# Run task only if build cache exists
**URL:** https://discuss.gradle.org/t/run-task-only-if-build-cache-exists/33407
**Category:** Help/Discuss
**Created:** [October 3, 2019, 1:56pm UTC](https://discuss.gradle.org/t/run-task-only-if-build-cache-exists/33407 "2019-10-03T13:56:32Z")
**Posts on this page:** 2
**Page:** 1
### Author:  [@gerhardol](https://discuss.gradle.org/u/gerhardol)
#### Post date: [October 3, 2019, 1:56pm UTC](https://discuss.gradle.org/t/run-task-only-if-build-cache-exists/33407/1 "2019-10-03T13:56:33Z")
I have a use case where a build takes up to 30 min. The build artifacts can be used to speed up next build when working in a GUI tool, so when a developer opens the tool to work with the product, it would be helpful if an existing build cache can be restored. If a build cache does not exist, a build should not be started.
This can kind of be handled by raising an exception when executing, but this gives an ugly error message.
I would like to check if a build cache exists for a task before executing the task or to exit execute gracefully without creating a cache.
Current draft below. The double invocation would not be needed if the first ‘deleteFiles’ task could evaluate if a cache exists for the genFiles task.
```
project.tasks.create('genFiles', GenerateFiles).configure {
description = "Generate files"
dependsOn('deleteFiles')
}
project.tasks.create('restoreCache', GenerateFiles).configure {
description = "Restore cache of generated files"
onlyIf { project.tasks.genFiles.getState().upToDate }
dependsOn('genFiles')
// TODO delete files if up to date, separate task needed?
//dependsOn('deleteFiles')
}
project.afterEvaluate {
project.tasks.create('deleteFiles') {
onlyIf { !ext.unpackCacheOnly.get() || project.tasks.genFiles.getState().upToDate }
doLast {
// Remove all output files/folders
// This is required to cache builds after output files are modified i.e. in a GUI generation (rebuild will not be cached otherwise)
TaskOutputs taskOutputs = project.tasks.genFiles.outputs
def str = ' Cleaned'
taskOutputs.getFiles().each { File f ->
str += ' ' + f
project.delete(f)
if (!f.parentFile.exists()) {
f.parentFile.mkdirs()
}
}
log.info str
}
}
```
}
//////////////////////////////////////////////////////////////////////////////////////
@TaskAction
void execute(IncrementalTaskInputs inputs) {
if (project.extensions.ext.unpackCacheOnly.get()){
// We need to throw to exit with error (to not cache)
throw new GradleException(‘No cache exists, exiting (ignore message about failure)’)
}
rest of build…
---
### Author:  [@gerhardol](https://discuss.gradle.org/u/gerhardol)
#### Post date: [October 10, 2019, 7:27am UTC](https://discuss.gradle.org/t/run-task-only-if-build-cache-exists/33407/2 "2019-10-10T07:27:14Z")
The solution I used
Users are confused about the failure, using ANSI escape to at least show the message clearly
```
def genCacheCheckTask = project.tasks.register('genCacheCheck', GenerateTask) {
group = Extension.name
description = "Try to restore cache to temp dir, to find if it exists"
unpackCacheOnly = true
testOutputDir = "tmp/tmpUnpackCache/"
}
def delGenCacheCheckTask = project.tasks.register('preGenCacheCheck', DeleteTask) {
deleteTask = genCacheCheckTask
}
genCacheCheckTask.configure {
dependsOn(delGenCacheCheckTask)
}
def genCacheTask = project.tasks.register('genCache', GenerateTask) {
group = Extension.name
description = "Extract existing cache"
dependsOn { genCacheCheckTask }
unpackCacheOnly = true
}
def delGenCacheTask = project.tasks.register('preGenCache', DeleteTask) {
deleteTask = genCacheTask
}
genCacheTask.configure {
dependsOn(delGenCacheTask)
}
/**
* Color output using ANSI escape
*/
def color = { color, text ->
def colors = [black: 30, red: 31, green: 32, yellow: 33, blue: 34, magenta: 35, cyan: 36, white: 37]
return new String((char) 27) + "[${colors[color]}m${text}" + new String((char) 27) + "[0m"
}
/*
* The action performed when at least one file has been updated
*/
@TaskAction
void execute(IncrementalTaskInputs inputs) {
if (unpackCacheOnly){
log.warn("${color 'green', 'No cache exists, exiting (ignore message about failure)'}")
// We need to throw to exit with error (to not cache)
throw new GradleException('No cache exists, exiting (ignore message about failure)')
}
```