Apple Xserve Up Mac OS X Server Specifications download pdf (Page 23)

Chapter 1 Executing Commands 23

Standard Pipes

Many commands can receive text input from the user and print text to the console.

They do so using standard pipes, which are created by the shell and passed to the

command.

Standard pipes include:

Â stdin—The standard input pipe is the means through which data enters a

command. By default, the user enters this from the command-line interface. You can

also redirect the output from files or other commands to stdin.

Â stdout—The standard output pipe is where the command output is sent. By default,

command output is sent to the command line. You can also redirect the output from

the command line to other commands and tools.

stderr—The standard error pipe is where error messages are sent. By default, errors

are displayed on the command line like standard output.

Redirecting Input and Output

From the command line, you can redirect input and output from a command to a file

or another command.

Redirecting output lets you capture the results of running the command and store it in

a file for later use. Similarly, providing an input file lets you provide a command with

preset input data, instead of needing to enter that data.

You can use the following characters to redirect input and output:

In addition to using file redirection, you can also redirect the output of one command

to the input of another using the vertical bar character, or pipe. You can combine

commands in this manner to implement more sophisticated versions of the same

commands.

For example, the command man bash | grep “commands” passes the formatted

contents of the bash man page to the grep tool, which searches those contents for

lines containing the word “commands.” The result is a listing of lines with the specified

text, instead of the entire man page.

For more information about redirection, see the bash man page.

Redirect Description

> Use the greater-than character to redirect command output to a file.

< Use the less-than character to use the contents of a file as input to the command.

>> Use a double greater-than to append output from a command to a file.

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Apple Xserve Up Mac OS X Server Specifications Page 23