The Linux Command Line by William Shotts
In this lesson, 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:
[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. If you want the new results to be appended to the file instead, 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 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:
[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 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.
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:
[me@linuxbox me]$ 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. I use this technique all the time.
By connecting commands together, you can acomplish amazing feats. Here are some examples you’ll want to try:
| 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 filters. 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:
| 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. |