Why does bash give the following result after brace expansion?

Question

I am using linux and the following version of the bash:

GNU bash, version 5.1.16(1)-release (x86_64-pc-linux-gnu)

When I type:

echo file{[1,2],3}.txt

I expect brace expansion to be done first, so:

echo file[1,2].txt file3.txt

After that, I expect it to stop there, since [1,2] is not a valid globbing. However, it gives the following as output:

file[1.txt file2].txt file3.txt

Why?

List of files in current directory (result of ls command):

go.mod  hello.sh  hi  main.go

Answer 1

Because you are telling it to :) This is what bash sees:

file{[1,2],3}.txt

The [ and ] have no special meaning in this context, globs aren't expanded within brace expansions: if a brace expansion results in a glob, that glob would be evaluated by the shell after the brace expansion.

So here, what happens is that the shell sees three comma separated elements in the brace: [1, 2] and 3. This is why it duly prints out file[1.txt, file2].txt and file3.txt. It's no different than using something like file{a,b,c}.txt.

You can see it happening with set -x:

$ set -x
$ echo file{[1,2],3}.txt
+ echo 'file[1.txt' 'file2].txt' file3.txt
file[1.txt file2].txt file3.txt

This behavior is documented in the EXPANSIONS section of man bash where the order of precedence is given:

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

As an aside, [1,2] is a perfectly valid globbing pattern, it just isn't expanded inside the braces. For example:

$ touch file{1,2,3}.txt
$ ls
file1.txt  file2.txt  file3.txt
$ ls file[1,2].txt
file1.txt  file2.txt

Note that in globs, [a,b] matches any of the characters a, , or b. It doesn't mean "a or b".