Uit: http://labor-liber.org/en/gnu-linux/introduction/all#input_output_redirection

In general, a command (a program):

• Gets data to process fromstandard input or stdin(default: keyboard).
• Returns processed data tostandard output or stdout(default: screen).
• If program execution causes errors, error messages are sent to standard error orstderr (default: screen).

These three are files, and are always open. As all open files, they are assigned to a file descriptor (an integer).

48Redirections

Redirection

Capturing output from a file or a command (or program or script) and sending it as input to another file or command (or program or script).

|

(pipe) redirects standard output of a program as standard input of another.

>

redirects standard output to a file (and overwrites the file if it already exists).

<

uses a file as standard input.

>>

Appends standard output to a file.

< Input_data_file  program1 | program2 > Output_data_file

A "filter" is a program which reads data from standard input, processes it in some way, and sends the processed data to standard output.

49Redirections Examples

2> file

Direct standard error to file

2>> file

Append standard error to file.

2>&1

Print standard error messages to standard output.

1>&2

Print standard output messages to standard error.

50Building Command Lines From Standard Input

xargs

Build and execute command lines from standard input.

Read remaining arguments from standard input instead of specifying them and execute the command.

cd /bin
ls | xargs whatis