Support process substitution?

[Paul Smith]

Does Ansible not support bash's process substitution?

My project maintains separate inventory files, and there are some playbooks that need hosts from certain combinations of them. Instead of having pre-concatenated files that could go out of sync, I'd like to do something like:

$ ansible-playbook -i <(cat inventorya inventoryb) playbook.yml

I get this error though when I try it:

ERROR: provided hosts list is empty

I haven't looked at the Ansible source, but this may be because process substitution uses an fifo under the hood, and Ansible may be strictly looking for a file, but this is just wild speculation.

(Ansible 1.7.1)

-Paul

[Michael DeHaan]

To be honest, I've never even seen that syntax used before -- redirection of input/output obviously yes...

It's a sign you can be writing systems management software for well over a decade and not learn all the bash things.

Basically right now it takes either a string - for a very deprecated use case we don't really document, or a filename or directory name.

A quick way to solve what you want to do, BTW, is just put both in the same directory

-i directoryname

will load all files int he one directory

and then use "--limit groupname" to limit ansible to just what you want to select.

(Be careful to not forget the limit!)

That's the way pretty much everyone approaches that, anyway :)

Another option would be use "-i directoryname" and just be particular about what your "hosts:" stanza would address.

I'm open to make this loading be more generic provided we be careful to not break any current possibilities, but am not entirely sure what is involved.

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[Brian Coca]

another form:

cat inventorya inventoryb |ansible-playbook -i /dev/stdin playbook.yml

It still won't work as you think, it will only work if inventorya and
inventoryb contain the hosts in comma separated lists, as -i looks for
an existing file or directory in the string unless there is a comma,
then it takes it as a list.

[Greg Andrews]

To clarify, process substitution is a Bash shell syntax that replaces the <( ... ) with a filename, but that filename is a named pipe (aka a FIFO) that a sub-shell has opened for writing by the commands inside the parentheses.  The command that you pass this to (in Paul's example, ansible-playbook) must open the given "file" and read it from start to end without trying to seek back and forth.

I tried this with the 'ansible' command a while ago and it failed the same way.  I assumed that the code wanted to do seeks around the file rather than just read it all, and that was the reason for the failure.  I didn't investigate any deeper.

I thought it could be a nice way to accommodate dynamic inventory sources, but there might be a good reason you wouldn't want to support it.

  -Greg

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[Michael DeHaan]

Yeah I'm obviously more familiar with that one.

Ultimately, I'm not sure it makes sense - there are *multiple* inputs to Ansible, and it would be unclear which thing got to be STDIN.

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[Michael DeHaan]

Ansible does already have a concept of dynamic inventory sources that has somewhat reserved that phrase:

http://docs.ansible.com/intro_dynamic_inventory.html

If a bit of tweaking is necessary to allow for reading files from named pipes, I'm ok with it -- provided everything else works as advertised.

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