In Linux, the
pipe command lets you sends the output of one command to another. Piping, as the term suggests, can redirect the standard output, input, or error of one process to another for further processing.
The syntax for the
unnamed pipe command is the
| character between any two commands:
Command-1 | Command-2 | …| Command-N
Here, the pipe cannot be accessed via another session; it is created temporarily to accommodate the execution of
Command-1 and redirect the standard output. It is deleted after successful execution.
In the example above,
contents.txt contains a list of all files in a particular directory—specifically, the output of the
ls -al command. We first
grep the filenames with the "file" keyword from
contents.txt by piping (as shown), so the output of the
cat command is provided as the input for the
grep command. Next, we add piping to execute the
awk command, which displays the 9th column from the filtered output from the
grep command. We can also count the number of rows in
contents.txt using the
wc -l command.
A named pipe can last until as long as the system is up and running or until it is deleted. It is a special file that follows the FIFO (first in, first out) mechanism. It can be used just like a normal file; i.e., you can write to it, read from it, and open or close it. To create a named pipe, the command is:
This creates a named pipe file that can be used even over multiple shell sessions.
Another way to create a FIFO named pipe is to use this command:
mknod p <pipe-name>
To redirect a standard output of any command to another process, use the
> symbol. To redirect a standard input of any command, use the
As shown above, the output of the
ls -al command is redirected to
contents.txt and inserted in the file. Similarly, the input for the
tail command is provided as
contents.txt via the
Here, we have created a named pipe,
my-named-pipe, and redirected the output of the
ls -al command into the named pipe. We can the open a new shell session and
cat the contents of the named pipe, which shows the output of the
ls -al command, as previously supplied. Notice the size of the named pipe is zero and it has a designation of "p".
So, next time you're working with commands at the Linux terminal and find yourself moving data between commands, hopefully a pipe will make the process quick and easy.