file descriptor 0 1 2 一切皆文件 stdout stderr stdin /dev/null 沉默是金 pipes

$>emtry_or_create_a_file.f

$ll>>append_a_file.f

standard output input error

$ls -l /usr/bin >ok.txt  标准输出文件有内容 运行正确的结果信息

$ls -l /bin/usr 2 >ok.txt 标准输出文件有内容 运行错误信息

0 input

1 output

2 error

$ls -l /bin/usr >emptry.txt 标准输出文件无内容  不会将结果输出  缺少文件描述符中的2

将标准输出和标准错误重定向到一个文件的2种办法

$ll /bin/usr >o.f 2>&1

$ll /bin/usr & > o.f

标准错误将重定向到屏幕

$ll /bin/usr 2>&1>o.f

不希望的到输出

$ll /bin/usr 2 >/dev/null

$x 2>/dev/null

https://en.wikipedia.org/wiki/Null_device

In programmer jargon, especially Unix jargon, it may also be called the bit bucket^[2] or black hole.

 

movie.mpeg.001 movie.mpeg.002 movie.mpeg.003 ... movie.mpeg.099

 

$cat movie.mpeg.0*>movie.mpeg

 

 

 

Examples:
cat f - g Output f's contents, then standard input, then g's contents.
cat Copy standard input to standard output.

$cat

标准输入默认从键盘读取内容，于是、、、、、、

 

$tail -f  实时查看文件

$ tail --help
Usage: tail [OPTION]... [FILE]...
Print the last 10 lines of each FILE to standard output.
With more than one FILE, precede each with a header giving the file name.

With no FILE, or when FILE is -, read standard input.

Mandatory arguments to long options are mandatory for short options too.
-c, --bytes=[+]NUM output the last NUM bytes; or use -c +NUM to
output starting with byte NUM of each file
-f, --follow[={name|descriptor}]
output appended data as the file grows;
an absent option argument means 'descriptor'
-F same as --follow=name --retry
-n, --lines=[+]NUM output the last NUM lines, instead of the last 10;
or use -n +NUM to output starting with line NUM
--max-unchanged-stats=N
with --follow=name, reopen a FILE which has not
changed size after N (default 5) iterations
to see if it has been unlinked or renamed
(this is the usual case of rotated log files);
with inotify, this option is rarely useful
--pid=PID with -f, terminate after process ID, PID dies
-q, --quiet, --silent never output headers giving file names
--retry keep trying to open a file if it is inaccessible
-s, --sleep-interval=N with -f, sleep for approximately N seconds
(default 1.0) between iterations;
with inotify and --pid=P, check process P at
least once every N seconds
-v, --verbose always output headers giving file names
-z, --zero-terminated line delimiter is NUL, not newline
--help display this help and exit
--version output version information and exit

NUM may have a multiplier suffix:
b 512, kB 1000, K 1024, MB 1000*1000, M 1024*1024,
GB 1000*1000*1000, G 1024*1024*1024, and so on for T, P, E, Z, Y.

With --follow (-f), tail defaults to following the file descriptor, which
means that even if a tail'ed file is renamed, tail will continue to track
its end. This default behavior is not desirable when you really want to
track the actual name of the file, not the file descriptor (e.g., log
rotation). Use --follow=name in that case. That causes tail to track the
named file in a way that accommodates renaming, removal and creation.

GNU coreutils online help: <http://www.gnu.org/software/coreutils/>
Report tail translation bugs to <http://translationproject.org/team/>
Full documentation at: <http://www.gnu.org/software/coreutils/tail>
or available locally via: info '(coreutils) tail invocation'

 

 

使用-f选项，tail将持续监视这个文件；一旦添加了新行，

新行将会立即显示在屏幕上。

 

【注意对文件更名的跟踪】

 

tee

【管道】

命令从标准输入到读取数据，并将数据发送到标准输出的能力，

是使用了shell特性中的管道。

 

 

Copy standard input to each FILE, and also to standard output.

 

一个T型管道。

 $ll|tee tee.t|grep t

 $ll|tee tee.t|grep t|sort|uniq|wc|tee tee.t.t

 

 

 

 

 

管道  

how to use pipes

http://tldp.org/HOWTO/Bash-Prog-Intro-HOWTO-4.html