1.12 Use input-output redirection (e.g. >, >>, |, 2>)

Input-output redirection

In this topic, we will explore a feature used by many command line programs called input/output redirection. Many commands such as ls print their output on the display. By using some special notation 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:

# ls > list.txt

In this example, the ls command is executed and the results are written in a file named 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. If you want the new results to be appended to the file instead, use >> like this:

# ls >> 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 you 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:

# sort < list.txt

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

# sort < list.txt > sorted_list.txt

As you 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.

Standard Error
Standard error is the default error output device, which is used to write all system error messages. It is denoted by two number (2). Also known as stderr. The default standard input is the screen or monitor. 2> is input redirection symbol and syntax is:

# ls -l /tnt 2> error.txt

Redirect stream

You can simple redirect stream by using operator >&. Is the syntax to redirect a stream to another file descriptor - 0 is stdin. 1 is stdout. 2 is stderr.

# ls -l /tnt 2>&1

Pipes
The most useful and powerful thing you can do with I/O redirection is to connect multiple commands together with what are called pipes. With pipes, the standard output of one command is fed into the standard input of another.

# ls -l | less

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

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