编写自己的cp命令

　　有时候要对整个目录做备份，修改cp1.c使得当两个参数都是目录时，把第一个目录中的所有文件复制到第二个目录中，文件名不变。那么该如何实现？
　　我们先来看看cp1.c的实现方式，它从一个文件中读取数据然后写到另一个文件中，通过系统调用open（或者creat）、read、wirte和close来完成。从上面我们看出，cp1.c只是针对于一个文件进行的复制操作，而现在我们需要完成对整个目录的备份。参数由一个单独的文件（file）变成一个目录（directory），所以我们首先想到的就是要先进入到这个目录下，然后对该目录下的文件依次进行cp操作，那么这就涉及到了目录的操作，而我们在第三章ls命令的实现过程中，用到的就是对目录的操作，涉及到的系统调用包含有opendir、readdir和closedir。所以现在我们需要把两者用到的技术联系起来，以便完成对目录的备份工作。
　　具体实现：
　　　　1、命令行参数
　　　　　　int argc、char *argv[]
　　　　2、cp源、目的的类型判断
　　　　　　src为dir，dst也必须是dir
　　　　　　src为file，dst可以是anything
　　　　3、目录操作
　　　　　　opendir进入src目录下;
　　　　　　while{
　　　　　　　　readdir获得当前目录下的文件（或目录），递归判断是否还是目录，如果是继续深入;
　　　　　　　　srcpath、dstpath获取，调用cp完成复制;
　　　　　　}
　　　　　　closedir完成目录复制
　　　　4、cp实现
　　　　　　in=open（src）;out=creat（dst）
　　　　　　while{
　　　　　　　　read（in）;
　　　　　　　　write（out）;
　　　　　　}
　　　　　　close（in）;close（out）

具体的代码如下：

/** cp1.c
 *     version 1 of cp - uses read and write with tunable buffer size
 *
 *     usage: cp1 src dest
 */
#include        <stdio.h>
#include        <unistd.h>
#include        <fcntl.h>

#define BUFFERSIZE      4096
#define COPYMODE        0644

void oops(char *, char *);

main(int ac, char *av[])
{
        int     in_fd, out_fd, n_chars;
        char    buf[BUFFERSIZE];
                        /* check args     */
        if ( ac != 3 ){
                fprintf( stderr, "usage: %s source destination
", *av);
                exit(1);
        }
                        /* open files    */

        if ( (in_fd=open(av[1], O_RDONLY)) == -1 )
                oops("Cannot open ", av[1]);

        if ( (out_fd=creat( av[2], COPYMODE)) == -1 )
                oops( "Cannot creat", av[2]);
    
                        /* copy files    */

        while ( (n_chars = read(in_fd , buf, BUFFERSIZE)) > 0 )
                if ( write( out_fd, buf, n_chars ) != n_chars )
                        oops("Write error to ", av[2]);
    if ( n_chars == -1 )
            oops("Read error from ", av[1]);

                        /* close files    */

        if ( close(in_fd) == -1 || close(out_fd) == -1 )
                oops("Error closing files","");
}

void oops(char *s1, char *s2)
{
        fprintf(stderr,"Error: %s ", s1);
        perror(s2);
        exit(1);
}

 改进（添加目录判断）之后的具体实现：

/** cp2.c
 ** ------------------------------------------------------------
    cp2.c is a
    revised version of cp1.c that can copy an entire directory
    file by file to a second directory.  It also supports some
    of the features required by earlier exercises.

 ** ------------------------------------------------------------
 **
 ** 
 *     A version of cp1.c that works if src or dest name directories
 *     (but not if src is a directory and dest is not)
 *
 *     usage: cp1 src dest
 *     If dest names a directory, then copy src to dest/src
 *     If src  names a directory, then copy all files in src to dest
 *        If src is a directory and dest is NOT a directory, quit
 *     Note: if src has a leading path, then only use last component
 *     
 *    build: cc sol03.14.c -o sol03.14
 */
#include        <stdio.h>
#include    <stdlib.h>
#include        <unistd.h>
#include        <fcntl.h>
#include    <sys/stat.h>
#include    <string.h>
#include    <dirent.h>

#define BUFFERSIZE      4096
/*
 * note: the real copy takes the mode of the copy from
 *       the mode of the source.  
 */
#define COPYMODE        0644

void oops(char *, char *);
void *emalloc(size_t);

int
main(int ac, char *av[])
{
        if ( ac != 3 ){
                fprintf( stderr, "usage: %s source destination
", *av);
                exit(1);
        }

    /*
     * if source is a dir, then the dest has to be, too
     */

    if ( isadir(av[1]) ){
        if ( isadir(av[2]) )
            copydir(av[1], av[2]);
        else {
            fprintf(stderr,"cp1: %s is not a directory
", av[2]);
            exit(1);
        }
    }
    /*
     * if source is not a dir, then the dest can be anything
     */
    else
        do_copy( av[1], av[2] );
    return 0;
}

/*
 * copydir()
 *   loops through all files in srcdir, copying each to destdir
 *   uses do_copy but builds the paths here
 *   Note: this function skips subdirectories of srcdir
 */
copydir(char *srcdir, char *destdir)
{
    char           *srcpath, *destpath;
    DIR           *dir_ptr;
    struct dirent  *direntp;

    srcpath  = (char *) emalloc(strlen(srcdir)+1+MAXNAMLEN+1);
    destpath = (char *) emalloc(strlen(destdir)+1+MAXNAMLEN+1);
    if ( (dir_ptr = opendir(srcdir)) == NULL )
        oops("Cannot open directory", srcdir);

    /*
     * loop through all items in src dir
     *  Do not copy directories, and report that so user
     *  realizes not all the items are copied.
     */
    while( ( direntp = readdir(dir_ptr)) != NULL )
    {
        sprintf(srcpath,"%s/%s", srcdir, direntp->d_name);
        if ( isadir(srcpath) ){
            if ( strcmp(direntp->d_name,".") != 0 &&
                 strcmp(direntp->d_name,"..") != 0 )
                printf("skipping directory %s
", srcpath);
            continue;
        }
        sprintf(destpath, "%s/%s", destdir, direntp->d_name);
        do_copy( srcpath, destpath );
    }
    closedir(dir_ptr);
    free(srcpath);
    free(destpath);
}

/*
 * copies a file from src to dest
 * If dest is a directory, then do_copy() copies to
 * a file in dest with the name taken from the filename for
 * src
 */
do_copy(char *src, char *dest)
{
        int     in_fd, out_fd, n_chars;
        char    buf[BUFFERSIZE];
    char    *destfilename;
    char    *make_destfilename(char*,char*);

    destfilename = make_destfilename(src, dest);

    /*
     * open files
     */

        if ( (in_fd=open(src, O_RDONLY)) == -1 )
                oops("Cannot open ", src);

        if ( (out_fd=creat( destfilename, COPYMODE)) == -1 )
                oops( "Cannot creat", destfilename);
    
    /*
     * copy files
     */

        while ( (n_chars = read(in_fd , buf, BUFFERSIZE)) > 0 )
                if ( write( out_fd, buf, n_chars ) != n_chars )
                        oops("Write error to ", destfilename);
    if ( n_chars == -1 )
            oops("Read error from ", src);

    /*
     * close files
     */

        if ( close(in_fd) == -1 || close(out_fd) == -1 )
                oops("Error closing files","");
}

void oops(char *s1, char *s2)
{
        fprintf(stderr,"Error: %s ", s1);
        perror(s2);
        exit(1);
}

/*
 * if dest is a directory, then combine src and dest
 * (see header to this program)
 */

char *
make_destfilename(char *src, char *dest)
{
    struct stat info;
    char   *srcfilename;
    char   *rv;

    if ( stat(dest, &info) == -1 )    /* let someone else handle this */
        return dest;

    if ( ! S_ISDIR(info.st_mode) )    /* ok to copy to other types    */
        return dest;

    /* find last component of source name */
    if ( (srcfilename = strrchr(src, '/')) != NULL )
        srcfilename++;
    else
        srcfilename = src;

    /* use that to construct target name */
    rv = emalloc(strlen(srcfilename) + strlen(dest) + 2);
    sprintf(rv, "%s/%s", dest, srcfilename);

    return rv;
}

void *
emalloc(size_t n)
{
    void *rv = malloc(n);
    if ( rv == NULL )
        oops("Out of memory","");
    return rv;
}
/*
 * boolean: tells if arg names a directory
 */
isadir(char *str)
{
    struct stat info;

    return ( stat(str,&info) != -1 && S_ISDIR(info.st_mode) );
}