POSIX

The POSIX (Portable Operating System Interface) standard defines strict conventions for command-line utilities to ensure consistency and interoperability (e.g., pipability). Key requirements from IEEE Std 1003.1-2017 include:

Option Syntax

• Short Options: Options should be single alphanumeric characters preceded by - (e.g., -a).
• Option Grouping: Multiple short options with no arguments should be groupable (e.g., -abc is equivalent to -a -b -c).
• Option Arguments: Options requiring arguments should accept them separated by space (e.g., -o file) or attached (e.g., -ofile).
• Long Options: While POSIX strictly defines short options, GNU conventions (double-hyphen --option) are widely accepted extensions in the Linux/Unix ecosystem. POSIX compliance usually implies supporting the short forms at minimum.

Standard Input/Output

• Stdin as Operator: If a file operand is notably absent, or if the operand is explicitly -, the utility should read from standard input.
• Stdout/Stderr Separation: Diagnostic messages must go to stderr, while actual data output goes to stdout.
• Pipability: Output format should be text-stream friendly unless a specific format (like JSON) is requested.

End of Options

• Delimiter: The -- argument must be recognized as the end of options. Everything following it is treated as an operand (filename), even if it starts with -.

Exit Status

• 0: Success.
• >0: Failure. Distinct codes should be used for different error types if possible (e.g., syntax error vs runtime error), though POSIX only strictly mandates 0 for success.