1. What does the shell ordinarily do while a command is executing? What should you do if you do not want to wait for a command to finish before running another command?

The shell works very fast. You might think you're waiting on it but it's finished... You can give it multiple commands, and they will execute in the order you type in. While it is working it will spit out some results and when done it will return you to $ prompt
2. What is a PID number? Why are these numbers useful when you run processes in the background?
A PID (i.e., process identification number) is an identification number that is automatically assigned to each process when it is created on a Unix-like operating system.

The PID is needed in order to terminate a frozen or otherwise misbehaving program with the kill command. This command makes it possible to end a program that cannot otherwise be stopped except by rebooting (i.e., restarting) the system, and it is thus an important element in the stability and robustness of Unix-like operating systems.

Assume that the following files are in the working directory:
$ ls intro notesb ref2 section1 section3 section4b notesa ref1 ref3 section2 section4a sentrev
Give commands for each of the following, using wildcards to express filenames with as few characters as possible.
a. List all files that begin with section.
$ ls section*
b. List the section1, section2, and section3 files only.
$ ls section[1-3]
c. List the intro file only.
$ ls i*
d. List the section1, section3, ref1, and ref3 files.
1. $ ls *[13]
2.
Give an example of a command that uses grep
a. With both input and output redirected. $ grep \$Id < *.c > id_list
Chapter 9
Number 1
Explain the following unexpected result:
$ whereis date date: /bin/date ...
$ echo $PATH
.:/usr/local/bin:/usr/bin:/bin
$ cat > date echo "This is my own version of date."
$