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The expression ' [A-Da-f]' as you have typed it (including the space) is to match against strings that simply contain the ranges of characters listed, that is A, B, C, D, a, b, c, d, e or f after a...

This is known as a useless use of cat

There is no point running the cat command. It costs you a process and it's much neater to write:-

grep '^[A-Da-f]' f1 >> f2

This will get all lines...

Yes, exactly right. Piping takes input from previous command output.
cat f1 >> f2 | grep '^[A-Da-F]' is all messed up.

It should be [^A-Da-f] or ^[A-Da-f] depending on what you are trying to do....

You need to use several commands and redirection to achieve what you want. One (amongst many) example were:$ head -5 file2 >fileX;cat file1 >>fileX; tail -n+6 file2 >>fileX, assuming you want to...

You can't do that with cat. "cat file1 file2" always places file2 at the end of file1. cat does not look for intermediate positions within files.

Could you provide a short example of file1 and...

In case the previous comments were too cryptic, the command:
find . –type d –name "?d*" –print
will look for files in or under the current directory (.) of type directory (-type d) with a name that...

Any single character.

See Shell Command Language (http://pubs.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html#tag_02_13) (and note that find uses pattern matching notation, not...

That's a wildcard, not a regular expression ("?" stands for a single character).

Some implementations of "find" support "regex" and "iregex" flags.