Process reference

This page lists the task properties, input/output methods, and directives available in process definitions.

Task properties

The following task properties are defined in the process body:

task.attempt

The current task attempt.

task.exitStatus

The exit code of the task script. Only applicable for processes with a script: or shell: block.

Since the exit code is only available after the task has been executed, it can only be used by certain process directives such as errorStrategy.

task.hash

Available only in exec: blocks

The task unique hash ID.

task.id

The pipeline-level task index. Corresponds to task_id in the execution trace.

task.index

The process-level task index.

task.name

Available only in exec: blocks

The current task name.

task.previousException

New in version 24.10.0.

The exception reported by the previous task attempt.

Since the exception is available after a failed task attempt, it can only be accessed when retrying a failed task execution, and therefore when task.attempt is greater than 1.

task.previousTrace

New in version 24.10.0.

The trace record associated with the previous task attempt.

Since the trace record is available after a failed task attempt, it can only be accessed when retrying a failed task execution, and therefore when task.attempt is greater than 1.

This is useful when retrying a task execution to access the previous task attempt runtime metrics e.g. used memory and CPUs.

task.process

The current process name.

task.workDir

Available only in exec: blocks

The task unique directory.

Additionally, the directive values for the given task can be accessed via task.<directive>.

Inputs

val( identifier )

Declare a variable input. The received value can be any type, and it will be made available to the process body (i.e. script, shell, exec) as a variable given by identifier.

file( identifier | stageName )

Deprecated since version 19.10.0: Use path instead.

Declare a file input. The received value can be any type, and it will be staged into the task directory. If the received value is not a file or collection of files, it is implicitly converted to a string and written to a file.

The argument can be an identifier or string. If an identifier, the received value will be made available to the process body as a variable. If a string, the received value will be staged into the task directory under the given alias.

path( identifier | stageName )

Declare a file input. The received value can be any type, and it will be staged into the task directory. If the received value is not a file or collection of files, it is implicitly converted to a string and written to a file.

The argument can be an identifier or string. If an identifier, the received value will be made available to the process body as a variable. If a string, the received value will be staged into the task directory under the given alias.

Available options:

arity

New in version 23.09.0-edge.

Specify the number of expected files. Can be a number, e.g. '1', or a range, e.g. '1..*'. If a task receives an invalid number of files for this path input, it will fail.

name

Specify how the file should be named in the task work directory. Can be a name or a pattern.

stageAs

Alias of name.

env( name )

Declare an environment variable input. The received value should be a string, and it will be exported to the task environment as an environment variable given by name.

stdin

Declare a stdin input. The received value should be a string, and it will be provided as the standard input (i.e. stdin) to the task script. It should be declared only once for a process.

tuple( arg1, arg2, ... )

Declare a tuple input. Each argument should be an input declaration such as val, path, env, or stdin.

The received value should be a tuple with the same number of elements as the tuple declaration, and each received element should be compatible with the corresponding tuple argument. Each tuple element is treated the same way as if it were a standalone input.

Outputs

val( value )

Declare a variable output. The argument can be any value, and it can reference any output variables defined in the process body (i.e. variables declared without the def keyword).

file( pattern )

Deprecated since version 19.10.0: Use path instead.

Declare a file output. It receives the output files from the task environment that match the given pattern.

Multiple patterns can be specified using the colon separator (:). The union of all files matched by each pattern will be collected.

path( pattern, [options] )

Declare a file output. It receives the output files from the task environment that match the given pattern.

Available options:

arity

New in version 23.09.0-edge.

Specify the number of expected files. Can be a number or a range. If a task produces an invalid number of files for this path output, it will fail.

If the arity is 1, a single file will be emitted. Otherwise, a list will always be emitted, even if only one file is produced.

Warning

If the arity is not specified, a single file or list will be emitted based on whether a single file or multiple files are produced at runtime, resulting potentially in an output channel with a mixture of files and file collections.

followLinks

When true, target files are returned in place of any matching symlink (default: true).

glob

When true, the specified name is interpreted as a glob pattern (default: true).

hidden

When true, hidden files are included in the matching output files (default: false).

includeInputs

When true and the output path is a glob pattern, any input files matching the pattern are also included in the output (default: false).

maxDepth

Maximum number of directory levels to visit (default: no limit).

type

Type of paths returned, either file, dir or any (default: any, or file if the specified file name pattern contains a double star (**)).

env( name )

Declare an environment variable output. It receives the value of the environment variable (given by name) from the task environment.

Changed in version 23.12.0-edge: Prior to this version, if the environment variable contained multiple lines of output, the output would be compressed to a single line by converting newlines to spaces.

stdout

Declare a stdout output. It receives the standard output of the task script.

eval( command )

New in version 24.02.0-edge.

Declare an eval output. It receives the standard output of the given command, which is executed in the task environment after the task script.

If the command fails, the task will also fail.

tuple( arg1, arg2, ... )

Declare a tuple output. Each argument should be an output declaration such as val, path, env, stdin, or eval. Each tuple element is treated the same way as if it were a standalone output.

Generic options

The following options are available for all process outputs:

emit: <name>

Defines the name of the output channel.

optional: true | false

When true, the task will not fail if the specified output is missing (default: false).

topic: <name>

New in version 23.11.0-edge.

Experimental: may change in a future release.

Defines the channel topic to which the output will be sent.

Directives

accelerator

New in version 19.09.0-edge.

The accelerator directive allows you to request hardware accelerators (e.g. GPUs) for the task execution. For example:

process foo {
    accelerator 4, type: 'nvidia-tesla-k80'

    script:
    """
    your_gpu_enabled --command --line
    """
}

The above examples will request 4 GPUs of type nvidia-tesla-k80.

Note

This directive is only used by certain executors. Refer to the Executors page to see which executors support this directive.

Note

Additional options may be required to fully enable the use of accelerators. When using containers with GPUs, you must pass the GPU drivers through to the container. For Docker, this requires the option --gpus all in the docker run command. For Apptainer/Singularity, this requires the option --nv. The specific implementation details depend on the accelerator and container type being used.

Note

The accelerator type option depends on the target execution platform. Refer to the platform-specific documentation for details on the available accelerators:

• Google Cloud

• Kubernetes

The accelerator type option is not supported for AWS Batch. You can control the accelerator type indirectly through the allowed instance types in your Compute Environment. See the AWS Batch FAQs for more information.

afterScript

The afterScript directive allows you to execute a custom (Bash) snippet immediately after the main process has run. This may be useful to clean up your staging area.

Note

When combined with the container directive, the afterScript will be executed outside the specified container. In other words, the afterScript is always executed in the host environment.

arch

The arch directive allows you to define the CPU architecture to build the software in use by the process’ task. For example:

process cpu_task {
    spack 'blast-plus@2.13.0'
    arch 'linux/x86_64', target: 'cascadelake'

    script:
    """
    blastp -query input_sequence -num_threads ${task.cpus}
    """
}

The example above declares that the CPU generic architecture is linux/x86_64 (X86 64 bit), and more specifically that the microarchitecture is cascadelake (a specific generation of Intel CPUs).

This directive is currently used by the following Nextflow functionalities:

• by the spack directive, to build microarchitecture-optimised applications;

• by the Wave containers service, to build containers for one of the generic families of CPU architectures (see below);

• by the spack strategy within Wave containers, to optimise the container builds for specific CPU microarchitectures.

Allowed values for the arch directive are as follows, grouped by equivalent family (choices available for the sake of compatibility):

• X86 64 bit: linux/x86_64, x86_64, linux/amd64, amd64

• ARM 64 bit: linux/aarch64, aarch64, linux/arm64, arm64, linux/arm64/v8

• ARM 64 bit, older generation: linux/arm64/v7

Examples of values for the architecture target option are cascadelake, icelake, zen2 and zen3. See the Spack documentation for the full and up-to-date list of meaningful targets.

array

New in version 24.04.0.

Warning

Experimental: may change in a future release.

The array directive allows you to submit tasks as job arrays for executors that support it.

A job array is a collection of jobs with the same resource requirements and the same script (parameterized by an index). Job arrays incur significantly less scheduling overhead compared to individual jobs, and as a result they are preferred by HPC schedulers where possible.

The directive should be specified with a given array size, along with an executor that supports job arrays. For example:

process cpu_task {
    executor 'slurm'
    array 100

    script:
    """
    your_command --here
    """
}

Nextflow currently supports job arrays for the following executors:

• AWS Batch

• Google Cloud Batch

• LSF

• PBS/Torque

• PBS Pro

• SGE

• SLURM

A process using job arrays will collect tasks and submit each batch as a job array when it is ready. Any “leftover” tasks will be submitted as a partial job array.

Once a job array is submitted, each “child” task is executed as an independent job. Any tasks that fail (and can be retried) will be retried without interfering with the tasks that succeeded. Retried tasks are submitted individually rather than through a job array, in order to allow for the use of dynamic resources.

The following directives must be uniform across all tasks in a process that uses job arrays, because these directives are specified once for the entire job array:

• accelerator

• clusterOptions

• cpus

• disk

• machineType

• memory

• queue

• resourceLabels

• resourceLimits

• time

For cloud-based executors like AWS Batch, or when using Fusion with any executor, the following additional directives must be uniform:

• container

• containerOptions

beforeScript

The beforeScript directive allows you to execute a custom (Bash) snippet before the main process script is run. This may be useful to initialise the underlying cluster environment or for other custom initialisation.

For example:

process foo {
  beforeScript 'source /cluster/bin/setup'

  script:
  """
  echo bar
  """
}

When the process is containerized (using the container directive), the beforeScript will be executed in the container only if the executor is container-native (e.g. cloud batch executors, Kubernetes). Otherwise, the beforeScript will be executed outside the container.

cache

The cache directive allows you to store the process results to a local cache. When the cache is enabled and the pipeline is launched with the resume option, any task executions that are already cached will be re-used. See the Caching and resuming page for more information about how the cache works.

The cache is enabled by default, but you can disable it for a specific process by setting the cache directive to false. For example:

process noCacheThis {
  cache false
  // ...
}

The following options are available:

false

Disable caching.

true (default)

Enable caching. Input file metadata (name, size, last updated timestamp) are included in the cache keys.

'deep'

Enable caching. Input file content is included in the cache keys.

'lenient'

Enable caching. Minimal input file metadata (name and size only) are included in the cache keys.

This strategy provides a workaround for incorrect caching invalidation observed on shared file systems due to inconsistent file timestamps.

clusterOptions

The clusterOptions directive allows the usage of any native configuration option accepted by your cluster submit command. You can use it to request non-standard resources or use settings that are specific to your cluster and not supported out of the box by Nextflow.

The cluster options can be a string:

process foo {
  clusterOptions '-x 1 -y 2'
  // ...
}

Changed in version 24.04.0: Prior to this version, grid executors that require each option to be on a separate line in the job script would attempt to split multiple options using a variety of different conventions. Multiple options can now be specified more clearly using a string list as shown below.

The cluster options can also be a string list:

process foo {
  clusterOptions '-x 1', '-y 2', '--flag'
  // ...
}

Grid executors that require one option per line will write each option to a separate line, while grid executors that allow multiple options per line will write all options to a single line, the same as with a string. This form is useful to control how the options are split across lines when it is required by the scheduler.

Note

This directive is only used by grid executors. Refer to the Executors page to see which executors support this directive.

Warning

While you can use the clusterOptions directive to specify options that are supported as process directives (queue, memory, time, etc), you should not use both at the same time, as it will cause undefined behavior. Most HPC schedulers will either fail or simply ignore one or the other.

conda

The conda directive allows for the definition of the process dependencies using the Conda package manager.

Nextflow automatically sets up an environment for the given package names listed by in the conda directive. For example:

process foo {
  conda 'bwa=0.7.15'

  script:
  """
  your_command --here
  """
}

Multiple packages can be specified separating them with a blank space e.g. bwa=0.7.15 fastqc=0.11.5. The name of the channel from where a specific package needs to be downloaded can be specified using the usual Conda notation i.e. prefixing the package with the channel name as shown here bioconda::bwa=0.7.15.

The conda directive also allows the specification of a Conda environment file path or the path of an existing environment directory. See the Conda environments page for further details.

container

The container directive allows you to execute the process script in a Docker container.

It requires the Docker daemon to be running in machine where the pipeline is executed, i.e. the local machine when using the local executor or the cluster nodes when the pipeline is deployed through a grid executor.

For example:

process runThisInDocker {
  container 'dockerbox:tag'

  script:
  """
  your_command --here
  """
}

Simply replace in the above script dockerbox:tag with the name of the Docker image you want to use.

Tip

Containers are a very useful way to execute your scripts in a reproducible self-contained environment or to run your pipeline in the cloud.

Note

This directive is ignored for processes that are executed natively.

containerOptions

The containerOptions directive allows you to specify any container execution option supported by the underlying container engine (ie. Docker, Singularity, etc). This can be useful to provide container settings only for a specific process e.g. mount a custom path:

process runThisWithDocker {
    container 'busybox:latest'
    containerOptions '--volume /data/db:/db'

    output:
    path 'output.txt'

    script:
    """
    your_command --data /db > output.txt
    """
}

Warning

This feature is not supported by the Kubernetes and Google Life Sciences executors.

cpus

The cpus directive allows you to define the number of (logical) CPU required by the process’ task. For example:

process big_job {
  cpus 8
  executor 'sge'

  script:
  """
  blastp -query input_sequence -num_threads ${task.cpus}
  """
}

This directive is required for tasks that execute multi-process or multi-threaded commands/tools and it is meant to reserve enough CPUs when a pipeline task is executed through a cluster resource manager.

See also: penv, memory, time, queue, maxForks

debug

By default the stdout produced by the commands executed in all processes is ignored. By setting the debug directive to true, you can forward the process stdout to the current top running process stdout file, showing it in the shell terminal.

For example:

process sayHello {
  debug true

  script:
  """
  echo Hello
  """
}

Hello

Without specifying debug true, you won’t see the Hello string printed out when executing the above example.

disk

The disk directive allows you to define how much local disk storage the process is allowed to use. For example:

process big_job {
    disk '2 GB'
    executor 'cirrus'

    script:
    """
    your_command --here
    """
}

The following memory unit suffix can be used when specifying the disk value:

Unit	Description
B	Bytes
KB	Kilobytes
MB	Megabytes
GB	Gigabytes
TB	Terabytes

See MemoryUnit for more information.

Note

This directive is only used by certain executors. Refer to the Executors page to see which executors support this directive.

See also: cpus, memory time, queue and Dynamic task resources.

echo

Deprecated since version 22.04.0: Use debug instead

errorStrategy

The errorStrategy directive allows you to define how the process manages an error condition. By default, when an error status is returned by the executed script (i.e. when it ends with a non-zero exit status), the process stops immediately, forcing the entire pipeline to terminate.

The following error strategies are available:

terminate (default)

When a task fails, terminate the pipeline immediately and report an error. Pending and running jobs are killed.

finish

When a task fails, wait for submitted and running tasks to finish and then terminate the pipeline, reporting an error.

ignore

When a task fails, ignore it and continue the pipeline execution. If the workflow.failOnIgnore config option is set to true, the pipeline will report an error (i.e. return a non-zero exit code) upon completion. Otherwise, the pipeline will complete successfully.

See Constants for more information on workflow.failOnIgnore.

retry

When a task fails, retry it.

When setting the errorStrategy directive to ignore the process doesn’t stop on an error condition, it just reports a message notifying you of the error event.

For example:

process ignoreAnyError {
  errorStrategy 'ignore'

  // ...
}

In this case, the workflow will complete successfully and return an exit status of 0. However, if you set workflow.failOnIgnore = true in your Nextflow configuration, the workflow will return a non-zero exit status and report the failed tasks as an error.

The retry error strategy allows you to re-submit for execution a process returning an error condition. For example:

process retryIfFail {
  errorStrategy 'retry'

  // ...
}

The number of times a failing process is re-executed is defined by the maxRetries and maxErrors directives.

Tip

More complex strategies depending on the task exit status or other parametric values can be defined using a dynamic errorStrategy. See Dynamic directives for details.

See also: maxErrors, maxRetries and Dynamic task resources.

executor

The executor defines the underlying system where processes are executed. By default a process uses the executor defined globally in the nextflow.config file.

The executor directive allows you to configure what executor has to be used by the process, overriding the default configuration.

The following executors are available:

Name	Executor
awsbatch	AWS Batch service
azurebatch	Azure Batch service
condor	HTCondor job scheduler
google-lifesciences	Google Genomics Pipelines service
k8s	Kubernetes cluster
local	The computer where Nextflow is launched
lsf	Platform LSF job scheduler
moab	Moab job scheduler
nqsii	NQSII job scheduler
oge	Alias for the sge executor
pbs	PBS/Torque job scheduler
pbspro	PBS Pro job scheduler
sge	Sun Grid Engine / Open Grid Engine
slurm	SLURM workload manager
uge	Alias for the sge executor

The following example shows how to set the process’s executor:

process doSomething {
  executor 'sge'

  // ...
}

Note

Each executor supports additional directives and executor configuration options. See Executors for more information.

ext

The ext is a special directive used as namespace for user custom process directives. This can be useful for advanced configuration options. For example:

process mapping {
  container "biocontainers/star:${task.ext.version}"

  input:
  path genome
  tuple val(sampleId), path(reads)

  script:
  """
  STAR --genomeDir $genome --readFilesIn $reads ${task.ext.args ?: ''}
  """
}

In the above example, the process container version is controlled by ext.version, and the script supports additional command line arguments through ext.args.

The ext directive can be set in the process definition:

process mapping {
  ext version: '2.5.3', args: '--foo --bar'

  // ...
}

Or in the Nextflow configuration:

process.ext.version = '2.5.3'
process.ext.args = '--foo --bar'

fair

New in version 22.12.0-edge.

The fair directive, when enabled, guarantees that process outputs will be emitted in the order in which they were received. For example:

process foo {
    fair true

    input:
    val x
    output:
    tuple val(task.index), val(x)

    script:
    """
    sleep \$((RANDOM % 3))
    """
}

workflow {
    channel.of('A','B','C','D') | foo | view
}

The above example produces:

[1, A]
[2, B]
[3, C]
[4, D]

label

The label directive allows the annotation of processes with mnemonic identifier of your choice. For example:

process bigTask {
  label 'big_mem'

  script:
  """
  your_command --here
  """
}

The same label can be applied to more than a process and multiple labels can be applied to the same process using the label directive more than one time.

Note

A label must consist of alphanumeric characters or _, must start with an alphabetic character and must end with an alphanumeric character.

Labels are useful to organize workflow processes in separate groups which can be referenced in the configuration file to select and configure a subset of processes having similar computing requirements. See Process selectors for more information.

See also: resourceLabels

machineType

New in version 19.07.0.

The machineType can be used to specify a predefined Google Compute Platform machine type when running using the Google Batch or Google Life Sciences executor, or when using the autopools feature of the Azure Batch executor.

This directive is optional and if specified overrides the cpus and memory directives:

process foo {
  machineType 'n1-highmem-8'

  script:
  """
  your_command --here
  """
}

See also: cpus and memory.

maxSubmitAwait

The maxSubmitAwait directive allows you to specify how long a task can remain in submission queue without being executed. Elapsed this time the task execution will fail.

When used along with retry error strategy, it can be useful to re-schedule the task to a difference queue or resource requirement. For example:

process foo {
  errorStrategy 'retry'
  maxSubmitAwait '10 mins'
  maxRetries 3
  queue "${task.submitAttempt==1 : 'spot-compute' : 'on-demand-compute'}"

  script:
  """
  your_command --here
  """
}

In the above example the task is submitted to the spot-compute on the first attempt (task.submitAttempt==1). If the task execution does not start in the 10 minutes, a failure is reported and a new submission is attempted using the queue named on-demand-compute.

maxErrors

The maxErrors directive allows you to specify the maximum number of times a process can fail when using the retry error strategy. By default this directive is disabled, you can set it as shown in the example below:

process retryIfFail {
  errorStrategy 'retry'
  maxErrors 5

  script:
  """
  echo 'do this as that .. '
  """
}

Note

This setting considers the total errors accumulated for a given process, across all instances. If you want to control the number of times a process instance (aka task) can fail, use maxRetries.

See also: errorStrategy and maxRetries.

maxForks

The maxForks directive allows you to define the maximum number of process instances that can be executed in parallel. By default this value is equals to the number of CPU cores available minus 1.

If you want to execute a process in a sequential manner, set this directive to one. For example:

process doNotParallelizeIt {
  maxForks 1

  script:
  """
  your_command --here
  """
}

maxRetries

The maxRetries directive allows you to define the maximum number of times a process instance can be re-submitted in case of failure. This value is applied only when using the retry error strategy. By default only one retry is allowed, you can increase this value as shown below:

process retryIfFail {
    errorStrategy 'retry'
    maxRetries 3

    script:
    """
    echo 'do this as that .. '
    """
}

Note

There is a subtle but important difference between maxRetries and the maxErrors directive. The latter defines the total number of errors that are allowed during the process execution (the same process can launch different execution instances), while the maxRetries defines the maximum number of times the same process execution can be retried in case of an error.

See also: errorStrategy and maxErrors.

memory

The memory directive allows you to define how much memory the process is allowed to use. For example:

process big_job {
    memory '2 GB'
    executor 'sge'

    script:
    """
    your_command --here
    """
}

The following memory unit suffix can be used when specifying the memory value:

Unit	Description
B	Bytes
KB	Kilobytes
MB	Megabytes
GB	Gigabytes
TB	Terabytes

See MemoryUnit for more information.

See also: cpus, time, queue and Dynamic task resources.

module

Environment Modules is a package manager that allows you to dynamically configure your execution environment and easily switch between multiple versions of the same software tool.

If it is available in your system you can use it with Nextflow in order to configure the processes execution environment in your pipeline.

In a process definition you can use the module directive to load a specific module version to be used in the process execution environment. For example:

process basicExample {
  module 'ncbi-blast/2.2.27'

  script:
  """
  blastp -query <etc..>
  """
}

You can repeat the module directive for each module you need to load. Alternatively multiple modules can be specified in a single module directive by separating all the module names by using a : (colon) character as shown below:

process manyModules {
  module 'ncbi-blast/2.2.27:t_coffee/10.0:clustalw/2.1'

  script:
  """
  blastp -query <etc..>
  """
}

penv

The penv directive allows you to define the parallel environment to be used when submitting a parallel task to the SGE resource manager. For example:

process big_job {
  cpus 4
  penv 'smp'
  executor 'sge'

  script:
  """
  blastp -query input_sequence -num_threads ${task.cpus}
  """
}

This configuration depends on the parallel environment provided by your grid engine installation. Refer to your cluster documentation or contact your admin to learn more about this.

See also: cpus, memory, time

pod

The pod directive allows the definition of pod specific settings, such as environment variables, secrets, and config maps, when using the Kubernetes executor.

For example:

process your_task {
  pod env: 'FOO', value: 'bar'

  script:
  """
  echo $FOO
  """
}

The above snippet defines an environment variable named FOO whose value is bar.

When defined in the Nextflow configuration file, a pod setting can be defined as a map:

process {
  pod = [env: 'FOO', value: 'bar']
}

Or as a list of maps:

process {
  pod = [
    [env: 'FOO', value: 'bar'],
    [secret: 'my-secret/key1', mountPath: '/etc/file.txt']
  ]
}

The following options are available:

affinity: <config>

New in version 22.01.0-edge.

Specifies the pod affinity with the given configuration.

annotation: '<name>', value: '<value>'

Can be specified multiple times

Defines a pod annotation with the given name and value.

automountServiceAccountToken: true | false

New in version 22.01.0-edge.

Specifies whether to automount service account token into the pod (default: true).

config: '<configMap>/<key>', mountPath: '</absolute/path>'

Can be specified multiple times

Mounts a ConfigMap with name and optional key to the given path. If the key is omitted, the path is interpreted as a directory and all entries in the ConfigMap are exposed in that path.

csi: '<config>', mountPath: '</absolute/path>'

New in version 22.11.0-edge.

Can be specified multiple times

Mounts a CSI ephemeral volume with the given configuration to the given path.

emptyDir: <config>, mountPath: '</absolute/path>'

New in version 22.11.0-edge.

Can be specified multiple times

Mounts an emptyDir with the given configuration to the given path.

env: '<name>', config: '<configMap>/<key>'

Can be specified multiple times

Defines an environment variable whose value is defined by the given ConfigMap and key.

env: '<name>', fieldPath: '<fieldPath>'

New in version 21.09.1-edge.

Can be specified multiple times

Defines an environment variable whose value is defined by the given field path value.

For example, the following pod option:

pod = [env: 'MY_NODE_NAME', fieldPath: 'spec.nodeName']

Maps to the following pod spec:

env:
  - name: MY_NODE_NAME
    valueFrom:
      fieldRef:
        fieldPath: spec.nodeName

env: '<name>', secret: '<secret>/<key>'

Can be specified multiple times

Defines an environment variable whose value is defined by the given Secret and key.

env: '<name>', value: '<value>'

Can be specified multiple times

Defines an environment variable with the given name and value.

hostPath: '/host/absolute/path', mountPath: '</pod/absolute/path>'

New in version 23.10.0.

Can be specified multiple times

Allows creating hostPath volume and access it with the specified mountPath in the pod.

imagePullPolicy: 'IfNotPresent' | 'Always' | 'Never'

Specifies the image pull policy used by the pod to pull the container image.

imagePullSecret: '<name>'

Specifies the image pull secret used to access a private container image registry.

label: '<name>', value: '<value>'

Can be specified multiple times

Defines a pod label with the given name and value.

nodeSelector: <config>

Specifies the node selector with the given configuration.

The configuration can be a map or a string:

// map
pod = [nodeSelector: [disktype: 'ssd', cpu: 'intel']]

// string
pod = [nodeSelector: 'disktype=ssd,cpu=intel']

priorityClassName: '<name>'

New in version 22.01.0-edge.

Specifies the priority class name for pods.

privileged: true | false

New in version 22.05.0-edge.

Specifies whether the pod should run as a privileged container (default: false).

runAsUser: '<uid>'

Specifies the user ID with which to run the container. Shortcut for the securityContext option.

schedulerName: '<name>'

Specifies which scheduler is used to schedule the container.

secret: '<secret>/<key>', mountPath: '</absolute/path>'

Can be specified multiple times

Mounts a Secret with name and optional key to the given path. If the key is omitted, the path is interpreted as a directory and all entries in the Secret are exposed in that path.

securityContext: <config>

Specifies the pod security context with the given configuration.

toleration: <config>

New in version 22.04.0.

Can be specified multiple times

Specifies the pod toleration with the given configuration.

The configuration should be a map corresponding to a single toleration rule. For example, the following pod options:

pod = [
    [toleration: [key: 'key1', operator: 'Equal', value: 'value1', effect: 'NoSchedule']],
    [toleration: [key: 'key1', operator: 'Exists', effect: 'NoSchedule']],
]

Maps to the following pod spec:

tolerations:
  - key: "key1"
    operator: "Equal"
    value: "value1"
    effect: "NoSchedule"
  - key: "key1"
    operator: "Exists"
    effect: "NoSchedule"

ttlSecondsAfterFinished

New in version 24.02.0-edge.

Specifies the TTL mechanism for finished jobs in seconds. Applies to both successful and failed jobs.

volumeClaim: '<name>', mountPath: '</absolute/path>' [, subPath: '<path>', readOnly: true | false]

Can be specified multiple times

Mounts a Persistent volume claim with the given name to the given path.

The subPath option can be used to mount a sub-directory of the volume instead of its root.

The readOnly option can be used to mount the volume as read-only (default: false)

publishDir

The publishDir directive allows you to publish the process output files to a specified folder. For example:

process foo {
    publishDir '/data/chunks'

    output:
    path 'chunk_*'

    script:
    """
    printf 'Hola' | split -b 1 - chunk_
    """
}

The above example splits the string Hola into file chunks of a single byte. When complete the chunk_* output files are published into the /data/chunks folder.

Note

Only files that match the declaration in the output block are published, not all the outputs of the process.

Tip

The publishDir directive can be specified more than once in order to publish output files to different target directories based on different rules.

By default files are published to the target folder creating a symbolic link for each process output that links the file produced into the process working directory. This behavior can be modified using the mode option, for example:

process foo {
    publishDir '/data/chunks', mode: 'copy', overwrite: false

    output:
    path 'chunk_*'

    script:
    """
    printf 'Hola' | split -b 1 - chunk_
    """
}

Warning

Files are copied into the specified directory in an asynchronous manner, so they may not be immediately available in the publish directory at the end of the process execution. For this reason, downstream processes should not try to access output files through the publish directory, but through channels.

Available options:

contentType

New in version 22.10.0.

Experimental: currently only supported for S3.

Allow specifying the media content type of the published file a.k.a. MIME type. If set to true, the content type is inferred from the file extension (default: false).

enabled

Enable or disable the publish rule depending on the boolean value specified (default: true).

failOnError

Changed in version 24.03.0-edge: The default value was changed from false to true

When true abort the execution if some file can’t be published to the specified target directory or bucket for any cause (default: true)

mode

The file publishing method. Can be one of the following values:

• 'copy': Copies the output files into the publish directory.

• 'copyNoFollow': Copies the output files into the publish directory without following symlinks ie. copies the links themselves.

• 'link': Creates a hard link in the publish directory for each output file.

• 'move': Moves the output files into the publish directory. Note: this is only supposed to be used for a terminal process i.e. a process whose output is not consumed by any other downstream process.

• 'rellink': Creates a relative symbolic link in the publish directory for each output file.

• 'symlink': Creates an absolute symbolic link in the publish directory for each output file (default).

overwrite

When true any existing file in the specified folder will be overridden (default: true during normal pipeline execution and false when pipeline execution is resumed).

path

Specifies the directory where files need to be published. Note: the syntax publishDir '/some/dir' is a shortcut for publishDir path: '/some/dir'.

pattern

Specifies a [glob][glob] file pattern that selects which files to publish from the overall set of output files.

saveAs

A closure which, given the name of the file being published, returns the actual file name or a full path where the file is required to be stored. This can be used to rename or change the destination directory of the published files dynamically by using a custom strategy. Return the value null from the closure to not publish a file. This is useful when the process has multiple output files, but you want to publish only some of them.

storageClass

New in version 22.12.0-edge.

Experimental: currently only supported for S3.

Allow specifying the storage class to be used for the published file.

tags

New in version 21.12.0-edge.

Experimental: currently only supported for S3.

Allow the association of arbitrary tags with the published file e.g. tags: [FOO: 'Hello world'].

queue

The queue directive allows you to set the queue where jobs are scheduled when using a grid based executor in your pipeline. For example:

process grid_job {
    queue 'long'
    executor 'sge'

    script:
    """
    your_command --here
    """
}

Tip

Some grid executors support multiple queue names as a comma-separated list:

queue 'short,long,cn-el6'

However, this does not generally apply to other executors such as AWS Batch, Azure Batch, Google Batch.

Note

This directive is only used by certain executors. Refer to the Executors page to see which executors support this directive.

resourceLabels

New in version 22.09.1-edge.

The resourceLabels directive allows you to specify custom name-value pairs that Nextflow applies to the computing resource used to carry out the process execution. Resource labels can be specified using the syntax shown below:

process my_task {
    resourceLabels region: 'some-region', user: 'some-username'

    script:
    """
    your_command --here
    """
}

The limits and the syntax of the corresponding cloud provider should be taken into consideration when using resource labels.

Resource labels are currently supported by the following executors:

• AWS Batch

• Azure Batch

• Google Cloud Batch

• Google Life Sciences

• Kubernetes

New in version 23.09.0-edge: Resource labels are supported for Azure Batch when using automatic pool creation.

Resource labels in Azure are added to pools, rather than jobs, in order to facilitate cost analysis. A new pool will be created for each new set of resource labels, therefore it is recommended to also set azure.batch.deletePoolsOnCompletion = true when using process-specific resource labels.

See also: label

resourceLimits

New in version 24.04.0.

The resourceLimits directive allows you to specify environment-specific limits for task resource requests. Resource limits can be specified in a process as follows:

process my_task {
  resourceLimits cpus: 24, memory: 768.GB, time: 72.h

  script:
  """
  your_command --here
  """
}

Or in the Nextflow configuration:

process {
    resourceLimits = [ cpus: 24, memory: 768.GB, time: 72.h ]
}

Resource limits can be defined for the following directives:

• cpus

• disk

• memory

• time

Resource limits are a useful way to specify environment-specific limits alongside tasks with dynamic resources. Normally, if a task requests more resources than can be provisioned (e.g. a task requests 32 cores but the largest node in the cluster has 24), the task will either fail or cause the pipeline to hang forever as it will never be scheduled. If the resourceLimits directive is defined with these limits, the task resources will be automatically reduced to comply with these limits before the job is submitted.

scratch

The scratch directive allows you to execute the process in a temporary folder that is local to the execution node.

This is useful when your pipeline is launched by using a grid executor, because it allows you to decrease the NFS overhead by running the pipeline processes in a temporary directory in the local disk of the actual execution node. Only the files declared as output in the process definition will be copied in the pipeline working area.

In its basic form simply specify true at the directive value, as shown below:

process simpleTask {
  scratch true

  output:
  path 'data_out'

  script:
  """
  your_command --here
  """
}

By doing this, it tries to execute the script in the directory defined by the variable $TMPDIR in the execution node. If this variable does not exist, it will create a new temporary directory by using the Linux command mktemp.

Note

Cloud-based executors use scratch = true by default, since the work directory resides in object storage.

The following values are supported:

false

Do not use a scratch directory.

true

Create a scratch directory in the directory defined by the $TMPDIR environment variable, or $(mktemp /tmp) if $TMPDIR is not set.

'$YOUR_VAR'

Create a scratch directory in the directory defined by the given environment variable, or $(mktemp /tmp) if that variable is not set. The value must use single quotes, otherwise the environment variable will be evaluated in the pipeline script context.

'/my/tmp/path'

Create a scratch directory in the specified directory.

'ram-disk'

Create a scratch directory in the RAM disk /dev/shm/.

shell

The shell directive allows you to define a custom shell command for process scripts. By default, script blocks are executed with /bin/bash -ue.

process doMoreThings {
    shell '/bin/bash', '-euo', 'pipefail'

    script:
    """
    your_command --here
    """
}

The same directive could be specified in your Nextflow configuration as follows:

process.shell = ['/bin/bash', '-euo', 'pipefail']

spack

The spack directive allows for the definition of the process dependencies using the Spack package manager.

Nextflow automatically sets up an environment for the given package names listed by in the spack directive. For example:

process foo {
    spack 'bwa@0.7.15'

    script:
    """
    your_command --here
    """
}

Multiple packages can be specified separating them with a blank space, e.g. bwa@0.7.15 fastqc@0.11.5.

The spack directive also allows the specification of a Spack environment file path or the path of an existing environment directory. See the Spack environments page for further details.

stageInMode

The stageInMode directive defines how input files are staged into the process work directory. The following values are allowed:

'copy'

Input files are staged in the process work directory by creating a copy.

'link'

Input files are staged in the process work directory by creating a hard link for each of them.

'rellink'

Input files are staged in the process work directory by creating a symbolic link with a relative path for each of them.

'symlink'

Input files are staged in the process work directory by creating a symbolic link with an absolute path for each of them (default).

stageOutMode

The stageOutMode directive defines how output files are staged out from the scratch directory to the process work directory. The following values are allowed:

'copy'

Output files are copied from the scratch directory to the work directory.

'fcp'

New in version 23.02.0-edge.

Output files are copied from the scratch directory to the work directory by using the fcp utility (note: it must be available in your cluster computing nodes).

'move'

Output files are moved from the scratch directory to the work directory.

'rclone'

New in version 23.01.0-edge.

Output files are copied from the scratch directory to the work directory by using the rclone utility (note: it must be available in your cluster computing nodes).

'rsync'

Output files are copied from the scratch directory to the work directory by using the rsync utility.

See also: scratch.

storeDir

The storeDir directive allows you to define a directory that is used as a permanent cache for your process results.

In more detail, it affects the process execution in two main ways:

1. The process is executed only if the files declared in the output block do not exist in the directory specified by the storeDir directive. When the files exist the process execution is skipped and these files are used as the actual process result.

2. Whenever a process is successfully completed the files listed in the output block are moved into the directory specified by the storeDir directive.

The following example shows how to use the storeDir directive to create a directory containing a BLAST database for each species specified by an input parameter:

process formatBlastDatabases {
  storeDir '/db/genomes'

  input:
  path species

  output:
  path "${dbName}.*"

  script:
  dbName = species.baseName
  """
  makeblastdb -dbtype nucl -in ${species} -out ${dbName}
  """
}

Warning

The storeDir directive is meant for long-term process caching and should not be used to publish output files or organize outputs into a semantic directory structure. In those cases, use the publishDir directive instead.

Note

The use of AWS S3 paths is supported, however it requires the installation of the AWS CLI (i.e. aws) in the target compute node.

tag

The tag directive allows you to associate each process execution with a custom label, so that it will be easier to identify them in the log file or in the trace execution report. For example:

process foo {
  tag "$code"

  input:
  val code

  script:
  """
  echo $code
  """
}

workflow {
  Channel.of('alpha', 'gamma', 'omega') | foo
}

The above snippet will print a log similar to the following one, where process names contain the tag value:

[6e/28919b] Submitted process > foo (alpha)
[d2/1c6175] Submitted process > foo (gamma)
[1c/3ef220] Submitted process > foo (omega)

See also Trace execution report

time

The time directive allows you to define how long a process is allowed to run. For example:

process big_job {
    time '1h'

    script:
    """
    your_command --here
    """
}

The following time unit suffixes can be used when specifying the duration value:

Unit	Description
ms, milli, millis	Milliseconds
s, sec, second, seconds	Seconds
m, min, minute, minutes	Minutes
h, hour, hours	Hours
d, day, days	Days

Multiple units can be used in a single declaration, for example: '1day 6hours 3minutes 30seconds'

See Duration for more information.

Note

This directive is only used by certain executors. Refer to the Executors page to see which executors support this directive.

See also: cpus, memory, queue and Dynamic task resources.