---
layout: default
navsection: userguide
title: "Tutorial: Debug a crunch script"
navorder: 25
---

h1. Tutorial: Debug a crunch script

The commit-push-submit cycle is too slow for developing and debugging crunch scripts.

Get quicker turnaround times by running jobs in your VM (rather than submitting to the job queue) using your working git tree (rather than committing and pushing every little change).

h3. Prerequisites

* Log in to a VM (see "Setting up SSH access":ssh-access.html)
* Check out your git tree (see "Intro: git":intro-git.html)
* Set up your API token environment variables (see "Getting an API token":api-tokens.html)

If everything is set up correctly, the command @arv -h user current@ will display your account information.

h3. Create a script

Change to your git repository working tree and make sure the @crunch_scripts@ directory exists.

<pre>
yourname@shell:~$ cd yourrepo
yourname@shell:~/yourrepo$ mkdir -p crunch_scripts
</pre>

{% include notebox-begin.html %}

The process described here should work regardless of whether @yourrepo@ contains a git repository. But normally you would only ever edit code in a git tree -- especially crunch scripts, which can't be used to run regular Arvados jobs until they're committed and pushed.

{% include notebox-end.html %}


Create a new crunch script called @hello-world@ (say, @edit crunch_scripts/hello-world@).

<pre>
#!/usr/bin/env python

import arvados

print "hello world"
</pre>

Save it.

h3. Run the job in your VM.

Instead of a git commit hash, we provide the path to the working copy in the _script_version_ parameter.

<pre>
read -rd $'\000' the_job <<'EOF'; arv-crunch-job --job "$the_job"
{
 "script":"bug-factory",
 "script_version":"/home/yourname/yourrepo"
}
EOF
</pre>

You should see:

<pre>
No script_parameters specified at arv-crunch-job line 156
</pre>

Let's try again.

<pre>
read -rd $'\000' the_job <<'EOF'; arv-crunch-job --job "$the_job"
{
 "script":"hello-world",
 "script_version":"/home/yourname/yourrepo",
 "script_parameters":{}
}
EOF
</pre>

You should see several attempts to run your script, each ending in tragedy:

<pre>
2013-09-24_23:14:38 qr1hi-8i9sb-s1ngps1dznzrn8j 12311 0 stderr bash: /home/yourname/yourrepo/crunch_scripts/hello-world: Permission denied
2013-09-24_23:14:38 qr1hi-8i9sb-s1ngps1dznzrn8j 12311 0 stderr bash: line 0: exec: /home/yourname/yourrepo/crunch_scripts/hello-world: cannot execute: Permission denied
2013-09-24_23:14:38 qr1hi-8i9sb-s1ngps1dznzrn8j 12311 0 child 12323 on localhost.1 exit 32256 success=false
2013-09-24_23:14:38 qr1hi-8i9sb-s1ngps1dznzrn8j 12311 0 failure in 1 seconds
</pre>

Indeed, @hello-world@ is not executable. Fix it:

<pre>
chmod +x crunch_scripts/hello-world
</pre>

Try again.

This time, the job will still fail, but "hello world" will appear in the log.

<pre>
2013-09-26_02:15:35 qr1hi-8i9sb-fbpcg2conrhnlxv 16179 0 job_task qr1hi-ot0gb-c3x9vcztahzdcol
2013-09-26_02:15:35 qr1hi-8i9sb-fbpcg2conrhnlxv 16179 0 child 16249 started on localhost.1
2013-09-26_02:15:36 qr1hi-8i9sb-fbpcg2conrhnlxv 16179  status: 0 done, 1 running, 0 todo
2013-09-26_02:15:36 qr1hi-8i9sb-fbpcg2conrhnlxv 16179 0 stderr hello world
2013-09-26_02:15:37 qr1hi-8i9sb-fbpcg2conrhnlxv 16179 0 child 16249 on localhost.1 exit 0 success=false
2013-09-26_02:15:37 qr1hi-8i9sb-fbpcg2conrhnlxv 16179 0 failure in 1 seconds
</pre>

There are two problems here:

1. The string "hello world" should be stored as the job's output instead of noted in the job log as "stderr".
2. The task exited 0, but was recorded as a failure.

The job should work properly after we fix these two problems.

h3. Store output as a collection

Replace the @print "hello world"@ line with the following code.

<pre>
out = arvados.CollectionWriter()
out.set_current_file_name('hello.txt')
out.write('hello world')
out_collection = out.finish()
</pre>

The return value of @out.finish()@ is the content address (hash) of a collection stored in Keep.

h3. Record successful completion

Append the following line.

<pre>
arvados.current_task().set_output(out_collection)
</pre>

The @set_output()@ method tells Arvados that
* The hash specified by the @out_collection@ parameter is the output of this task, and
* This task completed successfully.

h3. Run the working script.

<pre>
read -rd $'\000' the_job <<'EOF'; arv-crunch-job --job "$the_job"
{
 "script":"hello-world",
 "script_version":"/home/yourname/yourrepo",
 "script_parameters":{}
}
EOF
</pre>

If everything worked correctly, the end of the log in your terminal should look like this.

<pre>
2013-09-26_02:27:07 qr1hi-8i9sb-xsgms0qsf4ztm3l 17149  output dd202dc7b1297aa221c9160455ba43ac+61+K@qr1hi
2013-09-26_02:27:08 qr1hi-8i9sb-xsgms0qsf4ztm3l 17149  finish
2013-09-26_02:27:09 qr1hi-8i9sb-xsgms0qsf4ztm3l 17149  meta key is c186e6555b51ac6974d951cd2f387722+1764+K@qr1hi
</pre>