Command Prompt: Shell Scripting and Configuration

Reference skill for writing commands, scripts, and configuration across Unix shells. Detects the target shell from context and routes to the appropriate reference.

Target versions (May 2026):

• Zsh: 5.10
• Bash: 5.3
• Fish: 4.6
• Nushell: 0.111
• Tcsh: 6.24
• Dash: 0.5.13

When to use

• Writing shell commands, scripts, or one-liners
• Configuring dotfiles (.zshrc, .bashrc, .profile, config.fish)
• Writing completions, shell functions, or aliases
• Porting scripts between shells
• Debugging shell-specific behavior (globbing, arrays, expansion, quoting)
• Setting up oh-my-zsh, starship, p10k, or other shell frameworks
• Choosing which shell to target for a new script
• Writing interactive commands on the user's local machine (zsh)

When NOT to use

• Remote FreeBSD/OPNsense/pfSense commands - use firewall-appliance (handles tcsh/csh in the BSD context)
• Ansible shell/command modules - use ansible (module gotchas differ from raw shell)
• CI/CD pipeline shell blocks - use ci-cd (restricted environments, no interactive features)
• General Linux sysadmin that isn't shell-specific - just do the task directly

AI Self-Check

Before returning any generated shell script or command, verify:

• Shebang matches the detected target shell (not assumed bash)
• set -euo pipefail (bash/zsh) or set -eu (POSIX sh) present in scripts
• All variables double-quoted ("$var") unless word splitting is intentional
• No shell-isms from the wrong shell (no [[ ]] in #!/bin/sh, no BASH_SOURCE in zsh)
• Array indexing correct for the target shell (bash: 0-indexed, zsh: 1-indexed)
• printf used over echo for non-trivial output
• Glob safety guards in place (empty-glob case handled)
• No hardcoded paths for tools (/usr/bin/git) - use command -v or bare command names
• Temp files use mktemp with cleanup traps, not hardcoded /tmp/foo
• No secrets in command history (use read -s or environment variables)
• Current source checked: dated versions, CLI flags, API names, and support windows are verified against primary docs before repeating them
• Hidden state identified: local config, credentials, caches, contexts, branches, cluster targets, or previous runs are made explicit before acting
• Verification is real: final checks exercise the actual runtime, parser, service, or integration point instead of only linting prose or happy paths
• Routing overlap checked: overlapping skills, trigger terms, and "When NOT to use" boundaries are checked before returning guidance
• Spec claims verified: claims about tool behavior, output contracts, or repo conventions are checked against current docs, scripts, or skill files
• Shell identified: examples match POSIX sh, Bash, Zsh, or Fish semantics intentionally
• Quoting tested: paths with spaces, empty variables, and glob characters behave safely

Performance

• Use builtins and stream processing for large inputs; avoid command substitution that buffers entire files.
• Prefer rg, fd, and targeted file lists when available, with portable fallbacks noted.
• Avoid spawning subshells inside tight loops when xargs, arrays, or shell builtins fit.

Best Practices

• Default to set -euo pipefail only when the script is written to handle those semantics.
• Use -- before user-controlled paths for commands that support it.
• Preview destructive expansions before rm, mv, chmod, chown, or recursive edits.

Workflow

Step 1: Detect the target shell

Before writing any shell code, determine the target shell. Check these signals in order:

Task-based routing

Step 2: Load the right reference

Don't load all references. Pick the one that matches. If porting between two shells, load both.

Step 3: Write code, then verify

Use the cross-shell comparison below for quick lookups. After writing, run through the Verification Checklist at the bottom of this section.

Quick Cross-Shell Comparison

Universal Patterns (All POSIX Shells)

These work in sh, bash, and zsh. Fish has different syntax for most of these - see the alt-shells reference.

Piping and redirection

Chaining

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