Bash – Deduplicating Bash Brace Expansions

I couldn't figure out how to get this done using only curly braces. I don't see a way to achieve this either, so unless someone more clever than I can figure out a way I'd say it's not possible.

As an alternative

Sample Data

$ tree
.
|-- dir1
|   |-- file1
|   `-- file2
`-- dir2
    |-- file1
    `-- file2

Examples

$ seq 2 | xargs -i{} echo dir{}/file{}
dir1/file1
dir2/file2

This can be put into a command like this:

$ echo $(seq 2 | xargs -i{} echo dir{}/file{})
dir1/file1 dir2/file2

or this:

$ ls $(seq 2 | xargs -i{} echo dir{}/file{})
dir1/file1  dir2/file2

or this:

$ ls -l $(seq 2 | xargs -i{} echo dir{}/file{})
-rw-rw-r-- 1 saml saml 0 Sep  2 03:18 dir1/file1
-rw-rw-r-- 1 saml saml 0 Sep  2 03:31 dir2/file2

Why curly braces can't do this

If you look at your original example:

{foo,bar}Q{foo,bar}

The way this gets expanded is as follows:

fooQfoo fooQbar barQfoo barQbar

The mechanism that expanded this is called a Cartesian Product.

For example:

$ echo {A,B}{X,Y,Z}
AX AY AZ BX BY BZ

Or this:

$ echo {M,N}-{A,B}{X,Y,Z}
M-AX M-AY M-AZ M-BX M-BY M-BZ N-AX N-AY N-AZ N-BX N-BY N-BZ

There is no way to create a Cartesian Product that will result in:

fooQfoo barQbar

You're only option is to either resort to trickery such as this:

$ echo dir{1,2}/file{2,1}
dir1/file2 dir1/file1 dir2/file2 dir2/file1

And then put this into a Bash array:

$ a=(dir{1,2}/file{2,1})
$ echo ${a[@]:1:2}
dir1/file1 dir2/file2

The other option would be some "other method" such as the one I previously discussed above (using xargs) for example.

References

• Cartesian Products in Bash

What happens is that bash first expands *.djvu{,.bk} into *.djvu *.djvu.bk, and then does glob-expansion on those. This would explain what you observe: in your case, *.djvu, matches an existing file, say foo.djvu and expands into that, but *.djvu.bk matches no file, and thus expands as itself, *.djvu.bk.

The order of expansion is specified in the bash documentation:

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

I would suggest rewriting your copy command as:

for f in *.djvu; do cp -- "$f" "$f".bk; done

Or perhaps, to avoid the syntactic overhead of an explicit for loop:

parallel -j1 cp -- {} {}.bk ::: *.djvu

(On second thoughts... that's not really much shorter.)

To answer your sub-question "how could it be expanded", one could use a sub-command (example in a directory containing just foo.djvu and bar.djvu):

$ echo $(echo *.djvu){,.bk}
bar.djvu foo.djvu bar.djvu foo.djvu.bk

But that isn't as safe a solution as the for loop or parallel call above; it will break down on file names containing white space.