\0';
char output[MAXPATHLEN];
if (run_quote_cmd(cmd, output) >= 0) {
for (char *p = output; *p; p++) {
if (*p == '\n' || *p == '\r') {
if (dest - result < sizeof(result) - 1) {
*dest++ = ' ';
}
} else {
if (dest - result < sizeof(result) - 1) {
*dest++ = *p;
}
}
}
}
src++;
} else {
if (dest - result < sizeof(result) - 1) {
*dest++ = '`';
}
src = start;
}
} else {
if (dest - result < sizeof(result) - 1) {
*dest++ = *src;
}
src++;
}
}
*dest = '\0';
strcpy(line, result);
}
10. 一行多指令
- 递归创建新进程，执行右侧指令
case ';':
fork_id = fork();
if (fork_id < 0) {
exit();
} else if (fork_id == 0) {
return argc;
} else {
wait(fork_id);
return parsecmd(argv, rightpipe);
}
break;