Managing output with pipes

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When dealing with processes, the output of those processes is important. In this section, we will discuss how to manage the output of processes in different ways.

Standard file descriptors

When a program often write output to the console, this is called standard output, or stdout. The shell defines stdin, stdout, and stderr for every program.

#	Common name	Description
0	stdin	Attached to programs for input data
1	stdout	Attached to programs for output data
2	stderr	Attached as a secondary output for errors

You can also redirect these file descriptors to attach to files other than the console.

Redirection notation

Notation	Meaning
< FILE	Read stdin from FILE
> FILE	Write stdout to FILE
>> FILE	Append stdout to FILE
2> FILE	Write stderr to FILE
2>> FILE	Append stderr to FILE
| PROGRAM	Join stdout to stdin of PROGRAM

The following example redirects the stdout from our program (the "Hello, world!" text it prints) from the console into the file message.txt.

$ hello > message.txt

You can concatenate (print) the contents of files with the cat program:

$ cat message.txt
Hello, world!

We can also use the output (stdout) of the cat program (again, the "Hello, World!" text) to the input (stdin) of another program with a pipe(|). We'll pass the output to the wc "word count" program which can count the words, lines, chars, etc of the provided input.

$ cat message.txt | wc --chars
14

wc program

wc or the "word count program is a handy tool that allows us to count the number of words (--words), lines (--lines), or characters (--chars) from a file or stdin.

How might we see how many files we have in our current directory?

We could get a list of our files with ls, and pass the output to wc to count the number of lines.

ls | wc --lines
55

Next section: Processes