Script Tutorial

Roman Kuzmin edited this page Nov 24, 2017 · 56 revisions

Hello World example

task Hello {
    'Hello, World!'

If you name your script like *.build.ps1, e.g., then it can be run by a simple command:

# Run from the directory of

If no task name is passed, Invoke-Build tries to run the default task, named ".". If that is not defined, it runs the first task found, which is Hello in the example above.

Build output looks like:

Build Hello ...\
Task /Hello
Hello, World!
Done /Hello 00:00:00.01
Build succeeded. 1 tasks, 0 errors, 0 warnings 00:00:00.02

A more realistic example

This script defines three tasks:

  • "Build" builds a C# project
  • "Clean" removes temporary data
  • "." invokes Build and Clean
use 4.0 MSBuild

# Synopsis: Build the project.
task Build {
    exec { MSBuild Project.csproj /t:Build /p:Configuration=Release }

# Synopsis: Remove temp files.
task Clean {
    Remove-Item bin, obj -Recurse -Force -ErrorAction 0

# Synopsis: Build and clean.
task . Build, Clean

Invoke-Build without parameters runs the default task, ".", which builds the project and then cleans up.

Points of interest:

  • use (alias of Use-BuildAlias) lets you directly call external tools, like MSBuild in this case
  • exec (alias of Invoke-BuildExec) runs a command (usually .exe) and stops with error on unsuccessful exit code
  • Tasks Build and Clean each consist of one script job (their "bodies"), and do not reference other tasks
  • The task "." references two other tasks and does not have its own script job (no "body")
  • The task "." is invoked by default
  • Help comments # Synopsis: ...

You can pass multiple tasks to Invoke-Build, for example:

# This explicitly runs both tasks, "Clean" and "Build"
Invoke-Build Clean, Build

Help comments are shown by the special, built-in task named "?":

PS> Invoke-Build ?

Name  Jobs         Synopsis
----  ----         --------
Build {}           Build the project.
Clean {}           Remove temp files.
.     Build, Clean Build and clean.

Jobs are references and actions

Task jobs are defined by the parameter Jobs with its name often omitted. Jobs are references to other tasks and own script block actions. Jobs are invoked in the specified order.

Unlike in many other task runners, task references do not have to precede own actions. A task may reference tasks that it depends on and also tasks to be invoked after own actions, i.e. continuation tasks.

Examples of tasks with various jobs:

# Dummy task with no jobs
task Task1

# Alias of another task
task Task2 Task1

# Combination of tasks
task Task3 Task1, Task2

# Simple action task
task Task4 {
    # action

# Typical complex task: referenced task(s) and one own action
task Task5 Task1, Task2, {
    # action after referenced tasks

# Possible complex task: actions and tasks in any required order
task Task6 {
    # action before Task1
Task1, {
    # action after Task1 and before Task2
Task2, ...

Script parameters for tasks

Build script parameters are standard PowerShell script parameters. They are available to the script code for reading and writing as $ParamName, and to all tasks for reading as $ParamName and for writing as $script:ParamName.

Note: script parameters are usual script variables (see later). The only difference is that their values can be specified upon invoking a script.

In the previous example the task Build builds the Release configuration. This slightly modified script makes this configurable:

    $Configuration = 'Release'

use 4.0 MSBuild

task Build {
    exec { MSBuild Project.csproj /t:Build /p:Configuration=$Configuration }

The command Invoke-Build Build still builds the Release configuration due to the script parameter's default value, but it is now possible to specify and build Debug as well:

Invoke-Build Build -Configuration Debug

Yes, it's really that simple. Script parameters are specified to Invoke-Build, thanks to its dynamic parameters propagated from the build script.

Note that build scripts cannot use parameters reserved for the engine: Task, File, Result, Safe, Summary, WhatIf.

Script variables for tasks

Build script variables are standard PowerShell variables in the script scope. Variables are available to the script code for reading and writing as $VarName, and to all tasks for reading as $VarName and for writing as $script:VarName.

Example: The variable $Tests is defined in the script scope and available to all tasks, including tasks defined before the variable. This is because tasks are invoked after the whole script is evaluated:

$Tests = '', ''

task Test {
    foreach($_ in $Tests) {
        Invoke-Build * $_

Note: The special task * used in this example invokes all tasks starting from roots of task trees. It is normally used on testing with tests defined as build tasks. Tests are often invoked all together, hence the special task *.

Nested scripts invoked by Invoke-Build have their own script scopes with their own parameters and variables. Parent scripts' variables can be accessed normally, for reading only. For example, the variable $Tests is available in the parent scope for the nested scripts and

Tasks may use existing script variables, or may create new ones. New script variables are typically created for use in other tasks.

Example: The task Version gets the file version and stores it in a script variable with $script:Version = .... Tasks Zip and NuGet require this task before their own scripts. Thus, these tasks can reference the script variable $Version and use it for package names:

task Version {
    $script:Version = (Get-Item Project.dll).VersionInfo.FileVersion

task Zip Version, {
    exec { & 7za a Project.$ Project.dll }

task NuGet Version, {
    exec { NuGet pack Project.nuspec -Version $Version }

task PackAll Zip, NuGet

Tasks are invoked once

This is the main rule of build flows. A task can be referenced by other tasks many times. But as soon as it is invoked, its contribution to a build is over.

When the task PackAll is invoked in the previous example then Version is referenced twice by the tasks scheduled for the build, at first by Zip and then by NuGet. But in fact it is invoked only when Zip calls it.

To invoke something several times, use functions, see #79.

Build "properties"

Invoke-Build "properties" are usual PowerShell script variables and parameters, like MSBuild properties defined in XML scripts (variables) and properties that come from command lines (parameters), or environment variables.

MSBuild deals with environment variables using the same syntax. In contrast, Invoke-Build scripts do not use environment variables in the same way. They should be referenced as $env:var or obtained using property. The latter gets the value of session or environment variable or the optional default.

Example: $DevModuleDir or $Configuration may come to the script below in three ways: as script parameters, as variables defined in a parent scope, and as environment variables. (property DevModuleDir) throws an error if the property is not found. But (property Configuration Release) does not fail, it uses the default value Release.

    $DevModuleDir = (property DevModuleDir),
    $Configuration = (property Configuration Release)

task Install {
    Copy-Item Bin/$Configuration/MyModule.dll $DevModuleDir/MyModule


Build properties should be used sparingly with carefully chosen names which unlikely can exist unintentionally. Avoid them in Persistent Builds because properties rely on external data.

Conditional tasks

The task parameter If specifies a condition, either a value evaluated on creation or a script block evaluated on invocation. If it is present and evaluated to false then the task is not invoked.

In the following example the task MakeHelp is invoked only if the current configuration is Release:

    $Configuration = 'Release'

task MakeHelp -If ($Configuration -eq 'Release') {

task Build {

Note that the task MakeHelp is still defined even if its condition is not true. Thus, other tasks may refer to it, like the task Build does.

If the variable $Configuration is supposed to change during the build and the task depends on its current value then a script block should be used instead:

task MakeHelp -If {$Configuration -eq 'Release'} {

If a condition is a script block and a task is called more than once then it is possible that it is skipped at first due to its condition evaluated to false but still invoked later when its condition gets true.

Safe task references and errors

If a task throws an exception or writes a terminating error then the whole build fails unless the task is referenced as safe ?TaskName by the calling task and other tasks having a chance to be invoked. The same convention works for task names in command lines.

The helper error (alias of Get-BuildError) is used in order to get errors of tasks called safe. Note that error is useless with unsafe references because the whole build stops on failures.

In this example Task2 calls Task1 safe and then checks for its error:

task Task1 {
    # code with potential failures

task Task2 ?Task1, {
    if (error Task1) {
        # Task1 failed
    else {
        # Task1 succeeded

Task jobs altered by other tasks

The task parameters After and Before are used in order to alter build task jobs in special cases, for example if a build script is imported (dot-sourced) and its direct changes are not suitable.

task Build {
    # original Build code

task BeforeBuild -Before Build {
    # when Build is called this code is invoked before it

task AfterBuild -After Build {
    # when Build is called this code is invoked after it

When the build engine preprocesses tasks the task Build is transformed into a task which is equivalent to this:

task Build BeforeBuild, <original task jobs>, AfterBuild

Incremental Tasks

Other task parameters Inputs, Outputs, and the switch Partial are used in order to define incremental and partial incremental tasks. These techniques are described in here:

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