Typical pitfalls to avoid for BASH beginners. [closed] - bash

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The other day I answered a question where it was necessary to loop through all the sub-dictionaries in a given directory.
As a self-taught bash user I quickly answered the question. However, in my answer I used for DIR in $(find ...); do "something"; done to loop through the sub-directories. A polite bash guru pointed out that if I continue with bad habits like that will one day find myself in a lot of trouble.
While correcting my answer I found that I had (embarrassingly) many more bad habits. I could not find any topic like this on stackoverflow, but I am sure there are many people like me at a "beginner(++)" level with bad habits based on methods that "sort-of-works".
My question is generic albeit simple:
What are important/damaging pitfalls to avoid for someone who is using bash? (If you find the question to generic to answer I would appreciate a short description of the case with link to documentation or another SO question regarding the subject is appreciated.)
E.g. When performing loops to edit files/dirs, using globb characters, pipes, logicals, short hand notation (i.e. ``) and last but not least applying functions for purposes which they should not be used.

Answer 1

I found Google's Styleguide very instructive. I changed some old habits after reading it, and it has a lot of examples.
EDIT: Some points to watch beneath.
Working with variables: set value without $ and access with $
Avoid confusion by using ${..} when using the variable
Calling a command within your commandline with $()
The old syntax `` will work but you can make easy mistakes.
Avoid arrays
You normally do not really need them and they have a more difficult syntax
Start (first line) your script with #!/bin/bash
It will check and use bash when available and give an error when it is missing.
Learn regex
sed can become your close friend. Even when you only use -e 's/.../.../g'
Also other utilities use regex.
Use the case statement when checking against different possible values
Avoid too much if-then-else nesting
Do not skip the if keyword when testing with [[ .. ]]
The if statement is obsolete but easier to read
Do not use eval
When you finally understand the syntax you wil learn it is dangereous.
Shell builtin's are faster but more difficult to read
basename is a simple utility but slower than ${fullfile##*/}
Maybe you should store a note with some handy builtins somewhere.
Downwards compatible?
When you think your scripts might have to be migrated to Solaris/AIX with ksh someday, try avoiding bash-specific syntax and Linux utilities. That would be quite difficult (the -i flag in sed, additional options in find) so getting bash installed on Solaris/AIX might be a better way.
Question: E.g. When performing loops to edit files/dirs, using globb characters, pipes, logicals, short hand notation (i.e. ``) and last but not least applying functions for purposes which they should not be used.
When you have a lot of scripts using the same functions, store the functions in a file like common.h
Include this file with . common.h
Learn the difference between function xxx {..} and xxx() {..}
Use local var's and return values by stdout or the returnvalue