One of the fundamental features that makes Linux and other Unices useful is the “pipe”. Pipes allow separate processes to communicate without having been designed explicitly to work together. This allows tools quite narrow in their function to be combined in complex ways.
A simple example of using a pipe is the command:
ls | grep x
When bash examines the command line, it finds the vertical bar character | that separates the two commands. Bash and other shells run both commands, connecting the output of the first to the input of the second. The ls program produces a list of files in the current directory, while the grep program reads the output of ls and prints only those lines containing the letter x.
The above, familiar to most Unix users, is an example of an “unnamed pipe”. The pipe exists only inside the kernel and cannot be accessed by processes that created it, in this case, the bash shell. For those who don't already know, a parent process is the first process started by a program that in turn creates separate child processes that execute the program.
The other sort of pipe is a “named” pipe, which is sometimes called a FIFO. FIFO stands for “First In, First Out” and refers to the property that the order of bytes going in is the same coming out. The “name” of a named pipe is actually a file name within the file system.
Pipes are shown by ls as any other file with a couple of differences:
% ls -l fifo1
prw-r--r-- 1 andy users 0 Jan 22 23:11 fifo1|
The p in the leftmost column indicates that fifo1 is a pipe. The rest of the permission bits control who can read or write to the pipe just like a regular file. On systems with a modern ls, the | character at the end of the file name is another clue, and on Linux systems with the color option enabled, fifo| is printed in red by default.
On older Linux systems, named pipes are created by the mknod program, usually located in the /etc directory. On more modern systems, mkfifo is a standard utility. The mkfifo program takes one or more file names as arguments for this task and creates pipes with those names. For example, to create a named pipe with the name pipe1 give the command:
mkfifo pipe
The simplest way to show how named pipes work is with an example. Suppose we've created pipe as shown above. In one virtual console1, type: (read more)
