Workflow parameterisation
Last updated on 2024-08-23 | Edit this page
Overview
Questions
- How can I change the data a workflow uses?
- How can I parameterise a workflow?
- How can I add my parameters to a file?
Objectives
- Use pipeline parameters to change the input to a workflow.
- Add a pipeline parameters to a Nextflow script.
- Understand how to create and use a parameter file.
In the first episode we ran the Nextflow script,
word_count.nf, from the command line and it counted the
number of lines in the file data/yeast/reads/ref1_1.fq.gz.
To change the input to script we can make use of pipeline
parameters.
Pipeline parameters
The Nextflow word_count.nf script defines a pipeline
parameter params.input. Pipeline parameters enable you to
change the input to the workflow at runtime, via the command line or a
configuration file, so they are not hard-coded into the script.
Pipeline parameters are declared in the workflow by prepending the
prefix params, separated by the dot character, to a
variable name e.g., params.input.
Their value can be specified on the command line by prefixing the
parameter name with a double dash character, e.g.,
--input.
In the script word_count.nf the pipeline parameter
params.input was specified with a value of
"data/yeast/reads/ref1_1.fq.gz".
To process a different file,
e.g. data/yeast/reads/ref2_2.fq.gz, in the
word_count.nf script we would run:
OUTPUT
N E X T F L O W ~ version 21.04.0
Launching `word_count.nf` [gigantic_woese] - revision: 8acb5cb9b0
executor > local (1)
[26/3cf986] process > NUM_LINES (1) [100%] 1 of 1 ✔
ref2_2.fq.gz 81720We can also use wild cards to specify multiple input files (This will
be covered in the channels episode). In the example below we use the
* to match any sequence of characters between
ref2_ and .fq.gz. Note: If
you use wild card characters on the command line you must enclose the
value in quotes.
This runs the process NUM_LINES twice, once for each file it matches.
OUTPUT
N E X T F L O W ~ version 21.04.0
Launching `word_count.nf` [tender_lumiere] - revision: 8acb5cb9b0
executor > local (2)
[cc/b6f793] process > NUM_LINES (1) [100%] 2 of 2 ✔
ref2_2.fq.gz 81720
ref2_1.fq.gz 81720Change a pipeline’s input using a parameter
Re-run the Nextflow script word_count.nf by changing the
pipeline input to all files in the directory
data/yeast/reads/ that begin with ref and end
with .fq.gz:
The string specified on the command line will override the default value of the parameter in the script. The output will look like this:
OUTPUT
N E X T F L O W ~ version 20.10.0
Launching `word_count.nf` [soggy_miescher] - revision: c54a707593
executor > local (6)
[d3/9ca185] process > NUM_LINES (2) [100%] 6 of 6 ✔
ref3_2.fq.gz 52592
ref2_2.fq.gz 81720
ref1_1.fq.gz 58708
ref1_2.fq.gz 58708
ref3_1.fq.gz 52592
ref2_1.fq.gz 81720Adding a parameter to a script
To add a pipeline parameter to a script prepend the prefix
params, separated by a dot character ., to a
variable name e.g., params.input.
Let’s make a copy of the word_count.nf script as
wc-params.nf and add a new input parameter.
To add a parameter sleep with the default value
2 to wc-params.nf we add the line:
Note: You should always add a sensible default value
to the pipeline parameter. We can use this parameter to add another step
to our NUM_LINES process.
This step, sleep ${params.sleep}, will add a delay for
the amount of time specified in the params.sleep variable,
by default 2 seconds. To access the value inside the script block we use
{variable_name} syntax
e.g. ${params.sleep}.
We can now change the sleep parameter from the command line, For Example:
Add a pipeline parameter
If you haven’t already make a copy of the word_count.nf
as wc-params.nf.
Add the param sleep with a default value of 2 below the
params.input line. Add the line
sleep ${params.sleep} in the process NUM_LINES
above the line printf ${read}.
Run the new script wc-params.nf changing the sleep input
time.
What input file would it run and why?
How would you get it to process all .fq.gz files in the
data/yeast/reads directory as well as changing the sleep
input to 1 second?
This would use 1 as a value of sleep parameter instead
of default value (which is 2) and run the pipeline. The input file would
be data/yeast/reads/ref1_1.fq.gz as this is the default. To
run all input files we could add the param
--input 'data/yeast/reads/*.fq.gz'
Parameter File
If we have many parameters to pass to a script it is best to create a
parameters file. Parameters can be stored in JSON format. JSON is a data
serialization language, that is a way of storing data objects and
structures, such as the params object in a file.
The -params-file option is used to pass the parameters
file to the script.
For example the file wc-params.json contains the
parameters sleep and input in JSON format.
Create a file called wc-params.json with the above
contents.
To run the wc-params.nf script using these parameters we
add the option -params-file and pass the file
wc-params.json:
OUTPUT
N E X T F L O W ~ version 21.04.0
Launching `wc-params.nf` [nostalgic_northcutt] - revision: 2f86c9ac7e
executor > local (2)
[b4/747eaa] process > NUM_LINES (1) [100%] 2 of 2 ✔
etoh60_1_2.fq.gz 87348
etoh60_1_1.fq.gz 87348Create and use a Parameter file.
Create a parameter file params.json for the Nextflow
file wc-params.nf, and run the Nextflow script using the
created parameter file, specifying:
- sleep as 10
- input as
data/yeast/reads/ref3_1.fq.gz
OUTPUT
N E X T F L O W
version 21.04.0 Launching `wc-params.nf` [small_wiles] - revision:
f5ef7b7a01 executor \> local (1) [f3/4fa480] process \> NUM_LINES
(1) [100%] 1 of 1 ✔ ref3_1.fq.gz 52592 Key Points
- Pipeline parameters are specified by prepending the prefix
paramsto a variable name, separated by dot character. - To specify a pipeline parameter on the command line for a Nextflow
run use
--variable_namesyntax. - You can add parameters to a JSON formatted file and pass them to the
script using option
-params-file.