实现的功能：
１重定向： < 、>、>>， dup2(oldfd,newfd)，标准输入的文件描述符为０，标准输出的为１，标准错误输出的为２
２管道： | 也需要用到dup2(oldfd,newfd)，标准输出用１，标准输入用０
３后台运行程序(&)：父进程直接返回，不等待子进程结束
４ｓｈｅｌｌ内置命令：只实现ｃｄ
５ｔａｂ键补全和上下键：需要用到ｒｅａｄｌｉｎｅ库，sudo apt-get install libreadline6-dev安装好之后，编译时加上－ｌｒｅａｄｌｉｎｅ
６ctrl+c不能中断程序： signal(SIGINT, SIG_IGN)用这个函数将ctrl+c忽略掉
７环境变量：将可执行文件放置在/bin目录下，命令执行时，系统会在$ＰＡＴＨ路劲下寻找可执行的文件
８界面美观，尽量接近ｂａｓｈ
下面是代码：
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <dirent.h>
#include <signal.h>
#include <readline/readline.h>
#include <readline/history.h>
#include <pwd.h>
#define normal 0 //一般的命令
#define out_redirect 1 //输出重定向
#define in_redirect 2 //输入重定向
#define have_pipe 3 //命令中有管道
#define out_redirec 4 //命令中有>>
void do_cmd(int argcout, char arglist[100][256]); //执行ａｒｇｌｉｓｔ中保存的命令
int find_command(char *command); // 在当前目录，以及/bin、/use/bin下查找命令的可执行程序
void explain_input(char *buf, int *argcout, char arglist[100][256]); //解析ｂｕｆ中的命令，将每个选项存到ａｒｇｌｉｓｔ
char oldpwd[300]; //为实现ｃｄ －，用ｏｌｄｐｗｄ来保存上个路径
int main(int argc, char **argv)
{
struct passwd *name;
signal(SIGINT, SIG_IGN);
int i, len;
int argcout = 0;
char arglist[100][256], temp[256], pwd[100];
char *buf = NULL, *s = NULL;
buf=(char *)malloc(sizeof(char)*256);
if( buf == NULL ){
perror("malloc failed");
exit(-1);
}
getcwd(oldpwd, sizeof(oldpwd)); //获取当前路径
while(1){
name = getpwuid(getuid());
getcwd(pwd, sizeof(pwd) - 1);
sprintf(temp, "[%s @myshell:\033[0;34m%s\033[0m]$ ", name->pw_name, pwd); //使输出更加美观
memset(buf,0,256);
s = readline(temp); //用readline()函数读取输入命令
add_history(s);
write_history(NULL);
strcpy(buf, s); //将命令拷贝到ｂｕｆ中
if ( strcmp(buf, "cd") == 0 || strcmp (buf, "cd ") == 0 ) {
//若命令中有ｃｄ，则默认为ｃｄ ～
strcat(buf, " ~");
}
len = strlen(buf);
len++;
buf[len++] = '\n';
buf[len] = '\0';
if( strcmp(buf,"exit") == 0 || strcmp(buf,"logout") == 0 ) //输入ｅｘｉｔ或者ｌｏｇｏｕｔ退出程序
break;
for( i = 0; i < 100; i++ ) {
arglist[i][0]='\0';
}
argcout = 0;
explain_input(buf,&argcout,arglist);
do_cmd(argcout,arglist);
}
if( buf != NULL ){
free(buf);
buf = NULL;
}
exit(0);
}
void explain_input(char *buf,int *argcout,char arglist[100][256])
{
int number = 0;
char *p = buf;
char *q = buf;
while(1){
if( p[0] == '\n' )
break;
if( p[0] == ' ' )
p++;
else{
q = p;
number = 0;
while( (q[0] != ' ') && (q[0] != '\n') ) {
number++;
q++;
}
strncpy(arglist[*argcout],p,number+1);
arglist[*argcout][number] = '\0';
*argcout = *argcout + 1;
p = q;
}
}
}
void do_cmd(int argcout,char arglist[100][256])
{
int flag=0;
int how =0; //用于只是命令中是否含有>,<,>>,|
int backgroud = 0; //用于命令中是否有&
int status;
int i;
int fd;
char *arg[argcout+1];
char *argnext[argcout+1];
char *file;
pid_t pid;
for( i = 0; i < argcout; i++ )
arg[i]=(char *)arglist[i];
arg[argcout] = NULL;
for( i = 0; i < argcout; i++ ) {
if( strncmp(arg[i],"&",1) == 0 ) {
if( i == argcout-1 ) {
backgroud = 1;
arg[argcout - 1] =NULL;
break;
}
else {
printf("wrong command\n");
return ;
}
}
if ( strcmp(arg[i],"cd") == 0 ) {
//判断命令中是否有ｃｄ
if ( !strcmp(arg[i+1], "~" ) ) {
strcpy(arg[i+1], "/home/crista");
}
if ( strcmp(arg[i+1], "-") == 0 ) {
strcpy(arg[i+1], oldpwd);
}
getcwd(oldpwd, sizeof(oldpwd));
chdir(arg[i+1]);
return ;
}
}
for( i = 0; arg[i] != NULL; i++ ) {
if( strcmp(arg[i],">") == 0 ) {
flag++;
how = out_redirect;
if (arg[i+1] == NULL)
flag++;
}
if( strcmp(arg[i],"<") == 0 ) {
flag++;
how = in_redirect;
if(i == 0)
flag++;
}
if( strcmp(arg[i],"|") == 0 ) {
flag++;
how = have_pipe;
if(arg[i+1] == NULL)
flag++;
if( i == 0)
flag++;
}
if ( strcmp(arg[i],">>") == 0 ) {
flag++;
how = out_redirec;
if (arg[i+1] == NULL)
flag++;
}
}
if(flag > 1){
printf("wrong command !\n"); //命令中只支持一个<,>,>>,|
return ;
}
if( how == out_redirect ){
for( i = 0; arg[i]!=NULL; i++ ) {
if(strcmp(arg[i],">") == 0) {
file = arg[i+1];
arg[i] = NULL;
}
}
}
if(how == in_redirect){
for(i=0; arg[i] != NULL;i++){
if(strcmp(arg[i],"<") == 0){
file = arg[i+1];
arg[i] = NULL;
}
}
}
if(how == have_pipe){
for( i=0; arg[i]!=NULL ;i++){
if(strcmp(arg[i],"|") == 0){
arg[i] = NULL;
int j;
for(j=i+1;arg[j]!=NULL;j++){
argnext[j-i-1] = arg[j];
}
argnext[j-i-1] = arg[j];
break;
}
}
}
if( how == out_redirec ){
for( i = 0; arg[i]!=NULL; i++ ) {
if(strcmp(arg[i],">>") == 0) {
file = arg[i+1];
arg[i] = NULL;
}
}
}
if( (pid = fork()) < 0 ) {
printf("fork error\n");
return ;
}
switch(how){
case 0:
if(pid == 0){
if( !(find_command(arg[0])) ){
printf("%s :command not found !\n",arg[0]);
exit(0);
}
execvp(arg[0],arg);
exit(0);
}
break;
case 1:
if(pid == 0){
if( !(find_command(arg[0])) ){
printf("%s :command not found !\n",arg[0]);
exit(0);
}
fd = open(file,O_RDWR | O_CREAT | O_TRUNC,0644);
dup2(fd,1);
execvp(arg[0],arg);
exit(0);
}
break;
case 2:
if(pid == 0){
if( !(find_command(arg[0])) ){
printf("%s :command not found !\n",arg[0]);
exit(0);
}
fd = open(file,O_RDONLY);
dup2(fd,0);
execvp(arg[0],arg);
close(fd);
exit(0);
}
break;
case 3:
if(pid == 0){
int pid2;
int status2;
int fd2;
if( (pid2 = fork()) < 0){
printf("fork2 error\n");
return ;
}
else if(pid2 == 0){
if(!(find_command)(arg[0])){
printf("%s :command not found !\n",arg[0]);
exit(0);
}
fd2 = open("/tmp/youdonotknowfile",O_WRONLY | O_CREAT | O_TRUNC ,0644);
dup2(fd2,1);
execvp(arg[0],arg);
exit(0);
}
if(waitpid(pid2,&status2,0) == -1){
printf("wait for child process error\n");
}
if(!(find_command)(argnext[0])){
printf("%s :command not found !\n",argnext[0]);
exit(0);
}
fd2 = open("/tmp/youdonotknowfile",O_RDONLY);
dup2(fd2,0);
execvp(argnext[0],argnext);
if( remove("/tmp/youdonotknowfile") )
printf("remove error\n");
exit(0);
}
break;
case 4:
if(pid == 0){
if( !(find_command(arg[0])) ){
printf("%s :command not found !\n",arg[0]);
exit(0);
}
fd = open(file,O_RDWR | O_CREAT | O_APPEND,0644);
dup2(fd,1);
execvp(arg[0],arg);
exit(0);
}
break;
default :
break;
}
if( backgroud ==1 ) {
printf("[process id %d]\n",pid); //父进程退出，不等待子进程，表示后台执行
exit(0);
}
if(waitpid(pid,&status,0) == -1){
printf("wait for child process error!\n"); //父进程等待子进程结束
}
}
int find_command(char *command)
{
DIR *dp;
struct dirent* dirp;
char *path[] = {
"./","/bin","/usr/bin",NULL};
if(strncmp(command,"./",2) == 0)
command =command + 2;
int i=0;
while(path[i]!=NULL){
if((dp = opendir(path[i])) == NULL){
printf("can not open /bin\n");
}
while((dirp = readdir(dp)) != NULL){
if(strcmp(dirp -> d_name,command) == 0){
closedir(dp);
return 1;
}
}
closedir(dp);
i++;
}
return 0;
}