【背景】
折腾：
【记录】尝试分析Buildroot在make menuconfig后make时的内部执行逻辑
期间，通过：
而了解到了rsync的功能：
大概就是，相当于，从源，到目的，拷贝文件，并且可以设置参数，保持很多文件和文件夹的属性。
感觉和cp很类似。
所以想要搞清楚，rsync和cp的区别。
【折腾过程】
1.参考：
的解释是：
其中一人的解释：
对于拷贝文件，实现备份的话，rsync，比cp更好。
因为rsync只拷贝那些改动了的内容。
2.另外的解释，貌似更合理：
rsync未必是最好。取决于实际情况。
比如：
当拷贝的源和目标，内容很大，而差异很小，则用
cp -u
会更有效。
3.所以先去看看cp的用法：
CLi@PC-CLI-1 ~/develop/buildroot/buildroot-2013.05
$ cp --help
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 (redundant if
the -n option is 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
--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.
Report cp bugs to [email protected]
GNU coreutils home page: <http://www.gnu.org/software/coreutils/>
General help using GNU software: <http://www.gnu.org/gethelp/>
For complete documentation, run: info coreutils 'cp invocation'
CLi@PC-CLI-1 ~/develop/buildroot/buildroot-2013.05
$得知-u是update，更新的意思，只更新SOURCE中更新的内容，或者是目标中缺失的文件：
对于文件备份来说，已经足够用了：
每次备份，的确只要备份：
要么是内容更新
要么是原先没有（缺失的）文件
即可。
4.再参考了：
后，更加推荐用rsync了：
rsync可以显示进度条（虽然不是很完美）：对于拷贝大量文件，还是很有用的。
其举了个例子：
rsync -a --stats --progress /home/tim/stuff /home/tim/documents
而还支持ssh：
rsync -a --stats --progress /home/tim/stuff [email protected]:/home/tim/
或：
rsync -a --stats --progress [email protected]:/home/tim/stuff /home/tim/
5.另外有空也可以参考：
【总结】
rsync：只拷贝那些更新的文件；
cp -u：也可以实现类似效果；
两者都基本可以满足备份的需求；
只是一般情况下，用rsync做这类备份之类的事情，更多见；
总体来说：
备份文件，更推荐用rsync。
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