shell-scripting

Shell Scripting

Write defensively. Shell defaults are hostile — unquoted variables split, unset variables vanish silently, failed commands continue. Every rule here exists to counteract a specific shell default that causes bugs.

References

Extended examples, code patterns, and lookup tables for the rules below.

• Strict mode, error handling, traps, debugging — [${CLAUDE_SKILL_DIR}/references/strict-mode.md]: errexit caveats, pipefail examples, trap patterns, temp file safety, debugging techniques
• Quoting rules, word splitting, globbing — [${CLAUDE_SKILL_DIR}/references/quoting.md]: Three quoting mechanisms, "$@" vs "$*", array expansion, printf vs echo, nested quoting
• POSIX sh vs bash, portable constructs — [${CLAUDE_SKILL_DIR}/references/portability.md]: Feature comparison, GNU vs BSD tool differences, portable pattern catalog
• Argument parsing, getopts, validation — [${CLAUDE_SKILL_DIR}/references/arguments.md]: getopts template, manual long-option parsing, validation patterns, usage messages, stdin detection
• Common shell scripting mistakes — [${CLAUDE_SKILL_DIR}/references/pitfalls.md]: Iteration pitfalls, variable pitfalls, test pitfalls, pipeline pitfalls, arithmetic traps
• Pure bash/sh alternatives to external commands — [${CLAUDE_SKILL_DIR}/references/builtins.md]: Parameter expansion, replacing sed/cut/basename/expr, arrays, read patterns, arithmetic

Script Header

Every bash script starts with:

#!/usr/bin/env bash
set -euo pipefail

• Shebang: Use #!/usr/bin/env bash — not #!/bin/bash. The env lookup is more portable across systems where bash is not at /bin/bash.
• set -e (errexit): Exit on command failure. Understand the exceptions: commands in if/while conditions, left side of &&/||, and negated commands (!) do not trigger errexit.
• set -u (nounset): Error on unset variables. Use ${VAR:-default} for optional variables.
• set -o pipefail: Pipeline returns the rightmost failing command's exit code, not the last command's.
• For POSIX sh scripts: Use #!/bin/sh. Drop pipefail (not POSIX). Use set -eu with caution — set -e behavior varies across sh implementations.
• File header: A script's purpose goes in its name and its --help text, not a header comment. Add one only when the name cannot carry it and the script has no --help.