Data file with Newman in Postman - Build an Automation Script

Run Postman collection with data file using Newman

Preconditions (3)

Step 1: Open terminal or command prompt

Step 2: Run Newman command with the collection file and data file using --iteration-data option

Step 3: Wait for Newman to execute all iterations

Step 4: Observe the test run results for each data set

✅ Expected Result: Newman runs the collection once for each data set in the data file and shows pass/fail results for each iteration

Automation Requirements - Newman CLI

Assertions Needed:

Verify Newman runs all iterations equal to the number of data rows

Verify each iteration passes all tests in the collection

Best Practices:

Use --iteration-data option to specify data file

Use JSON or CSV format for data file

Check exit code of Newman command for success

Use descriptive names for data sets in the file

Automated Solution

Postman

const { exec } = require('child_process');

const collectionFile = 'path/to/collection.json';
const dataFile = 'path/to/data.json';

exec(`newman run ${collectionFile} --iteration-data ${dataFile} --reporters cli,json --reporter-json-export newman-report.json`, (error, stdout, stderr) => {
  if (error) {
    console.error(`Error executing Newman: ${error.message}`);
    process.exit(1);
  }
  if (stderr) {
    console.error(`Newman stderr: ${stderr}`);
  }
  console.log(`Newman output:\n${stdout}`);

  // Simple check for iterations run
  const iterationsMatch = stdout.match(/iterations:\s*(\d+)/i);
  if (iterationsMatch) {
    const iterations = parseInt(iterationsMatch[1], 10);
    console.log(`Iterations run: ${iterations}`);
  } else {
    console.warn('Could not find iterations count in output');
  }

  // Check for failures
  const failuresMatch = stdout.match(/failed:\s*(\d+)/i);
  if (failuresMatch) {
    const failures = parseInt(failuresMatch[1], 10);
    if (failures === 0) {
      console.log('All tests passed for all iterations.');
    } else {
      console.error(`There are ${failures} test failures.`);
      process.exit(1);
    }
  } else {
    console.warn('Could not find failures count in output');
  }
});

This Node.js script runs Newman CLI using child_process.exec to execute the Postman collection with the data file.

The --iteration-data option tells Newman to run the collection once per data row.

The script captures the output and parses it to find how many iterations ran and how many tests failed.

If there are any failures, the script exits with error code 1 to indicate test failure.

This approach automates running the collection with multiple data inputs and verifies the results programmatically.

Common Mistakes - 4 Pitfalls

Not using --iteration-data option and running collection only once

Using incorrect path or filename for collection or data file

Ignoring Newman exit code and output

Using data file with wrong format or invalid JSON/CSV

Bonus Challenge

Now add data-driven testing with 3 different input data sets in the data file and verify all iterations pass

Show Hint

Practice

1. What is the main purpose of using a data file with Newman when running Postman collections?

easy

A. To run the same tests multiple times with different input values

B. To speed up the execution of tests by caching responses

C. To generate reports automatically after test execution

D. To connect Newman to a database for live data

Solution

1. Step 1: Understand the role of data files in Newman

   Data files allow running the same test collection multiple times with different sets of input data.

2. Step 2: Identify the correct purpose

   This helps test how the API behaves with various inputs without rewriting tests.

3. Final Answer:

   To run the same tests multiple times with different input values -> Option A

4. Quick Check:

   Data files = multiple inputs for same tests [OK]

Hint: Data files = repeat tests with different inputs [OK]

Common Mistakes:

• Thinking data files speed up tests
• Confusing data files with report generation
• Assuming data files connect to databases

2. Which command correctly runs a Postman collection named api_tests.postman_collection.json with a data file named test_data.csv using Newman?

easy

A. newman run api_tests.postman_collection.json -d test_data.csv

B. newman run api_tests.postman_collection.json --data test_data.csv

C. newman execute api_tests.postman_collection.json -data test_data.csv

D. newman run api_tests.postman_collection.json -datafile test_data.csv

Solution

1. Step 1: Recall Newman syntax for data files

   Newman uses the -d option to specify a data file.

2. Step 2: Match the correct command

   newman run api_tests.postman_collection.json -d test_data.csv uses -d correctly with newman run and the file names.

3. Final Answer:

   newman run api_tests.postman_collection.json -d test_data.csv -> Option A

4. Quick Check:

   Use -d for data file in Newman [OK]

Hint: Use '-d' option to add data file in Newman command [OK]

Common Mistakes:

• Using '--data' instead of '-d'
• Using 'execute' instead of 'run'
• Typing '-datafile' which is invalid

3. Given a CSV data file users.csv with two rows:
username,password
alice,pass123
bob,pass456
and a Postman test collection that uses {{username}} and {{password}} variables, what will be the number of iterations Newman runs when executing with -d users.csv?

medium

A. 1 iteration

B. 0 iterations

C. 3 iterations

D. 2 iterations

Solution

1. Step 1: Count data rows excluding header

   The CSV has 3 lines: 1 header + 2 data rows (alice and bob).

2. Step 2: Understand iteration count in Newman

   Newman runs one iteration per data row, so 2 iterations for 2 users.

3. Final Answer:

   2 iterations -> Option D

4. Quick Check:

   Iterations = data rows (excluding header) [OK]

Hint: Iterations = number of data rows in file [OK]

Common Mistakes:

• Counting header row as iteration
• Assuming only 1 iteration always
• Confusing iterations with number of variables

4. You run Newman with -d data.json but get an error saying the data file format is invalid. Which of the following is the most likely cause?

medium

A. The CSV file was renamed to .json but still contains CSV data

B. The JSON file contains an array of objects but is missing a root array

C. The JSON file uses single quotes instead of double quotes for keys and strings

D. The data file is empty

Solution

1. Step 1: Check JSON format requirements

   JSON requires double quotes for keys and string values; single quotes cause parsing errors.

2. Step 2: Identify common JSON errors

   Using single quotes is a common mistake causing invalid format errors.

3. Final Answer:

   The JSON file uses single quotes instead of double quotes for keys and strings -> Option C

4. Quick Check:

   JSON needs double quotes, not single [OK]

Hint: JSON keys and strings must use double quotes [OK]

Common Mistakes:

• Ignoring JSON syntax rules
• Confusing CSV and JSON file contents
• Not checking for empty files

5. You want to run a Postman collection with Newman using a JSON data file that contains user details. The JSON file has this structure:

[
  {"username": "alice", "password": "pass123"},
  {"username": "bob", "password": "pass456"},
  {"username": "", "password": "pass789"}
]

What will happen during the third iteration when username is an empty string?

hard

A. Newman will throw a syntax error and stop execution

B. The test will run with an empty username, possibly causing failures if username is required

C. Newman will skip the third iteration automatically because username is empty

D. The third iteration will use the username from the previous iteration

Solution

1. Step 1: Understand how Newman handles empty values in data files

   Newman runs all iterations regardless of empty values; it does not skip or throw errors automatically.

2. Step 2: Consider test behavior with empty username

   If the test expects a username, an empty string may cause test failures or unexpected results.

3. Final Answer:

   The test will run with an empty username, possibly causing failures if username is required -> Option B

4. Quick Check:

   Empty data values run as-is, may cause test failures [OK]

Hint: Empty values run as-is; tests may fail if required [OK]

Common Mistakes:

• Assuming Newman skips empty data rows
• Expecting automatic errors for empty strings
• Thinking data repeats from previous iteration