Portable shell scripting is the future of modern Linux, OS X, and Unix command–line access. Beginning Portable Shell From Novice to Professional teaches shell scripting by using the common core of most shells and expands those principles to all of scripting. You will learn about portable scripting and how to use the same syntax and design principles for all shells. You'll discover about the interaction between shells and other scripting languages like Ruby and Python, and everything you learn will be shown in context for Linux, OS X, bash, and AppleScript.
For an experienced shell programmer, this is actually a good read. Seebach touches on a lot of new and improved thinking in UNIX shells, with a good focus on what is POSIX. This is a twenty first century book, unlike most shell programming books that I have read. He focuses on bash, but why not. Bash is the default shell on just about every system that I have logged into for the past decade. Solaris and the Korn shell are dying.