cp命令简单实现

/*cp命令简单实现
 *cp src dest
 *支持 -i选项 cp -i src dest
 *-i 询问是否覆盖,当src dest相同时
 */

#include<stdio.h>
#include<stdlib.h>
#include<fcntl.h>
#include<unistd.h>

#define    BUFFSIZE    4096
#define    COPYMODE    0644

void usage(void);
int file_exits(char *filename);        //文件存在返回1,失败返回0
int file_replace(char *filename);

int main(int argc,char *argv[])
{
    char *src = NULL,*dest = NULL;
    char buff[BUFFSIZE];
    int i_option = 0;
    int in_fd,out_fd;
    int n_chars;

    while(--argc)        //处理参数
    {
        if( strcmp("-i",*++argv) == 0 )
            i_option = 1;
        else if( !src )
            src = *argv;
        else if( !dest )
            dest = *argv;
        else
            usage();        //打印使用方法并退出
    }
    if( !src || !dest )
        usage();

    if( (in_fd = open(src,O_RDONLY)) == -1 )    //打开源文件
    {
        perror("open src file failed");
        exit(1);
    }

    if( !i_option || !file_exits(dest) || file_replace(dest) )        //目标文件
    {    //创建一个文件并以只写的方式打开,如果原来该文件存在,会将这个文件的长度截短为0
        if( (out_fd = creat(dest,COPYMODE)) == -1 )
        {
            perror("creat dest failed");
            exit(1);
        }
    }

    while( (n_chars = read(in_fd,buff,BUFFSIZE)) > 0 )
    {
        if( write(out_fd,buff,n_chars) != n_chars )
        {
            perror("write to dest file failed");
            exit(1);
        }
    }
    if( n_chars < 0 )
    {
        perror("read from src file failed");
        exit(1);
    }

    close(in_fd);
    close(out_fd);

    return 0;
}

int file_exits(char *filename)        //文件存在返回1,失败返回0
{
    int fd;
    fd = open(filename,O_RDONLY);
    if(fd == -1)
        return 0;
    else
    {
        if( close(fd) == -1 )
        {
            perror("close file failed");
            exit(1);
        }
        return 1;
    }
}

int file_replace(char *filename)
{
    char ans[10];
    int retval = 0;
    int ch;

    fprintf(stderr,"replace the %s ?",filename);
    if( scanf("%9s",ans) == 1 )
    {
        if( ans[0] == 'Y' || ans[0] == 'y' )
            retval = 1;
    }
    while( (ch=getchar()) != EOF && ch != '\n' )
        continue;
    return retval;
}

void usage(void)
{
    printf("usage:cp [-i] src dest\n");
    exit(1);
}