Bash Brace Expansion & Variables

Question

I have a find command:

find Directory/{Alpha,Bravo,Charlie} arg1 arg2

I want to replace Alpha,Bravo,Charlie with $find_dir

find Directory/{$find_dir} arg1 arg2

however the latter expands to

find Directory/{Alpha,Bravo,Charlie} arg1 arg2

rather then

find Directory/Alpha Directory/Bravo Directory/Charlie arg1 arg2

Why? It's part of a fairly complex bash script that may have more or less directories, not all which are relevant (so globbing Upload/* would not work). So if I setup three now and add another, I'll have to manually add it in later. Plus I need it run from the root of directories to keep finds outputs perspective (./Upload/Dir/file as opposed to ./Dir/file).

But using a variable would permit me to change that as needed and keep it relevant to other parts of the script.

Answer 1

From bash documentation, Brace Expansion section:

Brace expansion is performed before any other expansions, and any characters special to other expansions are preserved in the result. It is strictly textual. Bash does not apply any syntactic interpretation to the context of the expansion or the text between the braces. To avoid conflicts with parameter expansion, the string ‘${’ is not considered eligible for brace expansion.

Another note, in bash:

The order of expansions is: brace expansion, tilde expansion, parameter, variable, and arithmetic expansion and command substitution (done in a left-to-right fashion), word splitting, and filename expansion.

So in your case, bash saw brace expansion before variable expansion, it will do brace expansion first, produce result {Alpha,Bravo,Charlie}.

If you can control $find_dir variable content, you can use eval:

eval "find Directory/{$find_dir} arg1 arg2"