gr8linux wrote:

How does Shell invoke a process, when an application is executed?

Er, that's a little vague. Can you be more specific?

If I do not run any command, is any IDLE thread running?

If by IDLE thread, you mean a process pertaining to your shell, then yes. The shell process itself is running. E.g., on my box:

[joeblow@mybox ~]$ ps -fwwu $USER

UID        PID  PPID  C STIME TTY          TIME CMD

joeblow    16405 16404  4 00:31 pts/2    00:00:00 -tcsh

joeblow    16422 16405  0 00:31 pts/2    00:00:00 ps -fwwu joeblow

The output of this ps command shows all running process that are owned by user "joeblow". The shell (tcsh) and the ps command itself are the only two processes joeblow is running, though the ps command will have already disappeared, of course. The shell is using PID (Process ID) 16405. Notice that the PPID (Parent PID) of the ps command is the PID of the tcsh shell.

You can also use the tty command to see the filename of the terminal that your shell is using to connect to STDIN:

[joeblow@mybox ~]$ tty

/dev/pts/2

which correlates to the pts/2 in the TTY column of the ps output above.