Add tests/capture-output.sh to help with regression testing

Author: fred-lefebvre

tests/README.md

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+# ec2-metadata regression testing
+
+`capture-output.sh` runs every `ec2-metadata` subcommand and saves each output to a separate file. This lets you compare output before and after a code change to catch regressions.
+
+## Usage
+
+```bash
+# 1. Capture baseline output (before your changes)
+./capture-output.sh output-before
+
+# 2. Make your changes to ec2-metadata
+
+# 3. Capture output again
+./capture-output.sh output-after
+
+# 4. Compare
+diff -r output-before output-after
+```
+
+No diff output means the change is behavior-preserving. Any differences will show exactly which subcommand's output changed and how.
+
+## Notes
+
+- Must be run on an EC2 instance (requires IMDS access).
+- The subcommand list is derived dynamically from `ec2-metadata --help`, so new options are picked up automatically.
+- Individual subcommand failures are logged as warnings but don't stop the run, so you get partial results even if some metadata categories are unavailable.

tests/capture-output.sh

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+#!/usr/bin/bash
+# Capture ec2-metadata output for each subcommand into a directory.
+# Usage: ./capture-output.sh [output-dir]
+#   default output-dir: output
+
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+EC2_METADATA="${SCRIPT_DIR}/../ec2-metadata"
+OUTDIR="${1:-output}"
+
+[[ -x "$EC2_METADATA" ]] || { echo "ERROR: ec2-metadata not found at $EC2_METADATA" >&2; exit 1; }
+
+mapfile -t cmds < <("$EC2_METADATA" --help | grep -oP '(?<=/--)[a-z][-a-z0-9]+' | grep -v 'help\|quiet\|all\|path')
+if [[ ${#cmds[@]} -eq 0 ]]; then
+	echo "ERROR: No subcommands found in --help output" >&2
+	exit 1
+fi
+
+mkdir -p "$OUTDIR"
+
+for cmd in "${cmds[@]}"; do
+	"$EC2_METADATA" --"$cmd" > "$OUTDIR/$cmd.txt" 2>&1 || echo "WARN: --$cmd failed" >&2
+done
+
+"$EC2_METADATA" > "$OUTDIR/default.txt" 2>&1 || echo "WARN: default (no args) failed" >&2
+
+files=("$OUTDIR"/*.txt)
+echo "Captured ${#files[@]} files in $OUTDIR/"