C programming, run a command line or program

fpsasm · 02-07-2010, 05:14 PM

Hello,

I am wanting to write a program that runs a program or command-line. Is there are way of making a program that activates a command-line (for example executing 'ps -a -f' or '/home/shared/fah').

In addition to that, I want the program to do a 'ps -a -f' and put the results in a buff, how could I do this.

Cheers, thanks for the help in advanced,

Matthew

nadroj · 02-07-2010, 07:45 PM

You can call a program using either "system" or an "exec" function. You also need to know how fork and IPC (interprocess communication) work, so read up on those first if you dont already know. A simple method is with standard files, another method is with "pipes" (named or unnamed).

An example of how to do this might be

Code:

//open file/pipe

// fork

// child closes read end of fd/pipe, and later will write to it
// parent closes write end of fd/pipe, and later will read from it

// child redirects stdout to the write fd/pipe, using "dup2"
// note: if the program writes to stderr instead, then you must redirect that instead of stdin

// child calls the program, using an "exec" function

// parent reads from the fd/pipe, which should be the output of the program

// parent now has variable storing whatever the program outputted

This is a relatively complicated thing to do, so, again, if you arent familiar with the above topics then learn the basics of those. A good way to do that is by reading example code, versus digging through man or other pages. This is why my example above is very short, because if I wrote and explained the actual code, it would take--well, that isnt realistic for you to expect from us.

smeezekitty · 02-07-2010, 08:02 PM

system() will work.
exec() will also work but it will terminate your program.

nadroj · 02-07-2010, 08:15 PM

"exec" replaces the current process. If you "fork" and then "exec", as I described above, then there is no problem.

shishir · 02-09-2010, 05:03 AM

you could use popen(). It opens a pipe, forks a shell and then runs the command specified in the parameter. It dumps/refirects the output on the standard output of the command to a file stream that can be read from using fgets, etc.

man 3 popen

theNbomr · 02-09-2010, 08:07 AM

Whether to use system() vs. fork() + exec() vs. popen() should depend on whether your child process requires the services of a shell and whether you need to process the output of the child process. If you simply need to invoke another process, then fork()+exec() should suffice. If you need a shell to process a commandline, then system() is your friend. If you need to grab the standard output stream from the child process, then you want popen().
--- rod.