Redirect data streams with pipes This Way Please!

Pipes in the shell offer a surprising amount of versatility, including the ability to transfer data between computers.

- Jörg Schorn

Many users are only familiar with pipes as links between multiple flows, but they can do much more than that. Pipes can help you transfer data between computers. In this article, I will show you how to use pipes to redirect data streams in the shell.

Channels

Whenever a process starts under Linux, it is automatically assigned three channels. These channels have system assignments that let you address them, and each has a starting and end point. Channel 0 (STDIN) reads data, channel 1 (STDOUT) outputs data, and channel 2 (STDERR) outputs any error messages. Channel 2 typically points to the same device as channel 1 (Figure 1).

The shell itself, a Unix process, also uses these three channels. Each of them can be addressed via a file descriptor representing the respective channel number. On Linux, the channels used here physically reside in the /proc/PID/fd directory, where PID is equivalent to the process ID of the process being examined.

The Bash shell most commonly used on Linux also has channel 255. To make sure job control is retained when redirecting this channel, the shell sets it to STDERR at startup time.

Redirection

A redirection reads the channels of a process from a different source or outputs them to a different target. The most common use cases involve finding a string on the error channel and redirecting error messages to the /dev/ null device.