# Help understanding shell modules

**URL:** <https://forum.ansible.com/t/help-understanding-shell-modules/15601>\
**Category:** Ansible Project\
**Created:** [July 3, 2014, 7:10pm UTC](https://forum.ansible.com/t/help-understanding-shell-modules/15601 "2014-07-03T19:10:43Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Stuart\_Reynolds](https://avatars.discourse-cdn.com/v4/letter/s/90ced4/32.png) [@Stuart\_Reynolds](https://forum.ansible.com/u/Stuart_Reynolds)\
**Post date:** [July 3, 2014, 7:10pm UTC](https://forum.ansible.com/t/help-understanding-shell-modules/15601/1 "2014-07-03T19:10:43Z")

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I’m trying to understand what I can expect when writing my own modules. My hope would be to more quickly move and existing Makefile and shell script configuration to ansible.

However, just testing a simple shell module:

`  
#!/bin/bash  
set -e # Not so useful – return code is ignored by ansible.  
echo x=123  
echo a=789 1\>&2 # Output stderr  
echo y=456  
exit 1 # Ignored by ansible ☹ Succeeds if we get here.

`

there’s a lot I find confusing.

(1) Here, only x and y are returned, not a, which seems to disagree with the documentation. From: [http://docs.ansible.com/developing\_modules.html#common-pitfalls](http://docs.ansible.com/developing_modules.html#common-pitfalls)

> > Modules must not output anything on standard error, because the system will merge standard out with standard error and prevent the JSON from parsing. Capturing standard error and returning it as a variable in the JSON on standard out is fine, and is, in fact, how the command module is implemented.
> 
> > If a module returns stderr or otherwise fails to produce valid JSON, the actual output will still be shown in Ansible, but the command will not succeed.

(i.e. stdout and stderr are NOT merged). In fact, if I comment out the lines to print x and y, ansible notes that there’s no valid JSONy like output on stdout, reports a failure, which includes the values output to stderr. In fact if it looks like there’s valid variables in stdout, it stderr is never examined (ansible succeeds even if there is garbage output on stderr, and the return code is ignored). Did I misread the docs? Or or the docs out of date?

(2) The following…  
`  
#!/bin/bash  
set -e # NB. RETURN CODE IGNORED BY ANSIBLE  
echo x=123  
exit 1 # Simulate an error part way through my script  
echo y=123

`  
succeeds! (as far as Ansible is concerned). Ansible always ignores the return codes (really?!)

So, it seems that the only safe pattern when developing shell modules is:

`  
#!/bin/bash  
set -e # NB. RETURN CODE IGNORED BY ANSIBLE

{do all your work here. Exit on failure. Return code doesn’t matter}

# Print result variables here on success.

# Success must be guaranteed as soon as the first variable is output

echo y=123

# failures here while printing other variables as treated as a success by Ansible!

./outputVars # Danger, danger!

# Return code doesn’t matter

`

In other words… a lot of care is required when migrating shell modules in Ansible.  
A lot would be simpler if return code failures = ansible failures.

(3) Debugging. How? The docs suggest that I’m not generally allowed to output stdout or stderr from my commands (only variables to be reported to ansible may be reported). So… I need to add \>& /dev/null to every command?  
What’s the best practice here for getting command failures reported to ansible?

(4)  
`  
#!/bin/bash  
set -e  
echo “x=123” 1\>&2

`  
gives:

`  
failed: [localhost] =\> {“failed”: true, “parsed”: false}  
invalid output was: x=123

`

Which looks to me to be valid output (its just on stderr). Is there any way for a module to return variables in the event of a failure?

- Stu

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**Author:** ![Michael\_DeHaan1](https://avatars.discourse-cdn.com/v4/letter/m/5e9695/32.png) [@Michael\_DeHaan1](https://forum.ansible.com/u/Michael_DeHaan1)\
**Post date:** [July 3, 2014, 8:02pm UTC](https://forum.ansible.com/t/help-understanding-shell-modules/15601/2 "2014-07-03T20:02:37Z")

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This is more of a topic for ansible-devel, can you repost there?

Thanks!

---

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**Author:** ![Stuart\_Reynolds](https://avatars.discourse-cdn.com/v4/letter/s/90ced4/32.png) [@Stuart\_Reynolds](https://forum.ansible.com/u/Stuart_Reynolds)\
**Post date:** [July 3, 2014, 10:47pm UTC](https://forum.ansible.com/t/help-understanding-shell-modules/15601/3 "2014-07-03T22:47:50Z")

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Done. Moved to [https://groups.google.com/forum/#!topic/ansible-devel/hLfmzlsxggM](https://groups.google.com/forum/#!topic/ansible-devel/hLfmzlsxggM)