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 !=  ){
                 fprintf( stderr, "usage: %s source destination\n", *av);
                 exit();
         }
                         /* open files    */

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

         if ( (out_fd=creat( av[], COPYMODE)) == - )
                 oops( "Cannot creat", av[]);

                         /* copy files    */

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

                         /* close files    */

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

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

 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 !=  ){
                 fprintf( stderr, "usage: %s source destination\n", *av);
                 exit();
         }

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

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

 /*
  * 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)++MAXNAMLEN+);
     destpath = (char *) emalloc(strlen(destdir)++MAXNAMLEN+);
     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,".") !=  &&
                  strcmp(direntp->d_name,"..") !=  )
                 printf("skipping directory %s\n", 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)) == - )
                 oops("Cannot open ", src);

         if ( (out_fd=creat( destfilename, COPYMODE)) == - )
                 oops( "Cannot creat", destfilename);

     /*
      * copy files
      */

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

     /*
      * close files
      */

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

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

 /*
  * 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) == - )    /* 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) + );
     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) != - && S_ISDIR(info.st_mode) );
 }