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Shell Programming 
The Unix Shell Guide [Mirror] Unix KornShell Quick Reference
O'Reilly® Learning the Korn Shell [Mirror] [Mirror] [Mirror] [Mirror] [Mirror] [PDF] [2nd Edition] [2nd Edition]
The C Shell Tutorial C-shell Cookbook
Programming the Unix/Linux Shell (C. Cantin) Sed - An Introduction
Mastering Regular Expressions, 2nd Edition (J. Friedl) Learning to Use Regular Expressions
Teach Yourself Shell Programming in 24 Hours (S. Veeraraghavan) [Mirror] [PDF]
Effective AWK Programming [Mirror] AWK Programming Language
Bash Guide for Beginners [Mirror] [PDF] UNIX Bourne Shell Programming [Another]
Bash Reference Manual [Mirror] [Mirror] [Mirror] [Mirror] [Mirror]
UNIX/Linux Bourne/Bash Shell Scripting Tutorial  
Advanced Bash-Scripting Guide [Mirror] [Mirror] [Mirror] [Mirror] [Mirror] [PDF]