# The Snakemake Executable¶

This part of the documentation describes the snakemake executable. Snakemake is primarily a command-line tool, so the snakemake executable is the primary way to execute, debug, and visualize workflows.

## Useful Command Line Arguments¶

If called without parameters, i.e.

$snakemake  Snakemake tries to execute the workflow specified in a file called Snakefile in the same directory (instead, the Snakefile can be given via the parameter -s). By issuing $ snakemake -n


a dry-run can be performed. This is useful to test if the workflow is defined properly and to estimate the amount of needed computation. Further, the reason for each rule execution can be printed via

$snakemake -n -r  Importantly, Snakemake can automatically determine which parts of the workflow can be run in parallel. By specifying the number of available cores, i.e. $ snakemake -j 4


one can tell Snakemake to use up to 4 cores and solve a binary knapsack problem to optimize the scheduling of jobs. If the number is omitted (i.e., only -j is given), the number of used cores is determined as the number of available CPU cores in the machine.

## Cluster Execution¶

Snakemake can make use of cluster engines that support shell scripts and have access to a common filesystem, (e.g. the Sun Grid Engine). In this case, Snakemake simply needs to be given a submit command that accepts a shell script as first positional argument:

$snakemake --cluster qsub -j 32  Here, -j denotes the number of jobs submitted being submitted to the cluster at the same time (here 32). The cluster command can be decorated with job specific information, e.g. $ snakemake --cluster "qsub {threads}"


Thereby, all keywords of a rule are allowed (e.g. params, input, output, threads, priority, ...). For example, you could encode the expected running time into params:

rule:
input:  ...
output: ...
params: runtime="4h"
shell: ...


and forward it to the cluster scheduler:

$snakemake --cluster "qsub --runtime {params.runtime}"  If your cluster system supports DRMAA, Snakemake can make use of that to increase the control over jobs. E.g. jobs can be cancelled upon pressing Ctrl+C, which is not possible with the generic --cluster support. With DRMAA, no qsub command needs to be provided, but system specific arguments can still be given as a string, e.g. $ snakemake --drmaa " -q username" -j 32


Note that the string has to contain a leading whitespace. Else, the arguments will be interpreted as part of the normal Snakemake arguments, and execution will fail.

### Job Properties¶

When executing a workflow on a cluster using the --cluster parameter (see below), Snakemake creates a job script for each job to execute. This script is then invoked using the provided cluster submission command (e.g. qsub). Sometimes you want to provide a custom wrapper for the cluster submission command that decides about additional parameters. As this might be based on properties of the job, Snakemake stores the job properties (e.g. rule name, threads, input files, params etc.) as JSON inside the job script. For convenience, there exists a parser function snakemake.utils.read_job_properties that can be used to access the properties. The following shows an example job submission wrapper:

#!python

#!/usr/bin/env python3
import os
import sys

jobscript = sys.argv[1]

# do something useful with the threads

# access property defined in the cluster configuration file (Snakemake >=3.6.0)
job_properties["cluster"]["time"]



## Visualization¶

To visualize the workflow, one can use the option --dag. This creates a representation of the DAG in the graphviz dot language which has to be postprocessed by the graphviz tool dot. E.g. to visualize the DAG that would be executed, you can issue:

$snakemake --dag | dot | display  For saving this to a file, you can specify the desired format: $ snakemake --dag | dot -Tpdf > dag.pdf


To visualize the whole DAG regardless of the eventual presence of files, the forceall option can be used:

\$ snakemake --forceall --dag | dot -Tpdf > dag.pdf


Of course the visual appearance can be modified by providing further command line arguments to dot.

## All Options¶

All command line options can be printed by calling snakemake -h.

## Bash Completion¶

Snakemake supports bash completion for filenames, rulenames and arguments. To enable it globally, just append

snakemake --bash-completion


including the accents to your .bashrc. This only works if the snakemake command is in your path.