nesting brace expansion and command substitution

Question

I want to create a simple bash-script that checks whether a directory contains all the files whose names contain numbers from 1 to N.

# Creating some files for testing
$ cd /tmp/
$ mkdir test
$ touch test/a01x.dat
$ touch test/b02y.dat

# Display dir contents
$ ls test/*{01,02}*
test/a01x.dat  test/b02y.dat

But using seq command to generate numbers results in the following:

$ ls test/*{$(seq -s , -f "%02g" 1 2)}*
ls: cannot access 'test/*{01,02}*': No such file or directory

I understand that running the command by surrounding the path with single quotation marks must lead to the error because the wildcards don't expand

$ ls 'test/*{01,02}*'

But I didn't use them. What is the problem?

Answer 1

The problem is that the shell will do brace expansion (which processes {...,...}) before it does command substitution (the $(...) part.) So after your seq is expanded, the shell will not re-evaluate the {01,02} and will leave it as a literal.

You need to add an eval to have it re-evaluate the expression after the command substitution is performed:

$ eval "ls test/*{$(seq -s , -f "%02g" 1 2)}*"

In this case, the command substitution will be performed first, resulting in a string ls test/*{01,02}* and the eval will ask the shell to interpret this as a command, which will then perform the brace and glob expansion, resulting in the list of files you're expecting.