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.
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.
The call in line 1 of Listing 1 attempts to display the nonexistent /dev/pseudo/
directory, which generates an error message on channel 2. The call from line 5 adds a redirect from channel 2 to /dev/null
to the command. The error message now no longer appears on the screen, but the return value of the command remains unchanged.
Listing 1
Redirecting a Channel
01 # ls -ld /dev/pseudo 02 ls: cannot access /dev/pseudo: No such file or directory 03 # echo $? 04 2 05 # ls -ld /dev/pseudo 2>/dev/null 06 # echo $? 07 2
Pipes
A pipe is a special kind of file that acts as first in, first out (FIFO) memory in interprocess communication. With FIFO, one process writes to the pipe, while another reads from it. The reading process retrieves the characters in the same order as the writing process stored them. If, say, process 1 writes the values 1 Z 2 Y 3 X 4 W 5 V
to a pipe, process 2 reads them from the pipe in the same order. Linux uses two types of pipes: anonymous and named pipes.
Anonymous pipes connect commands using the pipe symbol (|
). These pipes are called anonymous because the user doesn't get to see them at runtime under normal circumstances. Anonymous pipes reside in the /proc/PID/fd/
directory like the standard channels. Calling a command sequence temporarily generates this kind of pipe.
Named pipes can be created in the filesystem using the mkfifo
command. They remain active until deleted again using the rm
command. When you use named pipes, you have to take care of the redirection work yourself, whereas the shell does this automatically for anonymous pipes.
Buy this article as PDF
(incl. VAT)
Buy Linux Magazine
Subscribe to our Linux Newsletters
Find Linux and Open Source Jobs
Subscribe to our ADMIN Newsletters
Support Our Work
Linux Magazine content is made possible with support from readers like you. Please consider contributing when you’ve found an article to be beneficial.
News
-
Canonical Bumps LTS Support to 12 years
If you're worried that your Ubuntu LTS release won't be supported long enough to last, Canonical has a surprise for you in the form of 12 years of security coverage.
-
Fedora 40 Beta Released Soon
With the official release of Fedora 40 coming in April, it's almost time to download the beta and see what's new.
-
New Pentesting Distribution to Compete with Kali Linux
SnoopGod is now available for your testing needs
-
Juno Computers Launches Another Linux Laptop
If you're looking for a powerhouse laptop that runs Ubuntu, the Juno Computers Neptune 17 v6 should be on your radar.
-
ZorinOS 17.1 Released, Includes Improved Windows App Support
If you need or desire to run Windows applications on Linux, there's one distribution intent on making that easier for you and its new release further improves that feature.
-
Linux Market Share Surpasses 4% for the First Time
Look out Windows and macOS, Linux is on the rise and has even topped ChromeOS to become the fourth most widely used OS around the globe.
-
KDE’s Plasma 6 Officially Available
KDE’s Plasma 6.0 "Megarelease" has happened, and it's brimming with new features, polish, and performance.
-
Latest Version of Tails Unleashed
Tails 6.0 is based on Debian 12 and includes GNOME 43.
-
KDE Announces New Slimbook V with Plenty of Power and KDE’s Plasma 6
If you're a fan of KDE Plasma, you'll be thrilled to hear they've announced a new Slimbook with an AMD CPU and the latest version of KDE Plasma desktop.
-
Monthly Sponsorship Includes Early Access to elementary OS 8
If you want to get a glimpse of what's in the pipeline for elementary OS 8, just set up a monthly sponsorship to help fund its continued existence.