file descriptor 0 1 2 一切皆文件 stdout stderr stdin /dev/null 沉默是金 pipes 禁止输出 屏蔽 stdout 和 stderr 输入输出重定向 重定向文件描述符

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.

Shell 输入/输出重定向

https://ipcmen.com/shell-input-output-redirection

大多数 UNIX 系统命令从你的终端接受输入并将所产生的输出发送回​​到您的终端。一个命令通常从一个叫标准输入的地方读取输入，默认情况下，这恰好是你的终端。同样，一个命令通常将其输出写入到标准输出，默认情况下，这也是你的终端。

重定向命令列表如下：

命令	说明
command > file	将输出重定向到 file。
command < file	将输入重定向到 file。
command >> file	将输出以追加的方式重定向到 file。
n > file	将文件描述符为 n 的文件重定向到 file。
n >> file	将文件描述符为 n 的文件以追加的方式重定向到 file。
n >& m	将输出文件 m 和 n 合并。
n <& m	将输入文件 m 和 n 合并。
<< tag	将开始标记 tag 和结束标记 tag 之间的内容作为输入。

需要注意的是文件描述符 0 通常是标准输入（STDIN），1 是标准输出（STDOUT），2 是标准错误输出（STDERR）。

 

 

 

[xiaole@localhost ~]$ wc -l << EOF
> 欢迎
> you
> 太阳
> EOF
3
[xiaole@localhost ~]$

　　

输出重定向

重定向一般通过在命令间插入特定的符号来实现。特别的，这些符号的语法如下所示:

command1 > file1

上面这个命令执行command1然后将输出的内容存入file1。

注意任何file1内的已经存在的内容将被新内容替代。如果要将新内容添加在文件末尾，请使用>>操作符。

实例

执行下面的 who 命令，它将命令的完整的输出重定向在用户文件中(users):

$ who > users

执行后，并没有在终端输出信息，这是因为输出已被从默认的标准输出设备（终端）重定向到指定的文件。

你可以使用 cat 命令查看文件内容：

$ cat users
_mbsetupuser console  Oct 31 17:35
tianqixin    console  Oct 31 17:35
tianqixin    ttys000  Dec  1 11:33

输出重定向会覆盖文件内容，请看下面的例子：

$ echo "IPCMEN：www.ipcmen.com" > users
$ cat users
IPCMEN：www.ipcmen.com
$

如果不希望文件内容被覆盖，可以使用 >> 追加到文件末尾，例如：

$ echo "IPCMEN：www.ipcmen.com" >> users
$ cat users
IPCMEN：www.ipcmen.com
IPCMEN：www.ipcmen.com
$

---

输入重定向

和输出重定向一样，Unix 命令也可以从文件获取输入，语法为：

command1 < file1

这样，本来需要从键盘获取输入的命令会转移到文件读取内容。

注意：输出重定向是大于号(>)，输入重定向是小于号(<)。

实例

接着以上实例，我们需要统计 users 文件的行数,执行以下命令：

$ wc -l users
       2 users

也可以将输入重定向到 users 文件：

$  wc -l < users
       2

注意：上面两个例子的结果不同：第一个例子，会输出文件名；第二个不会，因为它仅仅知道从标准输入读取内容。

command1 < infile > outfile

同时替换输入和输出，执行command1，从文件infile读取内容，然后将输出写入到outfile中。

重定向深入讲解

一般情况下，每个 Unix/Linux 命令运行时都会打开三个文件：

• 标准输入文件(stdin)：stdin的文件描述符为0，Unix程序默认从stdin读取数据。
• 标准输出文件(stdout)：stdout 的文件描述符为1，Unix程序默认向stdout输出数据。
• 标准错误文件(stderr)：stderr的文件描述符为2，Unix程序会向stderr流中写入错误信息。

默认情况下，command > file 将 stdout 重定向到 file，command < file 将stdin 重定向到 file。

如果希望 stderr 重定向到 file，可以这样写：

$ command 2 > file

如果希望 stderr 追加到 file 文件末尾，可以这样写：

$ command 2 >> file

2 表示标准错误文件(stderr)。

如果希望将 stdout 和 stderr 合并后重定向到 file，可以这样写：

$ command > file 2>&1
 
或者
 
$ command >> file 2>&1

如果希望对 stdin 和 stdout 都重定向，可以这样写：

$ command < file1 >file2

command 命令将 stdin 重定向到 file1，将 stdout 重定向到 file2。

---

Here Document

Here Document 是 Shell 中的一种特殊的重定向方式，用来将输入重定向到一个交互式 Shell 脚本或程序。

它的基本的形式如下：

command << delimiter
    document
delimiter

它的作用是将两个 delimiter 之间的内容(document) 作为输入传递给 command。

注意：

• 结尾的delimiter 一定要顶格写，前面不能有任何字符，后面也不能有任何字符，包括空格和 tab 缩进。
• 开始的delimiter前后的空格会被忽略掉。

实例

在命令行中通过 wc -l 命令计算 Here Document 的行数：

$ wc -l << EOF
    欢迎来到
    IPCMEN
    www.ipcmen.com
EOF
3          # 输出结果为 3 行
$

我们也可以将 Here Document 用在脚本中，例如：

#!/bin/bash
# author:IPCMEN
# url:www.ipcmen.com
 
cat << EOF
欢迎来到
IPCMEN
www.ipcmen.com
EOF

执行以上脚本，输出结果：

欢迎来到
IPCMEN
www.ipcmen.com

---

/dev/null 文件

如果希望执行某个命令，但又不希望在屏幕上显示输出结果，那么可以将输出重定向到 /dev/null：

$ command > /dev/null

/dev/null 是一个特殊的文件，写入到它的内容都会被丢弃；如果尝试从该文件读取内容，那么什么也读不到。但是 /dev/null 文件非常有用，将命令的输出重定向到它，会起到”禁止输出”的效果。

如果希望屏蔽 stdout 和 stderr，可以这样写：

$ command > /dev/null 2>&1

注意：0 是标准输入（STDIN），1 是标准输出（STDOUT），2 是标准错误输出（STDERR）。

https://www.tldp.org/LDP/abs/html/io-redirection.html

I/O Redirection

Chapter 20. I/O Redirection

Table of Contents

20.1. Using exec

20.2. Redirecting Code Blocks

20.3. Applications

There are always three default files [1] open, stdin (the keyboard), stdout (the screen), and stderr (error messages output to the screen). These, and any other open files, can be redirected. Redirection simply means capturing output from a file, command, program, script, or even code block within a script (see Example 3-1 and Example 3-2) and sending it as input to another file, command, program, or script.

Each open file gets assigned a file descriptor. [2] The file descriptors for stdin, stdout, and stderr are 0, 1, and 2, respectively. For opening additional files, there remain descriptors 3 to 9. It is sometimes useful to assign one of these additional file descriptors to stdin,stdout, or stderr as a temporary duplicate link. [3] This simplifies restoration to normal after complex redirection and reshuffling (see Example 20-1).

   COMMAND_OUTPUT >
      # Redirect stdout to a file.
      # Creates the file if not present, otherwise overwrites it.
 
      ls -lR > dir-tree.list
      # Creates a file containing a listing of the directory tree.
 
   : > filename
      # The > truncates file "filename" to zero length.
      # If file not present, creates zero-length file (same effect as 'touch').
      # The : serves as a dummy placeholder, producing no output.
 
   > filename
      # The > truncates file "filename" to zero length.
      # If file not present, creates zero-length file (same effect as 'touch').
      # (Same result as ": >", above, but this does not work with some shells.)
 
   COMMAND_OUTPUT >>
      # Redirect stdout to a file.
      # Creates the file if not present, otherwise appends to it.
 
      # Single-line redirection commands (affect only the line they are on):
      # --------------------------------------------------------------------
 
   1>filename
      # Redirect stdout to file "filename."
   1>>filename
      # Redirect and append stdout to file "filename."
   2>filename
      # Redirect stderr to file "filename."
   2>>filename
      # Redirect and append stderr to file "filename."
   &>filename
      # Redirect both stdout and stderr to file "filename."
      # This operator is now functional, as of Bash 4, final release.
 
   M>N
     # "M" is a file descriptor, which defaults to 1, if not explicitly set.
     # "N" is a filename.
     # File descriptor "M" is redirect to file "N."
   M>&N
     # "M" is a file descriptor, which defaults to 1, if not set.
     # "N" is another file descriptor.
 
      #==============================================================================
 
      # Redirecting stdout, one line at a time.
      LOGFILE=script.log
 
      echo "This statement is sent to the log file, \"$LOGFILE\"." 1>$LOGFILE
      echo "This statement is appended to \"$LOGFILE\"." 1>>$LOGFILE
      echo "This statement is also appended to \"$LOGFILE\"." 1>>$LOGFILE
      echo "This statement is echoed to stdout, and will not appear in \"$LOGFILE\"."
      # These redirection commands automatically "reset" after each line.
 
      # Redirecting stderr, one line at a time.
      ERRORFILE=script.errors
 
      bad_command1 2>$ERRORFILE       #  Error message sent to $ERRORFILE.
      bad_command2 2>>$ERRORFILE      #  Error message appended to $ERRORFILE.
      bad_command3                    #  Error message echoed to stderr,
                                      #+ and does not appear in $ERRORFILE.
      # These redirection commands also automatically "reset" after each line.
      #=======================================================================

   2>&1
      # Redirects stderr to stdout.
      # Error messages get sent to same place as standard output.
        >>filename 2>&1
            bad_command >>filename 2>&1
            # Appends both stdout and stderr to the file "filename" ...
        2>&1 | [command(s)]
            bad_command 2>&1 | awk '{print $5}'   # found
            # Sends stderr through a pipe.
            # |& was added to Bash 4 as an abbreviation for 2>&1 |.
 
   i>&j
      # Redirects file descriptor i to j.
      # All output of file pointed to by i gets sent to file pointed to by j.
 
   >&j
      # Redirects, by default, file descriptor 1 (stdout) to j.
      # All stdout gets sent to file pointed to by j.

   0< FILENAME
    < FILENAME
      # Accept input from a file.
      # Companion command to ">", and often used in combination with it.
      #
      # grep search-word <filename
 
   [j]<>filename
      #  Open file "filename" for reading and writing,
      #+ and assign file descriptor "j" to it.
      #  If "filename" does not exist, create it.
      #  If file descriptor "j" is not specified, default to fd 0, stdin.
      #
      #  An application of this is writing at a specified place in a file.
      echo 1234567890 > File    # Write string to "File".
      exec 3<> File             # Open "File" and assign fd 3 to it.
      read -n 4 <&3             # Read only 4 characters.
      echo -n . >&3             # Write a decimal point there.
      exec 3>&-                 # Close fd 3.
      cat File                  # ==> 1234.67890
      #  Random access, by golly.
 
   |
      # Pipe.
      # General purpose process and command chaining tool.
      # Similar to ">", but more general in effect.
      # Useful for chaining commands, scripts, files, and programs together.
      cat *.txt | sort | uniq > result-file
      # Sorts the output of all the .txt files and deletes duplicate lines,
      # finally saves results to "result-file".

Multiple instances of input and output redirection and/or pipes can be combined in a single command line.

command < input-file > output-file
# Or the equivalent:
< input-file command > output-file   # Although this is non-standard.
 
command1 | command2 | command3 > output-file

See Example 16-31 and Example A-14.

Multiple output streams may be redirected to one file.

ls -yz >> command.log 2>&1
#  Capture result of illegal options "yz" in file "command.log."
#  Because stderr is redirected to the file,
#+ any error messages will also be there.
 
#  Note, however, that the following does *not* give the same result.
ls -yz 2>&1 >> command.log
#  Outputs an error message, but does not write to file.
#  More precisely, the command output (in this case, null)
#+ writes to the file, but the error message goes only to stdout.
 
#  If redirecting both stdout and stderr,
#+ the order of the commands makes a difference.

Closing File Descriptors

n<&-

Close input file descriptor n.

0<&-, <&-

Close stdin.

n>&-

Close output file descriptor n.

1>&-, >&-

Close stdout.

Child processes inherit open file descriptors. This is why pipes work. To prevent an fd from being inherited, close it.

# Redirecting only stderr to a pipe.
 
exec 3>&1                              # Save current "value" of stdout.
ls -l 2>&1 >&3 3>&- | grep bad 3>&-    # Close fd 3 for 'grep' (but not 'ls').
#              ^^^^   ^^^^
exec 3>&-                              # Now close it for the remainder of the script.
 
# Thanks, S.C.

For a more detailed introduction to I/O redirection see Appendix F.

Notes

[1]	By convention in UNIX and Linux, data streams and peripherals (device files) are treated as files, in a fashion analogous to ordinary files.
[2]	A file descriptor is simply a number that the operating system assigns to an open file to keep track of it. Consider it a simplified type of file pointer. It is analogous to a file handle in C.
[3]	Using file descriptor 5 might cause problems. When Bash creates a child process, as with exec, the child inherits fd 5 (see Chet Ramey's archived e-mail, SUBJECT: RE: File descriptor 5 is held open). Best leave this particular fd alone.

---