POSIX Shell POSIX.1-2024

Use these rules for non-interactive scripts written to the POSIX.1-2024 Shell Command Language and standard utilities.

Invocation and interpreter scope

• Select a known POSIX-conforming interpreter explicitly and do not depend on the caller's interactive shell.
• Do not execute untrusted text through sh -c, eval, command substitution, or sourced files.
• Keep executable script files owned by a trusted account and non-writable by less-privileged users.
• Reject startup environments that can redirect interpreter behavior, command search, locale, or dynamic loading.
• Document every intentional non-POSIX extension and fail rather than silently running it under another shell.

Expansion and quoting

• Double-quote every parameter expansion and command substitution unless field splitting or pathname expansion is explicitly required.
• Use -- before untrusted operands when a utility supports it and reject leading hyphens where it does not.
• Avoid storing multiple arguments in one scalar; preserve argument boundaries through positional parameters.
• Do not use command substitution for arbitrary binary data because trailing newlines are removed and null-byte behavior is unspecified.
• Treat IFS, glob patterns, tilde expansion, arithmetic expansion, and here-document delimiters as security-sensitive syntax.

Input validation

• Parse options with getopts and reject unknown options, missing arguments, and excess positional operands.
• Validate each external value against a narrow allowlist before using it in a path, command, pattern, or arithmetic context.