I/O Redirection

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
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.

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
Program What it does
sort Sorts standard input then outputs the sorted result on standard output.
uniq Given a sorted stream of data from standard input, it removes duplicate lines of data (i.e., it makes sure that every line is unique).
grep Examines each line of data it receives from standard input and outputs every line that contains a specified pattern of characters.
fmt Reads text from standard input, then outputs formatted text on standard output.
pr Takes text input from standard input and splits the data into pages with page breaks, headers and footers in preparation for printing.
head Outputs the first few lines of its input. Useful for getting the header of a file.
tail Outputs the last few lines of its input. Useful for things like getting the most recent entries from a log file.
tr Translates characters. Can be used to perform tasks such as upper/lowercase conversions or changing line termination characters from one type to another (for example, converting DOS text files into Unix style text files).
sed Stream editor. Can perform more sophisticated text translations than tr.
awk An entire programming language designed for constructing filters. Extremely powerful.

Further Reading