I/O Redirection

In this we will explore a powerful feature used by many 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:

$ 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. If you want the new results to be appended to the file instead, use ">>" like this:

$ 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 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 < 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:

$ sort < file_list.txt > sorted_file_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 of a command can also be redirected in the way standard output are redirected.But it uses 2> for redirection.
Eg: cp a b 2>errorfile
Note: We can also redirect both standard output and standard error to a file by using
$command &>outfile

Pipelines

The most useful and powerful thing you can do with I/O redirection is to connect multiple commands together with what are called pipelines. With pipelines, the standard output of one command is fed into the standard input of another. Here is my absolute favorite:
$ ls -l | less
In this example, the output of the ls command is fed into less. By using this "| less" trick, you can make any command have scrolling output.
By connecting commands together, you can acomplish amazing feats. Here are some examples you'll want to try:

Examples of commands used together with pipelines

Command	What it does
ls -lt | head	Displays the 10 newest files in the current directory.
du | sort -nr	Displays a list of directories and how much space they consume, sorted from the largest to the smallest.
find . -type f -print | wc -l	Displays the total number of files in the current working directory and all of its subdirectories.