Shell Programming and Scripting

How to have a conversation between 2 processes using named pipes?

A very simple example... Open a shell and type:

Code:

mknod myfifo p
echo "test" > myfifo

At this point the shell hangs, because the fifo is waiting to be read.
So, open a second shell and type:

Code:

cat myfifo

The command reads (and empties) the pipe, you will see the output "test" and the first shell will return to the prompt.

What is the need for "p" after myfifo? what is it meant for?

pipes !!

command1 | command2

means the o/p of command1 will be given as an input to command2.
Ex: grep "abc" file1 | wc -l
Means the o/p of grep command will be given to wc -l as input to count the no. of lines where "abc" has found in the file1

Is that you were looking for ?? If not, then explain your Q, once again !!


Thnks. !!

And I must say...every command is treated as a saperate process with uniqueue process id. hence in the above example, there were 2 processes linked using pipe.

mknod is the unix command for creating special files. A pipe is a type of special file, and with the "p" option you tell mknod to create a FIFO (aka named pipe).

varungupta has explained the behaviour of normal command piping. In this case, the OS creates a temporary file for doing the work (a pipe), but you cannot reference explicitly this pipe by name, because is managed internally by the OS.
If you specifically need to put in communication two or more processes through NAMED pipes, you have to create a FIFO with mknod as explained before. In this manner you can reference explicitly the file on disk.

Generally, named pipes are used in the cases where a specific command line utility doesn't accept pipes as input, and you are forced to pass a phisical file to the utility as the input data. For these situations, named pipes are the answer.