Bash – Why arguments in braces do not expand in this case

bashbrace-expansioncommand line

For example, I want to put some file names inside braces for expansion like this:

$ ls
blue_texture  blue_waves  green_light
$ ls -m | tr -d ' \n'
blue_texture,blue_waves,green_light 
$ echo foo/bar/{`ls -m | tr -d ' \n'`}
foo/bar/{blue_texture,blue_waves,green_light}

My question why doesn't brace expansion work here? I expected it to work as following:

$ echo foo/bar/{blue_texture,blue_waves,green_light}
foo/bar/blue_texture foo/bar/blue_waves foo/bar/green_light

Best Answer

The reason this does not work is because bash performs brace expansion before command substitution (the Bash Reference Manual has some information about this). The substitution is not performed until after the brace expansion already occurred.

The only way you could do this is by using eval, but do not do that in this case, as you will allow arbitrary command execution.

Instead you have to seek some other method. This should produce the output you seek:

for file in *; do
    printf '%s ' "foo/bar/$file"
done; echo

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

Bash – Brace expansion does not work for less than 2 arguments

Yes, n > 1 is an explicit requirement:

A correctly-formed brace expansion must contain unquoted opening and closing braces, and at least one unquoted comma or a valid sequence expression. Any incorrectly formed brace expansion is left unchanged.

As for the why - historical reasons, to some extent (though it was copied from csh originally, which has the other behaviour). There are commands that take {} as a literal argument (find, parallel, and others with more complex arguments), and also other uses of {} in the shell language. Because brace expansions are only processed when written literally (and not from variables), there's really no motivation to support degenerate expansions, and some reasons not to.

Best Answer

Related Solutions

Bash – How to do multiple brace expansions without combinatorial explosion

As an alternative

Examples

Why curly braces can't do this

References

Bash – Brace expansion does not work for less than 2 arguments

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