Dale Dougherty and Arnold Robbins, Sed and Awk, 2nd edition, O'Reilly and Associates, 1997, 1-156592-225-5.
To unfold the full power of shell scripting, you need at least a passing familiarity with sed and awk. This is the standard tutorial. It includes an excellent introduction to "regular expressions". Read this book.
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Aeleen Frisch, Essential System Administration, 2nd edition, O'Reilly and Associates, 1995, 1-56592-127-5.
This excellent sys admin manual has a decent introduction to shell scripting for sys administrators and does a nice job of explaining the startup and initialization scripts. The book is long overdue for a third edition (are you listening, Tim O'Reilly?).
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Stephen Kochan and Patrick Woods, Unix Shell Programming, Hayden, 1990, 067248448X.
The standard reference, though a bit dated by now.
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Cameron Newham and Bill Rosenblatt, Learning the Bash Shell, 2nd edition, O'Reilly and Associates, 1998, 1-56592-347-2.
This is a valiant effort at a decent shell primer, but somewhat deficient in coverage on programming topics and lacking sufficient examples.
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Anatole Olczak, Bourne Shell Quick Reference Guide, ASP, Inc., 1991, 093573922X.
A very handy pocket reference, despite lacking coverage of Bash-specific features.
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Jerry Peek, Tim O'Reilly, and Mike Loukides, Unix Power Tools, 2nd edition, O'Reilly and Associates, Random House, 1997, 1-56592-260-3.
Contains a couple of sections of very informative in-depth articles on shell programming, but falls short of being a tutorial. It reproduces much of the regular expressions tutorial from the Dougherty and Robbins book, above.
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Ellen Siever, Linux in a Nutshell, 2nd edition, O'Reilly and Associates, 1999, 1-56592-585-8.
The all-around best Linux command reference, even has a Bash section.
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The man pages for bash and bash2, date, expect, expr, find, grep, gzip, ln, patch, tar, tr, xargs. The texinfo documentation on bash, dd, gawk, and sed.
The excellent "Bash Reference Manual", by Chet Ramey and Brian Fox, distributed as part of the "bash-2-doc" package (available as an rpm).
Ben Okopnik's well-written introductory Bash scripting articles in issues 53, 54, 55, 57, and 58 of the Linux Gazette .
Chet Ramey's Bash F.A.Q.
The sed F.A.Q.