How to excute shell command and return result string ?

Try "execve".  BTW why are you executing a "ifconfig" while you can
access the functionality via standard networking APIs?

system(3) is the high level version that runs the command
via a shell for full shell redirection, etc.
That sets up the pipes (file redirection) that the exec(2) family won't:
you'd have to dup() stdout before exec() to get the output.




Agreed, use the direct functions when possible
(no need for fork/exec other processes just to get the parameters,
works faster too), but is more work for the programmer.