how linux command works(1) - watch from procps

In the following days, or maybe years, I will go through the source code of all linux commands, to present you the principle of it's internal workflow.

 

Now the first one come here - watch:

 

Watch utility executes a program periodically, showing output fullscreen:

 

for(;;){
    if (!(p = popen(command, "r"))) {...}
    //… print output of the command to terminal, using ncurses
    usleep(interval * 1000000);
}