Pipeline script

The Nextflow scripting language is an extension of the Groovy programming language whose syntax has been specialized to ease the writing of computational pipelines in a declarative manner.

This means that Nextflow can execute any Groovy piece of code or use any library for the Java JVM platform.

For a detailed description of the Groovy programming language, references these links:

Below you can find a crash course in the most important language constructs used in the Nextflow scripting language.


Nextflow uses UTF-8 as default file character encoding for source and application files. Make sure to use the UTF-8 encoding when editing Nextflow scripts with your favourite text editor.

Language basics

Hello world

To print something is as easy as using the print or the println methods.

println "Hello, World!"

The only difference between the two is that the println method appends implicitly a new line character to the printed string.


To define a variable, simply assign a value to it. For example:

x = 1
println x

x = new java.util.Date()
println x

x = -3.1499392
println x

x = false
println x

x = "Hi"
println x


A List object can be defined by placing the list items in square brackets. For example:

myList = [1776, -1, 33, 99, 0, 928734928763]

You can access a given item in the list with square bracket notation (indexes start at 0):

println myList[0]

In order to get the length of the list use the size method:

println myList.size()

Learn more about lists:


Maps are used to store associative arrays or dictionaries. They are unordered collections of heterogeneous, named data. For example:

scores = [ "Brett":100, "Pete":"Did not finish", "Andrew":86.87934 ]

Note that each of the values stored in the map can be of a different type. Brett's is an integer, Pete's is a string, and Andrew's is a floating point number.

We can access the values in a map in two main ways, as shown below:

println scores["Pete"]
println scores.Pete

To modify or add data to a map, the syntax is similar to adding values to list.

scores["Pete"] = 3
scores["Cedric"] = 120

Learn more about maps at the following links:

Multiple assignment

An array or a list object can used to assign multiple variables at once. For example:

(a, b, c) = [10, 20, 'foo']
assert a == 10 && b == 20 && c == 'foo'

The three variables on the left of the assignment operator are initialized by the correspondent item in the list.

Read more about Multiple assignment on the Groovy documentation.

Conditional Execution

One of the most important features of any programming language is the ability to execute different code under different conditions. The simplest way to do this is to use the if construct. For example:

x = Math.random()
if( x < 0.5 ) {
    println "You lost."
else {
    println "You won!"


Strings can be defined by delimiting them with a single-quote ' or a double-quote ". Using either type of string allows you to use strings with quotations easily, as shown below:

println "he said 'cheese' once"
println 'he said "cheese!" again'

Strings may be concatenated with +. For example:

a = "world"
print "hello " + a + "\n"

String interpolation

There is an important difference between single ' and double " quoted strings. Double quoted strings supports variable interpolations, while single quoted strings do not.

In practice, strings that are declared inside double-quotes can contain arbitrary variables prefixing them with the $ character or any expressions by using the ${expression} syntax in a very similar way to Bash/shell scripts.

foxtype = 'quick'
foxcolor = ['b', 'r', 'o', 'w', 'n']
println "The $foxtype ${foxcolor.join()} fox"

x = 'Hello'
println '$x + $y'

It prints:

The quick brown fox
$x + $y

Multi-line strings

A block of text that span multiple lines can be defined by delimiting it with triple single or double quotes, as shown below:

text = """
    hello there James
    how are you today?


Like before, multi-line strings delimited by double-quotes characters supports variable interpolation, while single-quote string do not.

As in Bash/shell scripts, when terminating a multi-line text block with a \ character, the resulting string is not broken up by new line character(s):

myLongCmdline = """ blastp \
                -in $input_query \
                -out $output_file \
                -db $blast_database \

result = myLongCmdline.execute().text


In very few words a closure is a block of code that can be passed as an argument to a function. Thus you can define a chunk of code and then pass it around as if it were a string or an integer.

More formally, you can create functions that are defined first class objects.

square = { it * it }

The curly brackets around the expression it * it tells the script interpreter to treat this expression as code. In this case, the designator it refers to whatever value is given to the function. Then this compiled function is assigned to the variable 'square` much like those above. So now we can do something like this:

println square(9)

and get the value 81.

This is not very interesting until we find that we can pass this function square around as a method argument. There are some built in functions that take a function like this as an argument. One example is the collect method on lists. For example:

[ 1, 2, 3, 4 ].collect(square)

This expression says, create an array with the values 1,2,3 and 4, then call the collect method, passing in the closure we defined above. The collect method runs through each item in the array, calls the closure on the item, then puts the result in a new array, resulting in:

[ 1, 4, 9, 16 ]

For more methods that you can call with closures as arguments, see the Groovy GDK documentation.

By default closures take 1 parameter called it, you can also create closures with named parameters. For example the method Map.each() can take a closure with two variables, to which it binds the key and the associated value:

printMapClosure = { key, value ->
    println "$key = $value"

[ "Yue" : "Wu", "Mark" : "Williams", "sudha" : "Kumari" ].each(printMapClosure)



A closure has another two important features. First it can access variables in the scope where it is defined and so it can interact with them.

The second thing is that a closure can be defined in an anonymous manner, meaning that it is not given a name, and is defined in the place where it needs to be used.

As an example showing both these features see the following code fragment:

myMap = ["China": 1 , "India" : 2, "USA" : 3]

result = 0
myMap.keySet().each( { result+= myMap[it] } )

println result

Learn more about closures in the Groovy documentation

Regular expressions

Regular expressions are the Swiss Army knife of text processing. They provide the programmer with the ability to match and extract patterns from strings.

Regular expressions are supported by using the ~/pattern/ syntax and the =~ and the ==~ operators.

Use the =~ to check if there's any occurrence of a given pattern into a given string, thus:

assert 'foo' =~ /foo/       // return TRUE
assert 'foobar' =~ /foo/    // return TRUE

Use the ==~ to check whenever a string matches a given regular expression pattern.

assert 'foo' ==~ /foo/       // return TRUE
assert 'foobar' ==~ /foo/    // return FALSE

It is worth noting that the ~ operator creates a Java Pattern object from the given string, while the =~ creates a Java Matcher object.

x = ~/abc/
println x.class
// prints java.util.regex.Pattern

y = 'some string' =~ /abc/
println y.class
// prints java.util.regex.Matcher

Regular expression support is imported from Java. Java's regular expression language and API is documented in the Pattern Java documentation.

You may also be interested in this post Groovy: Don't Fear the RegExp.

String replacements

To replace pattern occurrences into a given string use the replaceFirst and replaceAll methods. For example:

x = "colour".replaceFirst(/ou/, "o")
println x
// prints: color

y = "cheesecheese".replaceAll(/cheese/,"nice")
println y
// prints: nicenice

Capturing groups

You can match a pattern that includes groups. First create a matcher object with the =~ operator. Then, you can index the matcher object to find the matches, matcher[0] returns a list representing the first match of the regular expression in the string. The first element is the string that matches the entire regular expression, and the remaining elements are the strings that match each group.

Here's how it works:

programVersion = '2.7.3-beta'
m = programVersion =~ /(\d+)\.(\d+)\.(\d+)-?(.+)/

assert m[0] ==  ['2.7.3-beta', '2', '7', '3', 'beta']
assert m[0][1] == '2'
assert m[0][2] == '7'
assert m[0][3] == '3'
assert m[0][4] == 'beta'

Applying some syntax sugar, you can do the same in just one line of code:

programVersion = '2.7.3-beta'
(full, major, minor, patch, flavor) = (programVersion =~ /(\d+)\.(\d+)\.(\d+)-?(.+)/)[0]

println full    // 2.7.3-beta
println major   // 2
println minor   // 7
println patch   // 3
println flavor  // beta

Removing part of a string

You can subtract a part of a String value using a regular expression pattern. The first match found is replaced with an empty String. For example:

// define the regexp pattern
wordStartsWithGr = ~/(?i)\s+Gr\w+/

// apply and verify the result
('Hello Groovy world!' - wordStartsWithGr) == 'Hello world!'
('Hi Grails users' - wordStartsWithGr) == 'Hi users'

Remove first match of a word with 5 characters:

assert ('Remove first match of 5 letter word' - ~/\b\w{5}\b/) == 'Remove  match of 5 letter word'

Remove first found numbers followed by a whitespace character:

assert ('Line contains 20 characters' - ~/\d+\s+/) == 'Line contains characters'

Files and I/O

In order to access and work with files, you need to use the file method which returns a file system object given a file path string. For example:

myFile = file('some/path/to/my_file.file')

The file method can reference either files or directories depending on what the string path is locating in the file system.

When using wildcard characters i.e. *, ?, [] and {} the argument is interpreted as a glob path matcher and the file method returns a list object holding the path of files whose name matches the specified pattern, or an empty list if no match is found. For example:

listOfFiles = file('some/path/*.fa')


Two asterisks, i.e. **, works like * but crosses directory boundaries. Also note that, by default wildcard characters do not match against hidden and directory files.

For example, if you want to include hidden files in the result list, add the optional parameter hidden as shown below:

listWithHidden = file('some/path/*.fa', hidden: true)

The list of available options is shown below:

Name Description
glob When true interprets characters *, ?, [] and {} as glob wildcards, otherwise handles them as normal characters (default: true)
type Type of paths returned, either file, dir or any (default: file)
hidden When true includes hidden files in the resulting paths (default: false)
maxDepth Maximum number of directory levels to visit (default: no limit)
followLinks When true it follows symbolic links during directories tree traversal, otherwise they are managed as files (default: true)


If you are a Java geek you will be interested to know that the file method returns a Path object, which allows you to use the usual methods as you would in a Java program.

See also: Channel.fromPath .

Basic read/write

Given a file variable, declared by using the file method as shown in the previous example, reading a file is as easy as getting the value of the file's text property, which returns the file content as a string value. For example:

print myFile.text

In the same way you can save a string value to a file by simply assigning it to the file's text property, as shown below:

myFile.text = 'Hello world!'


The actual file content is overwritten by the assignment operation. If the file does not exist, it is created implicitly by the assignment operation.

In order to append a string value to a file without erasing the actual content, you can use the append method:

myFile.append('Add this line\n')

or by using the left shift operator, which is just a more idiomatic way to append text content to a file:

myFile << 'Add a line more\n'

Binary data can managed in the same way, just using the file property bytes instead of text. Thus, the following example read the file and returns its content as a byte array:

binaryContent = myFile.bytes

Or you can save a byte array data buffer to a file, by simply writing:

myFile.bytes = binaryBuffer


The above methods read and write ALL the file content at once, in a single variable or buffer. For this reason they are not suggested when dealing with big files, which require a more memory efficient approach, for example reading a file line by line or by using a fixed size buffer.

Read a file line by line

In order to read a text file line by line you can use the method readLines() provided by the file object which returns the file content as a list of strings. For example:

myFile = file('some/my_file.txt')
allLines  = myFile.readLines()
for( line : allLines ) {
    println line

The same example can be written in a more idiomatic syntax, as shown below:

    .each { println it }


The method readLines() reads all the file content at once and returns a list containing all the lines. For this reason do not use it to read big files.

When you need to process a big file line by line, use the method eachLine which allows you to read a file processing each line one by one, thus avoiding the loading of all the file content in the memory. For example:

count = 0
myFile.eachLine {  str ->
        println "line ${count++}: $str"

Advanced file reading operations

The Reader and the InputStream classes allow you to gain fine control on read operations for text and binary files respectively.

The file method newReader creates a Reader object for the given file and allows you to read the content in single characters, lines or arrays of characters. For example:

myReader = myFile.newReader()
String line
while( line = myReader.readLine() ) {
    println line

The method withReader works in a similar manner but saves you from calling the method close when you have finished processing the file since it is managed automatically by the method itself. The same example can be rewritten as shown below:

myFile.withReader {
    String line
    while( line = myReader.readLine() ) {
        println line

The methods newInputStream and withInputStream work in a similar manner. The main difference is that they create an InputStream object that is useful to write binary data.

Table of the most important methods to read file content:

Name Description
getText Returns the file content as a string value
getBytes Returns the file content as byte array
readLines Reads the file line by line and returns the content as a list of strings
eachLine Iterates over the file line by line applying the specified closure
eachByte Iterates over the file by each single byte applying the specified closure
withReader Opens a file for reading and lets you access it with a Reader object
withInputStream Opens a file for reading and lets you access it with an InputStream object
newReader Returns a Reader object to read a text file
newInputStream Returns an InputStream object to read a binary file

Read the Java documentation for Reader and InputStream classes to learn more about the methods concerning these classes.

Advanced file writing operations

When you need to access low-level write operations to handle single bytes or characters, or if you are working with big files you will need to use the Writer and OutputStream classes which provide fine control on write operations.

For example, given two file objects sourceFile and targetFile, the following code snippet shows how to copy the file content from the first file into the second one replacing all the U characters with X:

sourceFile.withReader { source ->
    targetFile.withWriter { target ->
        String line
        while( line=source.readLine() ) {
            target << line.replaceAll('U','X')

Table of the most important methods to write data into a file:

Name Description
setText Saves a string value to a file
setBytes Saves a bytes array to a file
write Saves a string to a file truncating the actual content
append Appends a string value to a file without truncating the actual content
newWriter Creates a Writer object that allows you to save text data to a file
newPrintWriter Creates a PrintWriter object that allows you to write formatted text to a file
newOutputStream Creates an OutputStream object that allows you to write binary data to a file
withWriter Applies the specified closure to a Writer object, closing it when finished.
withPrintWriter Applies the specified closure to a PrintWriter object, closing it when finished.
withOutputStream Applies the specified closure to an OutputStream object, closing it when finished.

Read the Java documentation for the Writer, PrintWriter and OutputStream classes to learn more about methods available for writing data.

List directory content

Let's assume that you need to walk through a directory of your choice. You can define the myDir variable that points to it:

myDir = file('any/path')

The simplest way to get a directory list is by using the methods list or listFiles, that return a collection of first-level elements (files and directories) in a directory. For example:

allFiles = myDir.list()
for( def file : allFiles ) {
    println file


The only difference between list and listFiles is that the first returns a list of strings, while the latter a list of file objects, that allow you to access file dependent data e.g. size, last modified time, etc.

The eachFile method allows you to iterate through the first-level elements only (just like listFiles). As with other each- methods, they take a closure as an input parameter. For example:

myDir.eachFile { item ->
    if( item.isFile() ) {
        println "${item.getName()} - size: ${item.size()}"
    else if( item.isDirectory() ) {
        println "${item.getName()} - DIR"

Several variants of the above method are available. See the table below for a complete list.

Name Description
eachFile Iterates through first-level elements (files and directories). Read more
eachDir Iterates through first-level directories only. Read more
eachFileMatch Iterates through files and dirs whose name match the given filter. Read more
eachDirMatch Iterates through directories whose name match the given filter. Read more
eachFileRecurse Iterates through directory elements in a depth-first fashion. Read more
eachDirRecurse Iterates through directories in a depth-first fashion (regular files are ignored). Read more

See also: Channel fromPath method.

Create directories

Given a file variable representing a nonexistent directory, like the following:

myDir = file('any/path')

the method mkdir allows you to create a folder at the given path. It returns a true value if the folder is created successfully, of false otherwise.

result = myDir.mkdir()
println result ? "OK" : "Cannot create folder: $myDir"


If the parent directories do not exist, the above method will fail returning a false value.

The method mkdirs allows you to create the directory named by the file object, including any necessary but nonexistent parent directories. For example:


Copy files

The method copyTo allows you to copy a file into a new file or into a directory, or copy a directory to a new directory. Having a file variable myFile, the following example shows how to copy a file into a new file with a different name:



If the target file already exists, it will be replaced by the new one. Note also that if the target is a directory, the source file will be copied into that folder maintaining the original name.

When the source file is a directory, all the directory content is copied to the target folder. For example:

myDir = file('/some/path')

If the target path does not exist, it will be created automatically.


The copyTo method mimics the semantic of the Linux command cp -r <source> <target>, with the following caveats: Unix BASH distinguish between paths having or not having and ending slash, for example: /some/path/name and /some/path/name/. The first locates a regular file while the latter identifies a directory location. With Nextflow, due to Java files API implementation, this is not possibles and both strings represents the same path. If that path exists on the file systems it is handled accordingly (as a regular file or as a directory). If the path does not exist, it is supposed to locate a regular file (and any parent directory will be created automatically).

Move files

You can move a file by using the method moveTo as shown in the example below:

myFile = file('/some/path/file.txt')


When a file with the same name as the target already exists, it will be replaced by the new one. Note also that when the target specifies a folder name instead of a file, the source file is moved in that folder maintaining the original name.

When the source file is a directory, all the directory content is moved to the new the destination folder:

myDir = file('/any/dir_a')

Please note that the result of the above example depends on the existence of the destination folder. If the destination folder exists, the source is moved into the destination folder, thus the resulting path will be:


If the destination folder does not exist, the source is just renamed to the target name and so the resulting path of the above move operation would be:



The moveTo uses the same semantic as the Linux command mv <source> <target>. The same caveats as for the method copyTo are applied.

Rename files

You can rename a file or a directory by simply using the renameTo file method a shown below:

myFile = file('my_file.txt')

Delete files

The file method delete allows you to delete a file or a directory with a given path. It returns the value true when the operation is completed successfully or false if it fails to delete it. For example:

myFile = file('some/file.txt')
result = myFile.delete
println result ? "OK" : "Can delete: $myFile"


This method deletes a directory ONLY if it does not contain any file or sub-directory. In order to delete a directory and ALL its content, i.e. removing all the files and sub-directories it may contain, use the method deleteDir instead.

Check file attributes

The following methods can be used on a file variable created by using the file method.

Method Description
getName Gets the file name e.g. /some/path/file.txt -> file.txt
getBaseName Gets the file name e.g. /some/path/file.txt -> file
getExtension Gets the file extension e.g. /some/path/file.txt -> txt
getParent Gets the file parent path e.g. /some/path/file.txt -> /some/path
size Gets the file size in bytes
exists Returns true if the file exists, or false otherwise
isEmpty Returns true if the file is zero length or does not exist, false otherwise
isFile Returns true if it is a regular file e.g. not a directory
isDirectory Returns true if the file is a directory
isHidden Returns true if the file is hidden
lastModified Returns the file last modified timestamp i.e. a long as Linux epoch time

For example, the following line prints a file name and size:

println "File ${myFile.getName() size: ${myFile.size()}"

Get and modify file permissions

Given a file variable representing any file or a directory, the method getPermissions returns a string of nine characters representing the file permission using the Linux symbolic notation e.g. rw-rw-r--. For example:

permissions = myFile.getPermissions()

In the same way the method setPermissions allows you to set the file access permissions using the same string notation. For example:


A second version of the setPermissions method allows you to set file permissions specifying three digits, representing respectively the owner, group and other permissions. For example:


Learn more about File permissions numeric notation.

HTTP/FTP files

Nextflow provides a transparent integration for HTTP/S and FTP protocols that allows you to handle remote resources as local file system objects. Simply specify the resource URL as the argument of the file object. For example:

pdb = file('http://files.rcsb.org/header/5FID.pdb')

Then, you will be able to access it as a local file as described in the previous sections. For example:

println pdb.text

The above one-liner prints the content of the PDB file. See in the previous sections how to stream or copy the content of files.


Write and list operations are not supported by HTTP/S and FTP files.