∙ Pipes

Pipes

The pipe operator, looks like this | (to the left of the enter key on MacOS, and to the left of the number row on Windows). A pipe allows us to take the output of one command, and use it as the input for another command. For example, we can count the number of lines of each file in our folder of Shakespeare plays, using wc. But what if we wanted these in order, numerically? This is where we could use a pipe. A series of pipe and command operations is known as a “pipeline” (these might be saved as a small program, or a “script”).

We can use the output of wc as the input for sort. Try these commands in the folder some_plays

wc -l * | sort

Here the output of the command wc -l * is being piped into the command sort. Building up commands with pipes can lead to some very powerful automation.

Using pipes and redirects together

Combining redirects and pipes can be very powerful, as we can easily save the output of a series of commands into a new file. Here is a simple example where we pipe one command to another, then send the output of the next command into a file:

wc -l * | sort -r > ordered_by_line

Understanding this kind of flow is a first step towards programming. While the command line is a whole scripting language in itself, as things get more sophisticated we start to use proper programming languages rather than the command line, but the command line remains very useful and powerful. Let’s do some exercises using pipes and redirects.