Which of the following are output redirection that enables to redirect the standard output?

In this lesson, we will explore a powerful feature used by command line programs called input/output redirection. As we have seen, many commands such as ls print their output on the display. This does not have to be the case, however. By using some special notations we can redirect the output of many commands to files, devices, and even to the input of other commands.

Standard Output

Most command line programs that display their results do so by sending their results to a facility called standard output. By default, standard output directs its contents to the display. To redirect standard output to a file, the ">" character is used like this:

[me@linuxbox me]$ ls > file_list.txt

In this example, the ls command is executed and the results are written in a file named file_list.txt. Since the output of ls was redirected to the file, no results appear on the display.

Each time the command above is repeated, file_list.txt is overwritten from the beginning with the output of the command ls. To have the new results appended to the file instead, we use ">>" like this:

[me@linuxbox me]$ls >> file_list.txt

When the results are appended, the new results are added to the end of the file, thus making the file longer each time the command is repeated. If the file does not exist when we attempt to append the redirected output, the file will be created.

Standard Input

Many commands can accept input from a facility called standard input. By default, standard input gets its contents from the keyboard, but like standard output, it can be redirected. To redirect standard input from a file instead of the keyboard, the "<" character is used like this:

[me@linuxbox me]$ sort < file_list.txt

In the example above, we used the sort command to process the contents of file_list.txt. The results are output on the display since the standard output was not redirected. We could redirect standard output to another file like this:

[me@linuxbox me]$ sort < file_list.txt > sorted_file_list.txt

As we can see, a command can have both its input and output redirected. Be aware that the order of the redirection does not matter. The only requirement is that the redirection operators (the "<" and ">") must appear after the other options and arguments in the command.

Pipelines

The most useful and powerful thing we can do with I/O redirection is to connect multiple commands together to form what are called pipelines. With pipelines, the standard output of one command is fed into the standard input of another. Here is a very useful example:

[me@linuxbox me]$ ls -l | less

In this example, the output of the ls command is fed into less. By using this "| less" trick, we can make any command have scrolling output.

By connecting commands together, we can accomplish amazing feats. Here are some examples to try:

Examples of commands used together with pipelines

Filters

One kind of program frequently used in pipelines is called a filter. Filters take standard input and perform an operation upon it and send the results to standard output. In this way, they can be combined to process information in powerful ways. Here are some of the common programs that can act as filters:

Common filter commands

Further Reading

• Chapter 6 of The Linux Command Line covers this topic in more detail.
• Chapters 19 through 21 of The Linux Command Line provide an in-depth look at the text processing tools available in Linux.
• To learn more about the AWK programming language, consider the AWK adventure.

Which of the following is used to redirect the standard output of a command to a file?

Which of the following is used for output redirection?

Which number is used for the standard output redirection?

Which operator is used to redirect standard output and error in Linux?