ShellCheck

ShellCheck finds bugs in your shell scripts before they bite you in production. It's a linter that reads your shell scripts and tells you about quoting errors, unsafe variable expansions, and portability issues. The kind of bugs that work fine on your machine and explode on the server. GPL v3. Written in Haskell but you don't need to know or care about that; it's a standalone binary. Integrates with every major editor (VS Code, Vim, Emacs), runs in CI, and has a web version at shellcheck.net for quick checks.

Fully free. No paid tier, no premium rules, no hosted service beyond the free web checker. Every rule, every check, every integration, $0.

Every team that writes shell scripts should use this. Solo or enterprise, the answer is the same: install it, add it to CI, stop shipping broken bash. Takes 5 minutes to set up.

The catch: ShellCheck only handles sh/bash/dash/ksh. If you're writing Zsh scripts, coverage is limited. And it catches syntax and common pitfalls but won't validate your script's logic; it'll tell you your variable is unquoted, not that your deployment script deletes the wrong directory. Also, GPL v3 means you can't embed it in proprietary tools without licensing implications.