Exercise 1A 3 – BIOI2 – Integrative BIOInformatics platforme

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Exercise 1A - create your first snakefile

objective > setup > o1 > o2 > o3 > o4 > o5 > o6 > recap

Objective 3

We’ve seen how to add inputs, now let’s add a new rule! Create a new snakefile named ex1_o3.smk in which we add a new rule which will run MultiQC on a list of all output files of FastQC.

Where to start?

MultiQC is a tool used to aggregate the results of multiple other tools into a single html file.

Input files: FastQC’s zip file outputs.
MultiQC command: multiqc *fastqc.zip
Expected output: 2 files: multiqc_report.html and a multiqc_data repository
NB: when the output is a directory, you have to specify this using the directory() function. In this case, you would have to put: directory("multiqc_data")

Solution

Your code for ex1_o3.smk should look like this:

rule fastqc:
  input: 
    "Data/SRR3099585_chr18.fastq.gz",
    "Data/SRR3099586_chr18.fastq.gz",
  output: 
    "FastQC/SRR3099585_chr18_fastqc.zip", 
    "FastQC/SRR3099585_chr18_fastqc.html",
    "FastQC/SRR3099586_chr18_fastqc.zip", 
    "FastQC/SRR3099586_chr18_fastqc.html", 
  shell: "fastqc --outdir FastQC {input}"

rule multiqc: 
  input: 
    "FastQC/SRR3099585_chr18_fastqc.zip", 
    "FastQC/SRR3099586_chr18_fastqc.zip", 
  output:
    "multiqc_report.html", 
    directory("multiqc_data") 
  shell: 
    "multiqc {input}"

Test the script

Next, let’s check again if your pipeline works:

You should see something similar to the following output on your screen:

Observe the output

Wait, what??! What about my new multiqc rule?

Expected behaviour:

Current behaviour:

By default, Snakemake will only execute the first rule it encounters in your Snakefile, it’s called the target rule. If (and only if) the necessary input files to execute this rule are missing, will it scan the other rules in your Snakefile to generate them. In our case, the fastqc rule is the target rule as it’s written first. Since all the necessary input files are already available for the fastqc rule, Snakemake doesn’t execute any of the other rules in the file. Let’s see in the next objective how to fix this.