The Bash tool runs shell commands. It's the most capable and most-permissioned tool in Claude Code, which is why every session has at least one rule about it.

What it does

Bash runs a command in a persistent shell whose working directory sticks within the project. Output comes back as combined stdout+stderr, truncated if huge. You can run it in the background for long processes, pipe, use heredocs, and chain commands. Permission rules gate which commands are allowed.

When to use it

Running tests, builds, linters, and formatters.
Git operations, file system changes, and installs.
Anything that's faster as a one-liner than a tool sequence.
Invoking project-specific CLIs.

Gotchas

Interactive prompts hang the tool. Pass flags or pipe input instead of waiting for TTY.
Shell state (exported vars, cd in subshells) doesn't persist beyond a single call unless you chain with &&.
Newlines are blocked in command strings on some setups. Use quoted strings or here-docs for multi-line payloads.

Official docs: https://code.claude.com/docs/en/tools-reference.md#bash-tool-behavior