operators (>, >>, <, and |) ，pipe

摘自http://www.physionet.org/faq.shtml，有助于理解输入、输出、pipe等概念

What is a "standard input" or a "standard output"?

These concepts are common to all text mode applications (see the previous question). A program's standard input is whatever it reads from the keyboard (i.e., whatever you type into its terminal emulator window once the program begins to run). A program's standard output is whatever it prints in its terminal emulator window. There are (of course) exceptions, and the exceptions are what make these ideas useful!

First, it's possible to redirect either or both of the standard input and the standard output before the program begins to run, by adding appropriate parameters to the command line. So, for example, a program named pour can read its standard input from a file named teapot, and then write its standard output to another file named teacup, using a command such as:

    pour <teapot >teacup

(For an explanation of this command, see the answer to the next question.)

Second, most applications have an additional standard error output that is also printed in the terminal window, intermingled with the standard output. The standard error output is reserved for warning and error messages. If you redirect the standard output to a file, the standard error output still appears in the terminal window (and is not copied into the file). In most cases, this is useful behavior, since it allows you to see quickly if there have been any errors or warnings without the need to look through what may be lengthy output. If you wish, you can capture the standard error output in its own file using a command such as:

    frobnicate <input >output 2>errors

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If you are running programs from a command prompt (by typing commands into a terminal emulator window or an MS-DOS box), these things can be done easily.

If you have ever used GNU/Linux, Unix, or MS-DOS, you may have captured the output of a program by redirecting it to a file, like this:

    foo >bar

The > operator redirects foo's standard output (which would normally appear on-screen) into a file named bar. If bar exists already, its contents are replaced. If you wish to append foo's output to whatever is already contained in bar, use a command such as this instead:

    foo >>bar

There is an analogous operator that arranges for a program's standard input (which would normally be read from whatever you type on the keyboard) to be read from a file instead:

    baz <bar

Here, the < operator arranges for baz to read its input from a file named bar. If bar was created by foo, then this command allows baz to read foo's output.

You can combine input and output redirection in a single command using the pipe (|) operator:

    foo | baz

This command runs foo and sends its standard output directly to baz, without requiring an intermediate file. True multitasking operating systems such as Unix, GNU/Linux, and Mac OS X allow both programs to run (apparently) simultaneously; under MS-DOS or MS-Windows, the first program runs to completion before the second one begins execution.

You can use these techniques whenever you run programs from a command prompt, whether those programs are among those available here or obtained from some other source. You can use the same techniques with programs you write yourself; the only requirement is that your programs must read from the standard input and write to the standard output (i.e., they must not attempt to bypass the standard input/output mechanism by reading directly from the keyboard or writing directly to the screen).

These operators (>, >>, <, and |) are supported by all shells (command interpreters) under Unix, GNU/Linux, Mac OS X, and MS-DOS (including those that run within MS-DOS boxes or other types of terminal emulators under MS-Windows). For further information, please refer to the documentation for your shell or command interpreter.

 

 

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