分类： LINUX
2009-12-18 21:53:58
/dev/null：
In Unix-like operating systems, /dev/null or the null device is a special file that discards all data written to it, and provides no data to any process that reads from it (it returns EOF). In Unix programmer jargon, it may also be called the bit bucket or black hole.
The null device is typically used for disposing of unwanted output streams of a process, or as a convenient empty file for input streams. This is usually done by redirection.
This entity is a common inspiration for technical jargon expressions and metaphors by Unix programmers, e.g. "please send complaints to /dev/null" or "my mail got archived in /dev/null", being jocular ways of saying, respectively: "don't bother to send any complaints" and "my mail got deleted". A famous advertisement for the Titanium PowerBook G4 read [The Titanium Powerbook G4] Sends other UNIX boxes to /dev/null.
The null device is also a favorite subject of technical jokes, such as warning users that the system's /dev/null is already 98% full. The April Fool's, 1995 issue of the German magazine c't reported on an enhanced /dev/null chip that would efficiently dispose of the incoming data by converting it to flicker on an internal glowing LED.
/dev/null is a special file, not a directory (folder), so one cannot move files into it with the Unix mv command. See rm for the proper way to delete files in Unix.
The equivalent device in CP/M (and later DOS and Windows) is called NUL:, and on some versions of DOS just NUL (for example, one may hide output by directing it to NUL, e.g. PAUSE>NUL, which waits for the user to press any key without printing anything to the screen). Under classic Amiga operating systems, the device's name is NIL:. In Windows NT and its successors, it is named \Device\Null internally, though, the DOS NUL is a symbolic link to it. Similarly, in OpenVMS the device is named NL:.
这个就和路由里面的ip route 192.168.2.0 255.255.255.0 null 相通的。路由里面是把来自特定IP的数据包抛弃。因此这种路由也叫做路由黑洞。有时候为了避免路由环路，会用到这种配置。
linux中，在shell脚本编程时，有时不希望命令运行结果或错误信息输出到标准输出，可以将其重定向到日志文件
commond 2>&1 ,如果不希望错误信息写入文件便可以将其输出到dev/null.
/dev/zero：
In Unix-like operating systems, /dev/zero is a special file that provides as many null characters (ASCII NULL, 0x00; not ASCII character "digit zero", "0", 0x30) as are read from it. One of the typical uses is to provide a character stream for overwriting information. Another might be to generate a clean file of a certain size. Using mmap to map /dev/zero to RAM is the BSD way of implementing shared memory.
# Initialise partition (important note: trying out this command will eradicate
# any files that were on the partition, make sure you have a backup of any important data.)
dd if=/dev/zero of=/dev/hda7
# Create a large empty file called 'foobar'
dd if=/dev/zero of=foobar count=1000 bs=1000
Like /dev/null, /dev/zero acts as a source and sink for data. All writes to /dev/zero succeed with no other effects (the same as for /dev/null, although /dev/null is the more commonly used data sink); all reads on /dev/zero return as many NULs as characters requested.
