Short tutorial¶

Here we provide a short tutorial that guides you through the main features of Snakemake. Note that this is not suited to learn Snakemake from scratch, rather to give a first impression. To really learn Snakemake (starting from something simple, and extending towards advanced features), use the main Snakemake Tutorial.

This document shows all steps performed in the official Snakemake live demo, such that it becomes possible to follow them at your own pace. Solutions to each step can be found at the bottom of this document.

The examples presented in this tutorial come from Bioinformatics. However, Snakemake is a general-purpose workflow management system for any discipline. For an explanation of the steps you will perform here, have a look at Background. More thorough explanations are provided in the full Snakemake Tutorial.

Prerequisites¶

First, install Snakemake via Conda, as outlined in Installation via Conda. The minimal version of Snakemake is sufficient for this demo.

Second, download and unpack the test data needed for this example from here, e.g., via

mkdir snakemake-demo
cd snakemake-demo
wget https://github.com/snakemake/snakemake-tutorial-data/archive/v5.4.5.tar.gz
tar --wildcards -xf v5.4.5.tar.gz --strip 1 "*/data"

Step 1¶

First, create an empty workflow in the current directory with:

touch Snakefile

Once a Snakefile is present, you can perform a dry run of Snakemake with:

snakemake -n

Since the Snakefile is empty, it will report that nothing has to be done. In the next steps, we will gradually fill the Snakefile with an example analysis workflow.

Step 2¶

The data folder in your working directory looks as follows:

data
├── genome.fa
├── genome.fa.amb
├── genome.fa.ann
├── genome.fa.bwt
├── genome.fa.fai
├── genome.fa.pac
├── genome.fa.sa
└── samples
    ├── A.fastq
    ├── B.fastq
    └── C.fastq

You will create a workflow that maps the sequencing samples in the data/samples folder to the reference genome data/genome.fa. Then, you will call genomic variants over the mapped samples, and create an example plot.

First, create a rule called bwa, with input files

data/genome.fa
data/samples/A.fastq

and output file

mapped/A.bam

To generate output from input, use the shell command

"bwa mem {input} | samtools view -Sb - > {output}"

Providing a shell command is not enough to run your workflow on an unprepared system. For reproducibility, you also have to provide the required software stack and define the desired version. This can be done with the Conda package manager, which is directly integrated with Snakemake: add a directive conda: "envs/mapping.yaml" that points to a Conda environment definition, with the following content

channels:
  - bioconda
  - conda-forge
dependencies:
  - bwa =0.7.17
  - samtools =1.9

Upon execution, Snakemake will automatically create that environment, and execute the shell command within.

Now, test your workflow by simulating the creation of the file mapped/A.bam via

snakemake --use-conda -n mapped/A.bam

to perform a dry-run and

snakemake --use-conda mapped/A.bam --cores 1

to perform the actual execution.

Step 3¶

Now, generalize the rule bwa by replacing the concrete sample name A with a wildcard {sample} in input and output file the rule bwa. This way, Snakemake can apply the rule to map any of the three available samples to the reference genome.

Test this by creating the file mapped/B.bam.

Step 4¶

Next, create a rule sort that sorts the obtained .bam file by genomic coordinate. The rule should have the input file

mapped/{sample}.bam

and the output file

mapped/{sample}.sorted.bam

and uses the shell command

samtools sort -o {output} {input}

to perform the sorting. Moreover, use the same conda: directive as for the previous rule.

Test your workflow with

snakemake --use-conda -n mapped/A.sorted.bam

and

snakemake --use-conda mapped/A.sorted.bam --cores 1

Step 5¶

Now, we aggregate over all samples to perform a joint calling of genomic variants. First, we define a variable

samples = ["A", "B", "C"]

at the top of the Snakefile. This serves as a definition of the samples over which we would want to aggregate. In real life, you would want to use an external sample sheet or a config file for things like this.

For aggregation over many files, Snakemake provides the helper function expand (see the docs). Create a rule call with input files

fa="data/genome.fa"
bam=expand("mapped/{sample}.sorted.bam", sample=samples)

output file

"calls/all.vcf"

and shell command

samtools mpileup -g -f {input.fa} {input.bam} | bcftools call -mv - > {output}

Further, define a new conda environment file with the following content:

channels:
  - bioconda
  - conda-forge
dependencies:
  - bcftools =1.9
  - samtools =1.9

Step 6¶

Finally, we strive to calculate some exemplary statistics. This time, we don’t use a shell command, but rather employ Snakemake’s ability to integrate with scripting languages like R and Python.

First, we create a rule stats with input file

"calls/all.vcf"

and output file

"plots/quals.svg".

Instead of a shell command, we write

script:
    "scripts/plot-quals.py"

and create the corresponding script and its containing folder in our working directory with

mkdir scripts
touch scripts/plot-quals.py

We open the script in the editor and add the following content

import matplotlib
matplotlib.use("Agg")
import matplotlib.pyplot as plt
from pysam import VariantFile

quals = [record.qual for record in VariantFile(snakemake.input[0])]
plt.hist(quals)

plt.savefig(snakemake.output[0])

As you can see, instead of writing a command line parser for passing parameters like input and output files, you have direct access to the properties of the rule via a magic snakemake object, that Snakemake automatically inserts into the script before executing the rule.

Finally, we have to define a conda environment for the rule, say envs/stats.yaml, that provides the required Python packages to execute the script:

channels:
  - bioconda
  - conda-forge
dependencies:
  - pysam =0.15
  - matplotlib =3.1
  - python =3.8

Make sure to test your workflow with

snakemake --use-conda plots/quals.svg --cores 1

Step 7¶

So far, we have always specified a target file at the command line when invoking Snakemake. When no target file is specified, Snakemake tries to execute the first rule in the Snakefile. We can use this property to define default target files.

At the top of your Snakefile define a rule all, with input files

"calls/all.vcf"
"plots/quals.svg"

and neither a shell command nor output files. This rule simply serves as an indicator of what shall be collected as results.

Step 8¶

As a last step, we strive to annotate our workflow with some additional information.

Automatic reports¶

Snakemake can automatically create HTML reports with

snakemake --report report.html

Such a report contains runtime statistics, a visualization of the workflow topology, used software and data provenance information.

In addition, you can mark any output file generated in your workflow for inclusion into the report. It will be encoded directly into the report, such that it can be, e.g., emailed as a self-contained document. The reader (e.g., a collaborator of yours) can at any time download the enclosed results from the report for further use, e.g., in a manuscript you write together. In this example, please mark the output file "plots/quals.svg" for inclusion by replacing it with report("plots/quals.svg", caption="report/calling.rst") and adding a file report/calling.rst, containing some description of the output file. This description will be presented as caption in the resulting report.

Threads¶

The first rule bwa can in theory use multiple threads. You can make Snakemake aware of this, such that the information can be used for scheduling. Add a directive threads: 8 to the rule and alter the shell command to

bwa mem -t {threads} {input} | samtools view -Sb - > {output}

This passes the threads defined in the rule as a command line argument to the bwa process.

Temporary files¶

The output of the bwa rule becomes superfluous once the sorted version of the .bam file is generated by the rule sort. Snakemake can automatically delete the superfluous output once it is not needed anymore. For this, mark the output as temporary by replacing "mapped/{sample}.bam" in the rule bwa with temp("mapped/{sample}.bam").