Shell script sending some message to already running process(which written in c)

Can some one please help me on, communication mechanism between shell script and an daemon which is written C.

Want to exchange messages between shell script(bash) and an process.

Originally Posted by shivanagouda.rm Can some one please help me on, communication mechanism between shell script and an daemon which is written C. Want to exchange messages between shell script(bash) and an process.

Ok...so what, exactly, do you need help WITH?? You've posted no code, no details about what you're trying to do, etc. We can't guess. Shells typically have return codes on commands that can be read in a variety of ways.

Yes, more information is needed. However, as just a wild guess you might look into signals. See "man 7 signal".

Originally Posted by shivanagouda.rm Can some one please help me on, communication mechanism between shell script and an daemon which is written C. Want to exchange messages between shell script(bash) and an process.

Signals and pipes are some options. You should read Chapter 6.

If you wish, in my name on the left side, there's a link to my blog overall and in there are several ancient discussions with examples for things like a daemon communicating with child processes, and so forth, also the use of the select() call, pipes, signals. It's spread around across the Linux Application category, or whatever I named it, there's 3 or 4 from 8-10 years ago.

For shell to process, there are signals and also the return status. Say for instance a shell script is run from within a Linux C application, that application can monitor using wait or waitv for the termination signals from that child (the script) and it can also check exit status.

If the script runs continually, well when it gets spawned from the C code, (or vice versa, script spawning a C application), pipes can be created in advance and then used to communicate between each side. Note pipes are half duplex, so you'll need 2 of them. Examples shown in those blogs.

This is not some "one of + quick" thing, you'll need to study the topic and decide what works for you and develop plus test the results incrementally. All the code shown in those blogs was not written in an afternoon, more like a few weeks or longer.

As others have said, we're happy to help, but you'll need to study up on the topic and try some things on your own to be able to ask next level types of questions.

or you might want to check sockets, but there are a lot of different ways and without details hard to suggest anything

There’s also DBUS, which tends to be missed when people list IPC mechanisms.

My questions are :

Does the daemon already accept messages in a certain format?

Are you allowed to modify the daemon, the shell script, or both?

Are the daemon and the shell script running on the same machine?

Is the daemon an existing program that we can look at?

Can you give us an example of a message you would like to send and receive?

The classical way to do this is with "pipes" – implemented by the "|" character on the command-line.

Every Linux/Unix process automatically has three file-handles: STDIN, STDOUT, STDERR.

The "C" program can be written to simply expect to read commands from its STDIN file, and to write its outputs to one of the other two. Now, you can simply issue a console command which invokes several programs at the same time, "piping" their inputs and outputs from one to the next. If you set things up in this way, you don't even have to write a separate program: the shell will do it for you.

It was a great idea in the 1970's, and it's still a great idea today.

Originally Posted by shivanagouda.rm Can some one please help me on, communication mechanism between shell script and a daemon which is written C.

Originally Posted by sundialsvcs The classical way to do this is with "pipes"

Uh, no it's not. The very definition of a daemon is that it doesn't have a terminal, so you can't use pipes.

Named pipes, to be precise. The problem is they are unidirectional: you can send a request to a daemon via named pipe, but get no answer.

To clarify – command-line "|" piping simply uses STDIN and STDOUT. The shell parses the command line, creates the child processes, pipes the STDOUT of one to the STDIN of the next (redirection of STDERR takes a few more keystrokes on your part), then lets them go. The shell then sleeps until all of the children terminate.

No response from OP so far...

I would ask first if OP has access to the running binary's src or manual.
We are assuming he does and it is written to be communicated with this way (or can be changed to do so).

It's entirely possible it's a 3rd party binary and it's not built that way ...

/pessimistic