A continuous build tool watches your code repository. Every time someone pushes a change, it automatically builds the code, runs the tests, and tells you whether the change is safe to merge. If the build breaks or a test fails, the team finds out in minutes instead of days.
That's continuous integration (CI). Continuous delivery (CD) extends it: after the build and tests pass, the tool automatically deploys the code to a staging or production environment. Together, CI/CD turns "push code, wait for someone to manually test it, then manually deploy it" into "push code, everything else happens automatically."
Martin Fowler's original definition from his 2006 essay described CI as a practice where developers integrate code frequently, with each integration verified by an automated build. The tools below implement that practice.
The 7 tools
Jenkins
Type: Self-hosted, open-source Price: Free (you pay for the servers it runs on) Language: Java
Jenkins is the oldest and most widely used CI/CD tool. According to the 2024 Stack Overflow Developer Survey, Jenkins remains among the top CI tools despite its age. It runs on your own servers (or your own cloud VMs), and you configure it using pipelines defined in a Jenkinsfile.
Strengths: 1,800+ plugins for nearly any integration. Complete control over the build environment. No vendor lock-in. If you can install it on a server, you can run it.
Weaknesses: Requires maintenance. You manage the servers, updates, scaling, and security. The UI feels dated. Plugin compatibility issues are common after upgrades. Initial setup takes longer than any hosted alternative.
Best for: Large organizations that need full control over their build infrastructure, have dedicated DevOps staff, and run complex pipelines with custom requirements.
GitHub Actions
Type: Cloud-hosted (with self-hosted runner option) Price: Free for public repos. Private repos get 2,000 minutes/month free, then $0.008/minute (Linux) Integrated with: GitHub
GitHub Actions is built into GitHub. Workflows are defined in YAML files inside your repository (.github/workflows/). If your code is on GitHub, there's no separate tool to set up. You just add a YAML file and push.
Strengths: Zero setup for GitHub users. 20,000+ community actions in the marketplace. Matrix builds (test across multiple OS/language versions in parallel) are built in. The free tier is generous for small teams.
Weaknesses: Tied to GitHub. If you move your code to GitLab or Bitbucket, your pipelines don't come with you. Debugging failed workflows requires reading logs in a web UI (no SSH into build runners on the hosted plan). Minutes-based pricing can get expensive for long-running builds.
Best for: Teams already on GitHub who want CI/CD without managing infrastructure. Startups, open-source projects, and small-to-medium teams.
GitLab CI/CD
Type: Cloud-hosted or self-hosted Price: Free tier includes 400 CI/CD minutes/month. Paid plans start at $29/user/month Integrated with: GitLab
GitLab CI is built into GitLab, just as GitHub Actions is built into GitHub. Pipelines are defined in a .gitlab-ci.yml file. GitLab's CI is tightly integrated with its issue tracker, merge requests, container registry, and security scanning.
Strengths: Everything in one platform (code, CI/CD, issues, security, monitoring). Auto DevOps feature generates a pipeline automatically for common project types. Self-hosted option gives full control. Built-in container registry and package management.
Weaknesses: The UI has a steep learning curve. The free tier's 400 minutes is tight for active projects. Self-hosted GitLab demands heavy resources (CPU, RAM, storage).
Best for: Teams that want an all-in-one platform. Organizations that need self-hosted CI/CD with a modern interface. Companies with strict data residency requirements.
CircleCI
Type: Cloud-hosted (with self-hosted runner option) Price: Free tier includes 6,000 build minutes/month. Paid plans start at $15/month Integrated with: GitHub, Bitbucket, GitLab
CircleCI is a dedicated CI/CD platform that connects to your code repository (it's not tied to a single provider). Pipelines are defined in a .circleci/config.yml file.
Strengths: Fast build times (Docker layer caching, parallelism, resource classes). Generous free tier (6,000 minutes). Connects to multiple code hosts. SSH access to build runners for debugging. Orbs (reusable pipeline packages) reduce configuration duplication.
Weaknesses: Separate tool to manage (another dashboard, another set of credentials). The configuration syntax is unique to CircleCI, so switching away means rewriting pipelines. Pricing can escalate with heavy usage.
Best for: Teams that need fast builds and advanced caching. Projects spread across multiple code hosts. Teams that value build performance over all-in-one integration.
Bitbucket Pipelines
Type: Cloud-hosted Price: Free tier includes 50 build minutes/month. Paid plans start at $15/user/month Integrated with: Bitbucket (Atlassian)
Bitbucket Pipelines is Atlassian's CI/CD offering, built into Bitbucket. If your team uses Jira and Confluence, Bitbucket Pipelines fits naturally into the Atlassian ecosystem.
Strengths: Tight Jira integration (deployments linked to issues, build status on PRs). Simple YAML configuration. Deployment tracking built in.
Weaknesses: The free tier is very limited (50 minutes). Only works with Bitbucket. Fewer community integrations than GitHub Actions or Jenkins. Build performance is slower than CircleCI for complex pipelines.
Best for: Teams already in the Atlassian ecosystem (Jira + Confluence + Bitbucket).
Azure DevOps Pipelines
Type: Cloud-hosted or self-hosted Price: Free for public projects. Private projects get 1 free parallel job with 1,800 minutes/month. Additional jobs start at $40/month Integrated with: Azure, GitHub
Azure DevOps Pipelines is Microsoft's CI/CD platform. It connects to Azure repos, GitHub, or Bitbucket. The platform includes not just CI/CD but also boards (project management), test plans, and artifact management.
Strengths: Deep Azure cloud integration. Supports Windows, Linux, and macOS build agents natively. YAML and visual editor options. Good enterprise security and compliance features.
Weaknesses: The UI is complex and can feel overwhelming. Pipeline YAML syntax has a learning curve. Stronger for .NET/Windows workloads than for other stacks.
Best for: Enterprise teams on Microsoft Azure. Organizations with .NET/Windows workloads. Teams that need integrated project management and CI/CD.
Travis CI
Type: Cloud-hosted Price: Free for open-source projects. Paid plans start at $69/month Integrated with: GitHub, Bitbucket, GitLab
Travis CI was one of the first cloud CI services and was the default choice for open-source projects for years. It uses a .travis.yml configuration file.
Strengths: Simple configuration for standard projects. Good documentation. Multiple language support out of the box.
Weaknesses: Pricing increased after its 2019 acquisition (previously free for open-source with more generous limits). Many open-source projects migrated to GitHub Actions. Build speeds are slower than CircleCI. The community around Travis has shrunk.
Best for: Legacy projects already using Travis CI. Teams with simple build requirements that don't need advanced features.
Comparison table
How to pick one
Most teams overthink this decision. Here's a simple framework.
Where is your code? On GitHub → GitHub Actions. On GitLab → GitLab CI. On Bitbucket → Bitbucket Pipelines. Using the CI/CD tool integrated with your code host is the path of least resistance. No separate accounts, no webhook configuration, no access token management.
Do you need self-hosted? If compliance, data residency, or security policies require builds to run on your own servers → Jenkins or self-hosted GitLab CI. If you're fine with cloud-hosted builds → any of the others.
What's your budget? $0 and small team → GitHub Actions free tier (2,000 min) or CircleCI free tier (6,000 min). Medium budget → any cloud option. Large budget with dedicated DevOps staff → Jenkins (free tool, you pay for infrastructure and people).
Do you need advanced features? Complex pipelines with custom Docker images, matrix builds, manual approval gates, multi-environment deployments → Jenkins, GitLab CI, or CircleCI. Simple "build and test on every push" → GitHub Actions or Bitbucket Pipelines.
For mobile CI/CD pipelines specifically, the choice often comes down to whether you need macOS runners (for iOS builds). GitHub Actions, CircleCI, and Azure DevOps all offer macOS runners, but at higher per-minute pricing than Linux runners. If your team follows a shift-left testing approach, the CI tool should support running your full test automation suite on every pull request.
What a CI/CD pipeline actually does
If you're new to this, here's what happens when a developer pushes code and a continuous build tool picks it up:
- Trigger. The developer pushes a commit or opens a pull request. The CI tool detects the change (via webhook or polling).
- Build. The tool spins up a clean environment (a container or VM), clones the code, installs dependencies, and compiles the project (if applicable).
- Test. The tool runs the test suite: unit tests, integration tests, and any other automated checks. If a test fails, the pipeline stops and notifies the team.
- Report. The build status (pass/fail) appears on the pull request, in a Slack channel, or in an email. Developers know within minutes whether their change is safe.
- Deploy (CD). If all tests pass and the pipeline includes deployment stages, the tool deploys to staging or production automatically. Some teams add a manual approval gate before production deployment.
The key principle: no manual steps between "developer pushes code" and "team knows if the code is safe." The build tool automates everything in between. That's what "continuous" means. Every change gets built and tested, continuously.
FAQ
What is a continuous build tool?
A continuous build tool (also called a CI/CD tool) automatically builds your code, runs tests, and optionally deploys the application every time someone pushes a change. It catches bugs within minutes of introduction instead of days or weeks later.
Is Jenkins still the best CI/CD tool?
Jenkins remains the most flexible and widely adopted CI/CD tool, but it requires self-hosting and ongoing maintenance. For teams that want zero infrastructure management, cloud-hosted options like GitHub Actions, GitLab CI, or CircleCI are often a better fit. The "best" tool depends on your team's size, budget, and infrastructure preferences.
What is the difference between CI and CD?
CI (continuous integration) automatically builds and tests code on every change. CD (continuous delivery) extends CI by automatically deploying the tested code to staging or production. CI catches bugs. CD ships the fix. Most modern tools do both.
Which CI/CD tool is free?
Jenkins is completely free and open-source (you pay for servers). GitHub Actions offers 2,000 free minutes/month for private repos and unlimited for public repos. CircleCI offers 6,000 free minutes/month. GitLab CI offers 400 free minutes/month.
Can I switch CI/CD tools later?
Yes, but it requires rewriting pipeline configuration files. Jenkins uses Jenkinsfile, GitHub Actions uses YAML in .github/workflows/, GitLab uses .gitlab-ci.yml, CircleCI uses .circleci/config.yml. The concepts transfer (stages, jobs, triggers), but the syntax is different. Choose wisely upfront to avoid rework.
Do I need a CI/CD tool for a small project?
If you're a solo developer on a side project, you can deploy manually. But even for small projects, a basic CI pipeline (build + test on every push) catches bugs before they reach production. GitHub Actions' free tier makes this effectively zero-cost for GitHub-hosted projects.
