Skip to content

bash_vs_sh_explained.md

Latest commit

 

History

History
121 lines (82 loc) · 3.2 KB

bash_vs_sh_explained.md

File metadata and controls

121 lines (82 loc) · 3.2 KB
Raw

Difference Between bash and sh --- And Why Lightweight Images Include Only sh

1. What Is sh?

sh is the original Unix Bourne shell, designed to be small and simple.

Features:

  • Very small binary size
  • Minimal built‑in functions
  • No arrays
  • No advanced string operations
  • No [[ ]] syntax
  • Basic scripting only

In Containers:

  • BusyBox /bin/sh is extremely small (sometimes <200 KB)
  • Alpine's /bin/sh is actually ash, also lightweight

2. What Is bash?

bash (Bourne Again Shell) is a modern, feature‑rich shell used by most Linux distros.

Supports:

  • Arrays
  • Associative arrays
  • [[ … ]] conditional tests
  • Brace expansions ({1..5})
  • Command history
  • Tab completion
  • Rich scripting syntax

Larger size:

  • Usually ~1--1.5 MB binary
  • More dependencies

3. Key Differences (Quick Table)

Feature sh bash

Binary size Very small Larger Included in minimal images ✔ Yes ❌ No Arrays ❌ No ✔ Yes [[ … ]] test ❌ No ✔ Yes Brace expansion {1..5} ❌ No ✔ Yes Associative arrays ❌ No ✔ Yes Interactive features Minimal Rich Scripting complexity Basic Advanced

4. Why Lightweight Images Do NOT Include Bash

Minimal container images (BusyBox, Alpine, Distroless) are built for:

  • Small size
  • Fast startup
  • Low attack surface
  • Minimal dependencies

Including bash would: - Increase image size - Add unnecessary features - Increase potential vulnerabilities

Typical sizes:

Image Shell Size

BusyBox sh ~1--2 MB Alpine ash (sh) ~5 MB Ubuntu bash ~70 MB Debian bash ~120 MB

This is why cloud‑native best practices recommend using minimal base images.

5. Why Full Linux Images Include Bash

Full distributions (Ubuntu, CentOS, Debian) are designed for:

  • Human users
  • System management
  • Compatibility with existing scripts

So they include bash by default.

These images prioritize completeness, not size.

6. Why Kubernetes Usually Uses sh -c

Since minimal images do not provide bash, commands must be run through sh:

command: ["sh", "-c", "echo hello && sleep 1d"]

Using bash in Alpine/BusyBox will fail:

bash: not found

Summary

  • sh = minimal, lightweight, always available\
  • bash = powerful, large, only in full distros\
  • Lightweight images avoid bash to stay small and secure\
  • Kubernetes commands often use sh -c for portability