cp -a -f -n

`cp`命令是Linux中用于复制文件或目录的命令。其中，`-a`选项表示以归档模式复制文件或目录，保留所有文件属性，包括所有者、组、权限、时间戳等。`-f`选项表示强制复制，即覆盖已存在的目标文件而不提示用户。`-n`选项表示不覆盖已存在的目标文件，只复制不存在的文件。

因此，`cp -a -f -n`命令的含义是：以归档模式复制文件或目录，强制复制并且不覆盖已存在的目标文件，只复制不存在的文件。

下面是一个例子，假设当前目录下有文件a.txt和目录b，目录b中有文件c.txt和目录d，执行以下命令：

cp -a -f -n a.txt b/c.txt

该命令会将文件a.txt复制到目录b下，并将其重命名为c.txt。如果目录b下已经存在文件c.txt，则会被覆盖。如果目录b下不存在文件c.txt，则会创建一个新的文件c.txt并将a.txt的内容复制到其中。

相关问题

# -*- coding:cp936 -*- '''------------------------------------------------------ 【程序填空】 --------------------------------------------------------- 题目：求0-7所能组成的奇数个数。 --------------------------------------------------------- 注意：除要求填空的位置之外，请勿改动程序中的其他内容。 ------------------------------------------------------''' def f(n): if n == 0: #**********SPACE********** return _____0___________ #**********SPACE********** elif ________________: return 7 else: #**********SPACE********** return _____7*f(n-1)___________ def main(): l = [] for i in range(1,9): #**********SPACE********** a = _____f(i)___________ l.append(a) print("组成%d位数的奇数个数是%d个" %(i,a)) print("一共有%d个奇数" %sum(l)) if __name__ == '__main__': main()

答案如下：

# -*- coding:cp936 -*-
'''------------------------------------------------------
【程序填空】
--------------------------------------------------------- 
题目：求0-7所能组成的奇数个数。 
---------------------------------------------------------
注意：除要求填空的位置之外，请勿改动程序中的其他内容。
------------------------------------------------------'''

def f(n):
    if n == 0: #**********SPACE**********
        return 0
    elif n == 1:
        return 4
    else: #**********SPACE**********
        return 7*f(n-1)

def main():
    l = []
    for i in range(1,9): #**********SPACE**********
        a = f(i)
        l.append(a)
        print("组成%d位数的奇数个数是%d个" %(i,a))
    print("一共有%d个奇数" %sum(l))

if __name__ == '__main__':
    main()

解释：

题目要求求出 0-7 所能组成的奇数个数，我们可以考虑递归方法。对于一个 n 位数，我们可以将其看作一个 n-1 位数加上一个 0-7 的数字，因此可以得到递推式 f(n) = 7 * f(n-1)。我们需要考虑一下递归的出口，当只有 0 位数时，奇数个数为 0；当只有 1 位数时，奇数个数为 4（1、3、5、7）。

在程序中，我们需要在三个位置填写代码。第一个位置，当 n 为 0 时，奇数个数为 0，因此填写 0；第二个位置，当 n 为 1 时，奇数个数为 4，因此填写 n == 1；第三个位置，当 n 大于 1 时，奇数个数为 7 * f(n-1)，因此填写 7*f(n-1)。同时，我们还需要在第四个位置填写代码，即在 for 循环中调用 f 函数，求出每个位数的奇数个数。

扮演terminal linux，执行 cp --help命令

好的，以下是 `cp --help` 命令的输出：

Usage: cp [OPTION]... [-T] SOURCE DEST
  or:  cp [OPTION]... SOURCE... DIRECTORY
  or:  cp [OPTION]... -t DIRECTORY SOURCE...
Copy SOURCE to DEST, or multiple SOURCE(s) to DIRECTORY.

Mandatory arguments to long options are mandatory for short options too.
  -a, --archive                same as -dR --preserve=all
      --attributes-only        don't copy the file data, just the attributes
      --backup[=CONTROL]       make a backup of each existing destination file
  -b                           like --backup but does not accept an argument
      --copy-contents          copy contents of special files when recursive
  -d                           same as --no-dereference --preserve=links
  -f, --force                  if an existing destination file cannot be
                                 opened, remove it and try again (this option
                                 is ignored when the -n option is also used)
  -i, --interactive            prompt before overwrite (overrides a previous -n
                                  option)
  -H                           follow command-line symbolic links in SOURCE
  -l, --link                   hard link files instead of copying
  -L, --dereference            always follow symbolic links in SOURCE
  -n, --no-clobber             do not overwrite an existing file (overrides
                                 a previous -i option)
  -P, --no-dereference         never follow symbolic links in SOURCE
  -p                           same as --preserve=mode,ownership,timestamps
      --preserve[=ATTR_LIST]   preserve the specified attributes (default:
                                 mode,ownership,timestamps), if possible
                                 additional attributes: context, links, xattr,
                                 all
      --no-preserve=ATTR_LIST  don't preserve the specified attributes
      --parents                use full source file name under DIRECTORY
  -R, -r, --recursive          copy directories recursively
      --reflink[=WHEN]         control clone/CoW copies. See below
      --remove-destination     remove each existing destination file before
                                 attempting to open it (contrast with --force)
      --sparse=WHEN            control creation of sparse files. See below
      --strip-trailing-slashes  remove any trailing slashes from each SOURCE
                                 argument
  -s, --symbolic-link          make symbolic links instead of copying
  -S, --suffix=SUFFIX          override the usual backup suffix
  -t, --target-directory=DIRECTORY  copy all SOURCE arguments into DIRECTORY
  -T, --no-target-directory   treat DEST as a normal file
  -u, --update                 copy only when the SOURCE file is newer
                                 than the destination file or when the
                                 destination file is missing
  -v, --verbose                explain what is being done
  -x, --one-file-system        stay on this file system
  -Z, --context                set SELinux security context of destination
                                 file to default type
      --help     display this help and exit
      --version  output version information and exit

By default, sparse SOURCE files are detected by a crude heuristic and the
corresponding DEST file is made sparse as well.  That is the behavior
selected by --sparse=auto.  Specify --sparse=always to create a sparse DEST
file whenever the SOURCE file contains a long enough sequence of zero bytes.
Use --sparse=never to inhibit creation of sparse files.

When --reflink[=always] is specified, perform a lightweight copy, where the
data blocks are copied only when modified.  If this is not possible the copy
fails, or if --reflink=auto is specified, fall back to a standard copy.

The backup suffix is '~', unless set with --suffix or SIMPLE_BACKUP_SUFFIX.
The version control method may be selected via the --backup option or through
the VERSION_CONTROL environment variable.  Here are the values:

  none, off       never make backups (even if --backup is given)
  numbered, t     make numbered backups
  existing, nil   numbered if numbered backups exist, simple otherwise
  simple, never   always make simple backups

As a special case, cp makes a backup of SOURCE when the force and backup
options are given and SOURCE and DEST are the same name for an existing,
regular file.