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Every time you invoke an external binary in a shell script, the shell forks a new process—adding latency and consuming CPU/memory. Bash and other modern shells mitigate this overhead by providing built-in commands that execute inside the shell process. Leveraging built-ins can dramatically speed up your scripts and reduce resource usage. In this guide, we’ll cover:
  • Command categories and types
  • How to identify built-in commands
  • Performance benefits and benchmarks
  • Verifying process creation with strace
  • Listing all built-in commands and keywords
The image is a slide titled "Built-in" with a list of topics: command categories and types, how to identify built-in commands, and their performance benefits. It includes a checkmark icon next to each topic.

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The Chef Analogy

Think of your shell as a chef in a kitchen. If the chef delegated every simple task—chopping vegetables, stirring soup, plating food—to sous-chefs, the overhead would be enormous. Instead, the chef handles routine tasks directly and only calls for help on specialized jobs. Built-in commands work the same way: the shell “chef” handles them instantly, while external binaries require spawning a separate “assistant” process.

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Command Categories and Types

Shell commands fall into two main categories: Example:
In the first case, ls is an external binary loaded from disk; echo is handled directly by the shell.

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Identifying Built-ins vs. External Binaries

Use the type built-in to check how a command is implemented:
You can also use which or command -v, but type gives the most accurate distinction between built-in, keyword, and function.

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Performance Benefits of Built-ins

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Benchmarks: true vs /usr/bin/true

The true command simply returns a zero exit status. Compare its built-in version to the external binary:

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Command Execution Times at a Glance

Built-ins not only start faster but also complete quicker, especially when called repeatedly in loops.

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Verifying Process Creation with strace

To confirm built-ins don’t fork, trace execve system calls in your current shell:
  1. Find your shell’s PID
  2. Attach strace and filter for execve
  3. In another terminal, run a built-in vs an external binary
You’ll see execve only for cat, confirming echo runs inside the shell.

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External vs. Built-in Counterparts

Many built-ins have binary counterparts on disk:
Performance comparison:
Built-in echo completes instantly compared to the external /usr/bin/echo.

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Listing Built-ins and Keywords

  • List built-in commands:
  • List shell keywords:
  • Check if a word is a keyword:
Keywords (like time, if, for) are parsed by the shell and do not spawn new processes. Confusing them with external binaries can lead to unexpected behavior.
For a comprehensive list of shell built-ins and keywords, see the Bash manual:

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References

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