Differece between a logical and physical network.
at - causes the OS to execute commands it receives from its standard input. It executes them as a shell script in the working directory at the time you specify. When the OS executes commands using at, it uses electronic mail to send you the standard output and standard error output of the resulting process. You can redirect the output to avoid getting mail. cat - joins files end to end. It takes its input from files you specify on the command line or from its standard input. You can use cat to dispaly the contents of one or more text files on the terminal.