操作系统实验四：Shell的实现
一、实验内容
实现具有管道、重定向功能的shell
能够执行一些简单的基本命令，如进程执行、列目录等。
二、实验目的
1.学习并理解linux中shell的知识；
2.重点学会编程实现管道和重定向的功能；
3.实现自己的shell
三、设计思路和流程图
1.对输入的命令进行解析
实验内容主要是管道和重定向，这两个功能涉及shell“|”和“<”以及“>”等不同符号，所以要对输入的命令进行解析。初步按照空格来分，之后再按照<、>、|这些涉及管道和重定向的符号来分。
2.简单命令的执行
使用函数execvp可以实现简单的命令，这些命令暂时不涉及管道和重定向，函数原型为int execvp(const char *file ,char * const argv []);，execvp()会从PATH 环境变量所指的目录中查找符合参数file 的文件名，找到后便执行该文件，然后将第二个参数argv传给该欲执行的文件。为了不造成阻塞，这里启用了一个新线程实现它，最后父进程需等待子进程，以回收分配给它的资源。下面有些地方也用到这种方法。
3.输入输出重定向的实现
实现重定向的主要函数是freopen，FILE *freopen( const char *path, const char *mode, FILE *stream );path: 文件名，用于存储输入输出的自定义文件名。 mode: 文件打开的模式。和fopen中的模式（如r-只读, w-写）相同。 stream: 一个文件，通常使用标准流文件。函数实现重定向，把预定义的标准流文件定向到由path指定的文件中。要注意的是第二个参数，刚开始我是用的a+，结果每次输出都加到文件末尾。后来查了一下，改成w+可以先清空再写入文件。
4.管道功能的实现
命令之间通过|符号来分隔，使用pipe函数来建立管道。如何分隔这些命令呢？上面是写一个函数通过空格来分离每个字符串，这里通过strtok_r函数来分隔命令。利用pipe函数生成的的读取端和写入端，第一条命令的输出作为第二条命令的输入，从而实现管道的功能。
四、主要数据结构及说明
主要使用了数组和指针，存放相关的命令，通过字符串操作实现一些基本的逻辑。
五、源程序并附上注释
上面所说的设计思路与程序中的函数是对应的。
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <string.h>
#include <sys/stat.h>
#include<signal.h>
#include <fcntl.h>
#define hist_size 1024
char *hist[hist_size];
int f = 0; //to save change in directory
int head = 0, filled = 0;
//1-parse user's input
void parse(char *word, char **argv)
{
int count = 0;
memset(argv, 0, sizeof(char*)*(64));//copies 0 to the first 64 characters of the string pointed to by the argument argv.
char *lefts = NULL;
const char *split = " "; //setting delimeter
while (1)
{
char *p = strtok_r(word, split, &lefts);//ref-"strtok-r":http://baike.baidu.com/view/6991509.htm?fr=aladdin
if (p == NULL)
{
break;
}
argv[count] = p;//argv is an array, it stores each value of your input divided by " "
word = lefts;
//printf("%s\n",argv[count]);
//printf("%s\n",word);
count++;
}
if (strcmp(argv[0], "exit") == 0)
exit(0);
else if (strcmp(argv[0], "cd") == 0)
{
int ch = chdir(argv[1]); //ref-"chdir":http://baike.baidu.com/view/653970.htm?fr=aladdin
/*if(ch<0)
{
printf("No such file or directory %d\n.",ch);
}*/
f = 1;
}
}
//2-get the first word
char *trim(char *string)//remove extra spaces
{
int i = 0;
int j = 0;
char *ptr = malloc(sizeof(char*)*strlen(string));
for (i = 0; string[i] != '\0'; i++)
if (string[i] != ' ')
{
ptr[j] = string[i];
j++;
}
ptr[j] = '\0';
string = ptr;
//printf("%s\n",string);
//printf("%d\n",j);
return string;
}
//3-execute the basic order
void execute(char **argv)
{
pid_t pid;
int status;
//fork child process
if ((pid = fork()) < 0)
{
printf("error:fork failed.\n");
exit(1);
}
else if (pid == 0)
{
if (execvp(argv[0], argv) < 0 && strcmp(argv[0], "cd"))//ref-"execvp":http://baike.baidu.com/view/1745420.htm?fr=aladdin
printf("error:invalid command.\n");
exit(0);
}
else
{
while (wait(&status) != pid)//ref-"wait":http://see.xidian.edu.cn/cpp/html/289.html
;
}
}
//4-output redirect
//output:file in which we need to print the output
void execute_file(char **argv, char *output)
{
//printf("output:%s\n",output);
//printf("argv:%s\n",*argv);
pid_t pid;
int status, flag;
char *file = NULL;
if ((pid = fork()) < 0)
{
printf("error:fork failed.\n");
exit(1);//fclose(fd1);
}
else if (pid == 0)
{
if (strstr(output, ">")>0)//returns a pointer to first occurence after > or null
{
char *p = strtok_r(output, ">", &file);
output += 1; //change2
file = trim(file);//get the first word of file, that is to say, get the first word after >
flag = 1;
//printf("file : %s output : %s \n",*argv,output);
int old_stdout = dup(1);
//printf("mark\n");
//output redirect
FILE *fp1 = freopen(output, "w+", stdout);//ref-"freopen":http://baike.baidu.com/view/656692.htm?fr=aladdin
//printf("mark\n");
execute_file(argv, file);
fclose(stdout);
FILE *fp2 = fdopen(old_stdout, "w");//ref-"fdopen":http://baike.baidu.com/view/656646.htm?fr=aladdin
*stdout = *fp2;
exit(0);
}
if (strstr(output, "<") > 0)
{
char *p = strtok_r(output, "<", &file);
file = trim(file);
flag = 1;
int fd = open(file, O_RDONLY);//ref-"open":http://see.xidian.edu.cn/cpp/html/238.html
if (fd<0)
{
printf("No such file or directory.");
exit(0);
}
}
if (strstr(output, "|")>0)
{
fflush(stdout); printf("here"); fflush(stdout);
char *p = strtok_r(output, "|", &file);
file = trim(file);
flag = 1;
//fflush(stdout);printf("%s",file);fflush(stdout);
char *args[64];
parse(file, args);
execute(args);
}
int old_stdout = dup(1);
FILE *fp1 = freopen(output, "w+", stdout);
if (execvp(argv[0], argv) < 0)
printf("error:in exec");
fclose(stdout);
FILE *fp2 = fdopen(old_stdout, "w");
*stdout = *fp2;
exit(0);
}
else
{
while (wait(&status) != pid)
;
}
}
//5-input redirect
void execute_input(char **argv, char *output)
{
pid_t pid;
int fd;
char *file;
int flag = 0;
int status;
if ((pid = fork()) < 0)
{
printf("error:fork failed\n");
exit(1);
}
else if (pid == 0)
{
if (strstr(output, "<")>0)
{
char *p = strtok_r(output, "<", &file);
file = trim(file);
flag = 1;
//printf("file : %s output : %s \n",file,output);
fd = open(output, O_RDONLY);
if (fd<0)
{
printf("No such file or directory.");
exit(0);
}
output = file;
}
if (strstr(output, ">")>0)
{
char *p = strtok_r(output, ">", &file);
file = trim(file);
flag = 1;
fflush(stdout);
//printf("file : %s output : %s \n",file,output);
fflush(stdout);
int old_stdout = dup(1);
FILE *fp1 = freopen(file, "w+", stdout);
execute_input(argv, output);
fclose(stdout);
FILE *fp2 = fdopen(old_stdout, "w");
*stdout = *fp2;
exit(0);
}
if (strstr(output, "|") > 0)
{
char *p = strtok_r(output, "|", &file);
file = trim(file);
flag = 1;
char *args[64];
parse(file, args);
int pfds[2];
pid_t pid, pid2;
int status, status2;
pipe(pfds);
int fl = 0;
if ((pid = fork()) < 0)
{
printf("error:fork failed\n");
exit(1);
}
if ((pid2 = fork()) < 0)
{
printf("error:fork failed\n");
exit(1);
}
if (pid == 0 && pid2 != 0)
{
close(1);
dup(pfds[1]);
close(pfds[0]);
close(pfds[1]);
fd = open(output, O_RDONLY);
close(0);
dup(fd);
if (execvp(argv[0], argv) < 0)
{
close(pfds[0]);
close(pfds[1]);
printf("error:in exec");
fl = 1;
exit(0);
}
close(fd);
exit(0);
}
else if (pid2 == 0 && pid != 0 && fl != 1)
{
close(0);
dup(pfds[0]);
close(pfds[1]);
close(pfds[0]);
if (execvp(args[0], args) < 0)
{
close(pfds[0]);
close(pfds[1]);
printf("error:in exec");
exit(0);
}
}
else
{
close(pfds[0]);
close(pfds[1]);
while (wait(&status) != pid);
while (wait(&status2) != pid2);
}
exit(0);
}
fd = open(output, O_RDONLY);
close(0);
dup(fd);
if (execvp(argv[0], argv) < 0)
{
printf("error:in exec");
}
close(fd);
exit(0);
}
else
{
while (wait(&status) != pid);
}
}
//6-implement pipe
void execute_pipe(char **argv, char *output)
{
int pfds[2], pf[2], flag;
char *file;
pid_t pid, pid2, pid3;
int status, status2, old_stdout;
pipe(pfds);//create pipe
//pfds[0]:read pfds[1]:write
int blah = 0;
char *args[64];
char *argp[64];
int fl = 0;
if ((pid = fork()) < 0)
{
printf("error:fork failed\n");
exit(1);
}
if ((pid2 = fork()) < 0)
{
printf("error:fork failed\n");
exit(1);
}
if (pid == 0 && pid2 != 0)
{
close(1);
dup(pfds[1]);
close(pfds[0]);
close(pfds[1]);
if (execvp(argv[0], argv) < 0)//run the command
{
close(pfds[0]);
close(pfds[1]);
printf("error:in exec");
fl = 1;
kill(pid2, SIGUSR1);
exit(0);
}
}
else if (pid2 == 0 && pid != 0)
{
if (fl == 1){ exit(0); }
if (strstr(output, "<") > 0)
{
char *p = strtok_r(output, "<", &file);
file = trim(file);
flag = 1;
parse(output, args);//divide output to the array args
execute_input(args, file);
close(pfds[0]);
close(pfds[1]);
exit(0);
}
if (strstr(output, ">") > 0)
{
char *p = strtok_r(output, ">", &file);
file = trim(file);
flag = 1;
//fflush(stdout);printf("file : %s output : %s \n",file,output);fflush(stdout);
parse(output, args);
blah = 1;
}
else
{
parse(output, args);
}
close(0);
dup(pfds[0]);
close(pfds[1]);
close(pfds[0]);
if (blah == 1)
{
old_stdout = dup(1);
FILE *fp1 = freopen(file, "w+", stdout);
}
if (execvp(args[0], args) < 0)
{
fflush(stdout);
printf("error:in exec %d", pid);
kill(pid, SIGUSR1);
close(pfds[0]);
close(pfds[1]);
}
fflush(stdout);
printf("HERE");
//kill (pid, SIGUSR1);
if (blah == 1)
{
fclose(stdout);
FILE *fp2 = fdopen(old_stdout, "w");
*stdout = *fp2;
}
}
else
{
close(pfds[0]);
close(pfds[1]);
while (wait(&status) != pid);
while (wait(&status2) != pid2);
}
}
//7-implement pipe
void execute_pipe2(char **argv, char **args, char **argp)
{
int status;
int i;
int pipes[4];
pipe(pipes);
pipe(pipes + 2);
if (fork() == 0)
{
dup2(pipes[1], 1);
close(pipes[0]);
close(pipes[1]);
close(pipes[2]);
close(pipes[3]);
if (execvp(argv[0], argv) < 0)
{
fflush(stdout);
printf("error:in exec");
fflush(stdout);
close(pipes[0]);
close(pipes[1]);
close(pipes[2]);
close(pipes[3]);
exit(1);
}
}
else
{
if (fork() == 0)
{
dup2(pipes[0], 0);
dup2(pipes[3], 1);
close(pipes[0]);
close(pipes[1]);
close(pipes[2]);
close(pipes[3]);
if (execvp(args[0], args) < 0)
{
fflush(stdout);
printf("error:in exec");
fflush(stdout);
close(pipes[0]);
close(pipes[1]);
close(pipes[2]);
close(pipes[3]);
exit(1);
}
}
else
{
if (fork() == 0)
{
dup2(pipes[2], 0);
close(pipes[0]);
close(pipes[1]);
close(pipes[2]);
close(pipes[3]);
if (execvp(argp[0], argp) < 0)
{
fflush(stdout);
printf("error:in exec");
fflush(stdout);
close(pipes[0]);
close(pipes[1]);
close(pipes[2]);
close(pipes[3]);
exit(1);
}
}
}
}
close(pipes[0]);
close(pipes[1]);
close(pipes[2]);
close(pipes[3]);
for (i = 0; i < 3; i++)
wait(&status);
}
/*
int main()
{
char word[1024]="ls>a.txt";
char *argv[1024];
char *file=NULL;
char *p=strtok_r(word ,">", &file);
file=trim(file);
parse(word,argv);
execute_file(argv,file);
}*/
int main()
{
char line[1024];
char *argv[64];
char *args[64];
char *left;
size_t size = 0;
char ch;
int count = 0;
char *tri;
char *second;
char *file;
int i;
for (i = 0; i < hist_size; i++)
{
hist[i] = (char *)malloc(150);
}
while (1)
{
count = 0;
int flag = 0;
char *word = NULL;
char *dire[] = { "pwd" };
fflush(stdout);
printf("SHELL~");
fflush(stdout);
execute(dire); //print the current directory, we can also use getcwd()
printf("$");
int len = getline(&word, &size, stdin);
if (*word == '\n')
continue;
word[len - 1] = '\0';
char *file = NULL;
int i = 0;
char *temp = (char *)malloc(150);
strcpy(temp, word);
parse(temp, argv);
strcpy(hist[(head + 1) % hist_size], word); //storing an entry in history
head = (head + 1) % hist_size;
filled = filled + 1;
for (i = 0; word[i] != '\0'; i++)
{
if (word[i] == '>')
{
//printf("%s\n",word); //has the initial command
char *p = strtok_r(word, ">", &file);
file = trim(file);
//printf("%s\n",file);
flag = 1;
break;
}
else if (word[i] == '<')
{
char *p = strtok_r(word, "<", &file);
file = trim(file);
//printf("file : %s",file);
flag = 2;
break;
}
else if (word[i] == '|')
{
char *p = strtok_r(word, "|", &left);
flag = 3;
break;
}
}
if (strcmp(word, "exit") == 0)
{
exit(0);
}
if (flag == 1)
{
parse(word, argv); //parsed command stored in argv
//printf("At the right place");
execute_file(argv, file);
}
else if (flag == 2)
{
parse(word, argv);
execute_input(argv, file);
}
else if (flag == 3)
{
char *argp[64];
char *output, *file;
if (strstr(left, "|") > 0)
{
char *p = strtok_r(left, "|", &file);
parse(word, argv);
parse(left, args);
parse(file, argp);
execute_pipe2(argv, args, argp);
}
else
{
parse(word, argv);
execute_pipe(argv, left);
}
}
else
{
parse(word, argv);
execute(argv);
}
}
}
六、程序运行结果及分析
如图，为了便于观察，我用红色框框对命令和结果做了分隔。
可以看出，基本的管道和重定向功能已经实现。
七、实验体会
shell的实现是四个实验中需要写代码最多的一个，因为之前对这个部分的编程知识了解甚少，所以我找了很多资料学习，尤其是其中很多函数的用法，比如pipe、execvp等等有所了解。做完实验最大的感触就是很多书讲得都太笼统，只讲了一些概念和原理，但自己真正要动手实现起来，是有一定难度的。
