ls命令的实现

转自：http://blog.chinaunix.net/space.php?uid=25909722&do=blog&id=2856481

http://blog.chinaunix.net/uid-25909722-id-2876133.html

ls - list directory contents (man ls)

ls的功能是列出目录中的内容。那目录中都有写什么内容呢？我们知道磁盘上的文件系统由文件和目录组成，文件的内容可以是任意的数据，而目录的内容只能是文件名的列表和子目录名的列表。

也就是说：目录中的内容是该目录下的文件和子目录它们的名字的列表。

我们先来看一看关于目录的结构体：(/usr/include/bits/dirent.h)

struct dirent {

ino_t d_ino;
/* inode number */

off_t d_off;
/* offset to the next dirent */

unsigned short d_reclen;
/* length of this record */

unsigned char d_type;
/* type of file; not supportedby
all file system types */

char d_name[256]; /* filename */

};

d_ino是目录中的文件的i-node节点号，通过d_ino可以找到该文件的i-node节点，而i-node节点中保存了文件的属性等信息。通过i-node节点就可以访问到文件的内容。

关于目录访问的一些函数：(/usr/include/dirent.h)

#include <sys/types.h>

#include <dirent.h>

DIR *opendir(const char *name);

DIR *fdopendir(int fd);

struct dirent *readdir(DIR *dirp);

int readdir_r(DIR *dirp, struct
dirent *entry, struct dirent **result);

int closedir(DIR *dirp);

void rewinddir(DIR *dirp);

关于目录中文件的属性等信息使用struct
stat结构体来描述的，其结构的定义如下： （/usr/include/bits/stat.h）

struct stat

{

__dev_t st_dev; /* Device. */

unsigned short int __pad1; /* 用于填充对齐 */

__ino_t st_ino; /* 32bit
file serial number. */

__mode_t st_mode; /* File
mode. */

__nlink_t st_nlink; /* Link count. */

__uid_t st_uid; /* User
ID of the file's owner. */

__gid_t st_gid; /* Group
ID of the file's group.*/

__dev_t st_rdev; /* Device
number, if device. */

unsigned short int __pad2; /* 用于填充对齐 */

__off_t st_size; /* Size
of file, in bytes. */

__blksize_t st_blksize; /* Optimal
block size for I/O. */

__blkcnt_t st_blocks; /* Number
512-byte blocks allocated. */

struct timespec st_atim; /* Time of
last access. */

struct timespec st_mtim; /* Time of
last modification. */

struct timespec st_ctim; /* Time of
last status change. */

# define st_atime st_atim.tv_sec /* Backward
compatibility. */

# define st_mtime st_mtim.tv_sec

# define st_ctime st_ctim.tv_sec

__ino64_t st_ino; /* File
serial number. */

};

该结构体提供了关于文件(或者设备)的如下重要信息：

st_mode 文件类型和许可权限

st_nlink 文件链接数

st_uid 文件属主id

st_gid 文件属主所在组的id

st_size 文件的字节数

st_blocks 文件所占的块数

“ACM”三个时间：

st_atime 文件最后访问时间 (access time)

st_mtime 文件最后修改时间 (modification time)

st_ctime 文件属性/状态最后改变的时间 (change status time)

另外在/usr/include/sys/stat.h中提供了下列关于文件信息的宏定义：

typedef __dev_t dev_t;

typedef __gid_t gid_t;

typedef __ino_t ino_t;

typedef __mode_t mode_t;

typedef __nlink_t nlink_t;

typedef __off_t off_t;

typedef __uid_t uid_t;

typedef __blkcnt_t blkcnt_t;

typedef __blksize_t blksize_t;

/* Test macros for file
types. */

#define __S_ISTYPE(mode, mask) (((mode) & __S_IFMT) == (mask))

#define S_ISDIR(mode) __S_ISTYPE((mode), __S_IFDIR)
// directory 目录文件类型

#define S_ISCHR(mode) __S_ISTYPE((mode), __S_IFCHR)
// char 字文件类型

#define S_ISBLK(mode) __S_ISTYPE((mode), __S_IFBLK)
// block 块文件类型

#define S_ISREG(mode) __S_ISTYPE((mode), __S_IFREG)
// regular 普通文件类型

#define S_ISFIFO(mode) __S_ISTYPE((mode), __S_IFIFO)
// FIFO 管道文件类型

#define S_ISLNK(mode) __S_ISTYPE((mode), __S_IFLNK)
// link 链接文件类型

#define S_ISSOCK(mode) __S_ISTYPE((mode), __S_IFSOCK) // sock
网络套接字文件类型

/* Protection bits. */

#define S_ISUID __S_ISUID /* Set user
ID on execution. */

#define S_ISGID __S_ISGID /* Set group
ID on execution. */

#define S_IRUSR __S_IREAD /* Read by owner. */

#define S_IWUSR __S_IWRITE /* Write by owner. */

#define S_IXUSR __S_IEXEC /* Execute by owner. */

/* Read, write, and execute
by owner. */

#define S_IRWXU (__S_IREAD|__S_IWRITE|__S_IEXEC)

#define S_IRGRP (S_IRUSR >> 3) /* Read
by group. */

#define S_IWGRP (S_IWUSR >> 3) /* Write
by group. */

#define S_IXGRP (S_IXUSR >> 3) /* Execute
by group. */

/* Read, write, and execute
by group. */

#define S_IRWXG (S_IRWXU >> 3)

#define S_IROTH (S_IRGRP >> 3) /* Read
by others. */

#define S_IWOTH (S_IWGRP >> 3) /* Write
by others. */

#define S_IXOTH (S_IXGRP >> 3) /* Execute
by others. */

/* Read, write, and execute
by others. */

#define S_IRWXO (S_IRWXG >> 3)

# define S_BLKSIZE 512 /* Block size for `st_blocks'. */

在上一篇博文http://blog.chinaunix.net/space.php?uid=25909722&do=blog&id=2856481中，我们学习了实现ls命令要涉及到的相关的结构体和函数。有了前面的基础，本文我们实实在在的实现ls命令：

#include <stdio.h>

#include <sys/types.h>

#include <dirent.h>

#include <sys/stat.h>

#include <string.h>

#include <time.h>

#include <pwd.h>

#include <grp.h>

void do_ls(char[]);

void do_stat(char*);

void show_file_info(char*, struct
stat*);

void mode_str(int, char[]);

char *uid_str(uid_t);

char *gid_str(gid_t);

int main(int argc, char *argv[])

{

if(argc == 1)

do_ls(".");

else

while(--argc){

printf("%s:\n", *++argv);

do_ls(*argv);

}

return 0;

}

void do_ls(char dirname[])

{

DIR *dir_ptr; /* the
directory */

struct dirent *direntp; /* each entry */

char full_path[256];

if((dir_ptr = opendir(dirname)) == NULL){

fprintf(stderr, "ls2:
cannot open %s\n", dirname);

}else{

while((direntp = readdir(dir_ptr)) != NULL){

strcpy(full_path, dirname);

int dir_len = strlen(dirname);

if(dirname[dir_len - 1] != '/'){
/* 处理目录字符串最后没有‘/’的情况 */

full_path[dir_len] = '/';

strcpy(full_path + dir_len + 1, direntp->d_name);

}else

strcpy(full_path + dir_len, direntp->d_name);

do_stat(full_path);

}

closedir(dir_ptr);

}

}

void do_stat(char *filename) /* 获得目录中文件的相关信息 */

{

struct stat info;

/* 如果filename最后没有‘/’的话，stat调用失败 */

if(stat(filename, &info) == -1){ /* cannot
stat */

perror("stat"); /* say
why */

printf("filename:%s\n", filename);

}else{

char *pname = strrchr(filename, '/');

show_file_info(pname + 1, &info);

}

}

void show_file_info(char *filename, struct
stat *info_p) /* 打印文件的相关信息 */

{

char modestr[11];

mode_str(info_p->st_mode, modestr);

printf("%s", modestr);

printf("%3d ", (int)info_p->st_nlink);

printf("%-8s", uid_str(info_p->st_uid));

printf("%-8s", gid_str(info_p->st_gid));

printf("%4ld ", (long)info_p->st_size);

printf("%.12s ", 4 + ctime(&info_p->st_mtime));

printf("%s\n", filename);

}

void mode_str(int mode, char
str[]) /* 将文件的相关信息转化成
“crw-rw---"的形式 */

{

strcpy(str, "----------"); /* default = no
perms */

if(S_ISDIR(mode)) str[0] = 'd'; /* directory */

if(S_ISCHR(mode)) str[0] = 'c'; /* char
device */

if(S_ISBLK(mode)) str[0] = 'b'; /* block
device */

if(S_ISLNK(mode)) str[0] = 'l';

if(mode & S_IRUSR) str[1] = 'r'; /* 3
bits for user */

if(mode & S_IWUSR) str[2] = 'w';

if(mode & S_IXUSR) str[3] = 'x';

if(mode & S_IRGRP) str[4] = 'r'; /* 3
bits for group */

if(mode & S_IWGRP) str[5] = 'w';

if(mode & S_IXGRP) str[6] = 'x';

if(mode & S_IROTH) str[7] = 'r'; /* 3
bits for other */

if(mode & S_IWOTH) str[8] = 'w';

if(mode & S_IXOTH) str[9] = 'x';

}

char *uid_str(uid_t uid) /* 将uid转化成username */

{

static char numstr[10];

struct passwd *pw_ptr;

if((pw_ptr = getpwuid(uid)) == NULL){

sprintf(numstr, "%d", uid);

return numstr;

}else

return pw_ptr->pw_name;

}

char *gid_str(gid_t gid) /* 将gid转化成groupname */

{

static char numstr[10];

struct group *grp_ptr;

if((grp_ptr = getgrgid(gid)) == NULL){

sprintf(numstr, "%
d", gid);

return numstr;

}else

return grp_ptr->gr_name;

}

编译和运行结果：

digdeep@ubuntu:~/uulp$ gcc -Wall -o ls1 ls1.c

digdeep@ubuntu:~/uulp$ ./ls1 /etc/network/

/etc/network/:

drwxr-x--- 2 root root 4096 Apr 26 07:03 if-down.d

drwxr-x--- 2 root root 4096 Jul 14 10:47 if-up.d

drwxr-x--- 2 root root 4096 Apr 26 07:05 if-pre-up.d

drwxr-x--- 6 root root 4096 Jun 14 05:34 .

drwxr-x--- 2 root root 4096 Apr 26 07:05 if-post-down.d

-rwrr----- 1 root root 196 Jun 14 05:33 interfaces

drwxr-x---147 root root 12288 Sep 19 20:33 ..