Why Stash and unstash for passing data in Jenkins? - Purpose & Use Cases

The Big Idea

What if you could magically pass files between Jenkins steps without lifting a finger?

The Scenario

Imagine you are building a software project with multiple steps in Jenkins. One step creates files, and the next step needs those files to continue. Without a way to share data, you have to manually copy files between steps or store them somewhere else.

The Problem

Manually copying files between steps is slow and easy to forget. It can cause errors if files are missing or outdated. Also, managing file locations and cleanup becomes a headache, making your pipeline fragile and hard to maintain.

The Solution

Using stash and unstash lets Jenkins automatically save files from one step and restore them in another. This keeps your pipeline clean and reliable, so you don't worry about where files are or if they are available.

Before vs After

✗ Before

sh 'cp build/output.zip ../next-step/'
sh 'ls ../next-step/output.zip'

✓ After

stash name: 'buildFiles', includes: 'build/output.zip'
unstash 'buildFiles'
sh 'ls build/output.zip'

What It Enables

It enables smooth and error-free sharing of files between different parts of your Jenkins pipeline.

Real Life Example

When building a mobile app, you compile the app in one step and then run tests in another. Stash and unstash let you pass the compiled app files easily without manual copying.

Key Takeaways

Manual file sharing between steps is slow and error-prone.

Stash and unstash automate saving and restoring files in Jenkins pipelines.

This makes pipelines cleaner, faster, and more reliable.

Practice

1. What is the main purpose of stash and unstash in Jenkins pipelines?

easy

A. To temporarily save and later restore files between pipeline stages

B. To permanently store files on the Jenkins master server

C. To delete files after a pipeline stage completes

D. To send notifications when a stage finishes

Solution

1. Step 1: Understand the role of stash

   The stash command saves files temporarily during a pipeline run.

2. Step 2: Understand the role of unstash

   The unstash command restores those saved files in later stages or nodes.

3. Final Answer:

   To temporarily save and later restore files between pipeline stages -> Option A

4. Quick Check:

   stash + unstash = temporary file sharing [OK]

Hint: Remember stash saves, unstash restores files between stages [OK]

Common Mistakes:

• Thinking stash permanently stores files
• Confusing stash with deleting files
• Assuming stash sends notifications

2. Which of the following is the correct syntax to stash files named output.log in a Jenkins pipeline?

easy

A. stash { name: 'logs', files: 'output.log' }

B. stash('logs', 'output.log')

C. stash name = 'logs' include = 'output.log'

D. stash name: 'logs', includes: 'output.log'

Solution

1. Step 1: Recall stash syntax

   The correct stash syntax uses named parameters: stash name: '...', includes: '...'.

2. Step 2: Match options to syntax

   stash name: 'logs', includes: 'output.log' matches the correct syntax exactly.

3. Final Answer:

   stash name: 'logs', includes: 'output.log' -> Option D

4. Quick Check:

   stash uses named params: name and includes [OK]

Hint: Use named parameters: name and includes for stash [OK]

Common Mistakes:

• Using parentheses like a function call
• Using incorrect parameter names like files
• Using assignment (=) instead of colon (:)

3. Given this Jenkins pipeline snippet:

stage('Build') {
  steps {
    sh 'echo Hello > message.txt'
    stash name: 'msg', includes: 'message.txt'
  }
}
stage('Test') {
  steps {
    unstash 'msg'
    sh 'cat message.txt'
  }
}

What will be the output of the cat message.txt command in the Test stage?

medium

A. Hello

B. No such file or directory

C. Empty output

D. Error: stash not found

Solution

1. Step 1: Understand stash saves message.txt

   The Build stage creates message.txt with content 'Hello' and stashes it as 'msg'.

2. Step 2: Unstash restores message.txt in Test stage

   The Test stage unstashes 'msg', restoring message.txt, then runs cat message.txt.

3. Final Answer:

   Hello -> Option A

4. Quick Check:

   unstash restores files, so cat shows 'Hello' [OK]

Hint: Unstash restores files exactly as stashed [OK]

Common Mistakes:

• Assuming unstash does not restore files
• Expecting an error because stages run separately
• Thinking stash deletes files

4. You wrote this Jenkins pipeline snippet:

stage('Build') {
  agent { label 'build' }
  steps {
    sh 'touch file.txt'
    stash name: 'data', includes: 'file.txt'
  }
}
stage('Deploy') {
  agent { label 'deploy' }
  steps {
    unstash 'artifacts'
    sh 'cat file.txt'
  }
}

But the Deploy stage fails with cat: file.txt: No such file or directory. What is the most likely cause?

medium

A. The unstash command is missing in the Deploy stage

B. The file.txt was not created before stashing

C. The stash name used in unstash does not match the stash name

D. The pipeline syntax for stash is incorrect

Solution

1. Step 1: Check stash and unstash names

   The stash is named 'data' but unstash uses 'artifacts', so names do not match.

2. Step 2: Verify file creation and stash syntax

   file.txt is created before stash, unstash command is present, and stash syntax looks correct. Different agents require proper stash/unstash to pass files.

3. Step 3: Identify the cause

   Mismatched names cause unstash to fail silently, so file.txt is not available in Deploy workspace.

4. Final Answer:

   The stash name used in unstash does not match the stash name -> Option C

5. Quick Check:

   stash and unstash names must match exactly [OK]

Hint: Check stash and unstash names match exactly [OK]

Common Mistakes:

• Using different names in stash and unstash
• Forgetting to create the file before stash
• Omitting unstash command in later stage

5. You want to pass multiple files from a Build stage to a Test stage in Jenkins. Which is the best way to stash and unstash these files?

hard

A. stash name: 'allFiles', includes: '*.log, *.txt'
...
unstash 'allFiles'

B. stash name: 'allFiles', includes: '**/*.log, **/*.txt'
...
unstash 'allFiles'

C. stash name: 'allFiles', includes: '**/*.log **/*.txt'
...
unstash 'allFiles'

D. stash name: 'allFiles', includes: '*.log *.txt'
...
unstash 'allFiles'

Solution

1. Step 1: Understand includes pattern syntax

   Jenkins stash supports comma-separated glob patterns with '**/' for recursive matching.

2. Step 2: Compare options

   stash name: 'allFiles', includes: '**/*.log, **/*.txt' ... unstash 'allFiles' uses comma-separated recursive globs, which is correct. Options A, B, and C either lack recursion (A, B) or use spaces instead of commas (B, C). For example, '**/*.log **/*.txt' misses comma separator.

3. Final Answer:

   stash name: 'allFiles', includes: '**/*.log, **/*.txt' ... unstash 'allFiles' -> Option B

4. Quick Check:

   Use comma-separated globs with '**/' for multiple file types [OK]

Hint: Use comma-separated globs with '**/' for multiple files [OK]

Common Mistakes:

• Using spaces instead of commas to separate globs
• Missing recursive '**/' causing files to be skipped
• Incorrect glob syntax causing no files to stash