实现ls命令

概述

开始学习Unix/Linux上的系统编程，首先实现一个简易的ls命令.

知识点

stat: 得到文件的属性,头文件: #include <sys/stat.h>, 函数原型:int result = stat(char *fname, struct stat *bufp).
opendir: 打开一个目录,头文件:#include <sys/types.h> 和 #include <dirent.h>，函数原型:DIR *opendir(const char *name);
readdir: 读一个目录,头文件:#include <dirent.h>,函数原型:struct dirent *readdir(DIR *dirp);
closedir: 关闭一个目录,头文件:#include <sys/types.h> 和 #include <dirent.h> 函数原型:int closedir(DIR *dirp);

ls工作原理

main()
    opendir(char *)
       create a connection
       return a DIR *
    while(readdir(DIR *))
       reads next records,
       return a pointer to
       a struct dirent
    closedir(DIR *)
       close a connection

简易ls代码实现

/* ls2.c
 *    purpose  list contents of directory or directories
 *    action   if no args, use .  else list files in args
 *    note     uses stat and pwd.h and grp.h 
 *    BUG: try ls2 /tmp  
 */
#include    <stdio.h>
#include    <sys/types.h>
#include    <dirent.h>
#include    <sys/stat.h>
#include    <string.h>
#include    <time.h>
#include    <grp.h>
#include    <pwd.h>

void do_ls(char[]);
void dostat(char *);
void show_file_info( char *, struct stat *);
void mode_to_letters( int , char [] );
char *uid_to_name( uid_t );
char *gid_to_name( gid_t );

int main(int ac, char *av[])
{
    if ( ac == 1 )
        do_ls( "." );
    else
        while ( --ac ){
            printf("%s:\n", *++av );
            do_ls( *av );
        }
    return 0;
}

void do_ls( char dirname[] )
/*
 *    list files in directory called dirname
 */
{
    DIR        *dir_ptr;        /* the directory */
    struct dirent    *direntp;        /* each entry     */

    if ( ( dir_ptr = opendir( dirname ) ) == NULL )
        fprintf(stderr,"ls1: cannot open %s\n", dirname);
    else
    {
        while ( ( direntp = readdir( dir_ptr ) ) != NULL )
            dostat( direntp->d_name );
        closedir(dir_ptr);
    }
}

void dostat( char *filename )
{
    struct stat info;

    if ( stat(filename, &info) == -1 )        /* cannot stat     */
        perror( filename );            /* say why     */
    else                    /* else show info     */
        show_file_info( filename, &info );
}

void show_file_info( char *filename, struct stat *info_p )
/*
 * display the info about 'filename'.  The info is stored in struct at *info_p
 */
{
    void    mode_to_letters();
        char    modestr[11];

    mode_to_letters( info_p->st_mode, modestr );

    printf( "%s"    , modestr );
    printf( "%4d "  , (int) info_p->st_nlink);
    printf( "%-8s " , uid_to_name(info_p->st_uid) );
    printf( "%-8s " , gid_to_name(info_p->st_gid) );
    printf( "%8ld " , (long)info_p->st_size);
    printf( "%.12s ", 4+ctime(&info_p->st_mtime));
    printf( "%s\n"  , filename );

}

/*
 * utility functions
 */

/*
 * This function takes a mode value and a char array
 * and puts into the char array the file type and the
 * nine letters that correspond to the bits in mode.
 * NOTE: It does not code setuid, setgid, and sticky
 * codes
 */
void mode_to_letters( int mode, char str[] )
{
    strcpy( str, "----------" );           /* default=no perms */

    if ( S_ISDIR(mode) )  str[0] = 'd';    /* directory?       */
    if ( S_ISCHR(mode) )  str[0] = 'c';    /* char devices     */
    if ( S_ISBLK(mode) )  str[0] = 'b';    /* block device     */

    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_to_name( uid_t uid )
/*
 *    returns pointer to username associated with uid, uses getpw()
 */
{
    struct    passwd *getpwuid(), *pw_ptr;
    static  char numstr[10];

    if ( ( pw_ptr = getpwuid( uid ) ) == NULL ){
        sprintf(numstr,"%d", uid);
        return numstr;
    }
    else
        return pw_ptr->pw_name ;
}


char *gid_to_name( gid_t gid )
/*
 *    returns pointer to group number gid. used getgrgid(3)
 */
{
    struct group *getgrgid(), *grp_ptr;
    static  char numstr[10];

    if ( ( grp_ptr = getgrgid(gid) ) == NULL ){
        sprintf(numstr,"%d", gid);
        return numstr;
    }
    else
        return grp_ptr->gr_name;
}

编译并运行上面的代码:

cc -g -o myls myls.c
./myls