Shebang

Shebang

The #! line picks a script's interpreter. Depth, mechanism, and citations in [reference/shebang.md](../../reference/shebang.md); runnable pairs in [examples/shebang/](../../examples/shebang/).

Rules at a glance

| Rule | Good | Bad | Caught by | |------|------|-----|-----------| | Prefer env for portable version | #!/usr/bin/env bash | #!/bin/bash | judgment | | Pass flags with env -S | #!/usr/bin/env -S bash -euo pipefail | #!/usr/bin/env bash -euo pipefail | SC2096 | | Absolute interpreter path | #!/usr/bin/env python3 | #!bin/python3 | SC2239 | | #! first, LF endings, no BOM | #!… on line 1 | #…, CRLF, blank line | SC1113/SC1017/SC1128 | | Executable scripts need a shebang | present | missing | SC2148 | | Keep the line < 127 bytes | short | long env -S chains | kernel (silent truncation) | | Pin absolute path when privileged | #!/bin/sh | #!/usr/bin/env (PATH-hijackable) | judgment | | Match the dialect you write | #!/usr/bin/env bash + bashisms | #!/bin/sh + [[ ]] | runtime (dash) |

Why the kernel forces these

The interpreter path is used verbatim (no PATH search, so it must be absolute), everything after it is a single argument (hence env -S for flags), and the line is length-limited and silently truncated past the kernel's buffer. See the reference for execve(2) details.

Do not

Polyglot/self-re-exec headers, ${ORIGIN}/-relative shebangs, arbitrary programs as interpreters, and relying on setuid — all fragile or non-standard.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: posidoni
• Source: posidoni/shell-skill
• License: MIT

Install and usage instructions live in the source repository linked above.