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Lab — Monitor Disk Usage

Lab Overview

Purpose: Write a disk usage monitor suitable for cron.

Scenario: On-call wants a simple alert when any filesystem crosses 80% before the full monitoring stack is ready.

Expected outcome: A working script under ~/rebash-lab-shell with clear exit codes, stderr logging, and validation steps you can re-run.

This is a lab, not a tutorial

Apply Shell Scripting skills. Prefer small verified steps over rewriting everything at once.

Business Scenario

A startup runs a few VMs. They need a disk check script that exits non-zero when thresholds are breached.

Learning Objectives

By the end of this lab, you will be able to:

  • Parse df output safely
  • Compare usage against a threshold
  • Emit human logs on stderr and RESULT on stdout
  • Exclude tmpfs/devtmpfs noise

Prerequisites

Knowledge

Software

Tool Notes
Bash required
df/awk coreutils

Estimated cost: £0.

Environment

Any Linux host.

Initial State

mkdir -p ~/rebash-lab-shell/monitor
cd ~/rebash-lab-shell/monitor

Lab Tasks

Task 1 — Implement check-disk.sh

cat > check-disk.sh << 'EOF'
#!/usr/bin/env bash
set -euo pipefail
THRESH="${1:-80}"
alert=0
while read -r use mnt; do
  use="${use%\%}"
  if (( use >= THRESH )); then
    echo "ALERT disk ${use}% on ${mnt}" >&2
    alert=1
  else
    echo "OK disk ${use}% on ${mnt}" >&2
  fi
done < <(df -P -x tmpfs -x devtmpfs | awk 'NR>1 {print $5, $6}')
if (( alert )); then
  echo "RESULT status=alert metric=disk"
  exit 2
fi
echo "RESULT status=ok metric=disk"
exit 0
EOF
chmod +x check-disk.sh
./check-disk.sh 80
./check-disk.sh 1 || true

Task 2 — Document exit codes

Code Meaning
0 All filesystems under threshold
2 One or more alerts
3 Usage / parse error

Validation

  • Script exits 0 on a healthy host with a high threshold
  • Low threshold forces exit 2 and RESULT status=alert
  • tmpfs mounts are excluded

Troubleshooting

Symptom Possible cause Resolution
df columns differ BusyBox df Use df -P
Arithmetic errors Percent sign not stripped Strip % before (( ))

Cleanup

rm -rf ~/rebash-lab-shell

Stretch Goals

  • Add a --dry-run mode that prints actions without changing the system
  • Emit a machine-readable RESULT status=... line on stdout for CI
  • Schedule the script with cron or a systemd timer

Production Discussion

In production, wrap scripts with lock files, structured logging, explicit PATH, and documented exit codes. Prefer configuration files over hard-coded hosts and thresholds. Never embed secrets in scripts — use environment variables or a secrets manager.

Best Practices

  • Use #!/usr/bin/env bash and set -euo pipefail
  • Quote every expansion that may contain spaces
  • Log diagnostics to stderr; keep stdout for data
  • Prefer absolute paths in scheduled jobs
  • Validate inputs before destructive actions

Common Mistakes

Mistake Why it happens Correct approach
Unquoted paths Habit from interactive shell Always "$var"
Missing pipefail Default Bash pipeline behaviour set -o pipefail
Interactive-only PATH Cron/systemd minimal env Set PATH= at top
Skipping dry-run Time pressure Default to dry-run for risky ops

Success Criteria

  • Script runs under Bash with strict mode
  • Validation and failure paths are tested
  • Exit codes are documented
  • Cleanup leaves no lab artefacts (or documents what remains)

Reflection Questions

  1. What would break if this ran under /bin/sh (dash) instead of Bash?
  2. How would you make the script idempotent?
  3. How would you secure credentials and host inventories?
  4. How would you observe failures in production?

Interview Connection

Interviewers often ask about quoting, exit codes, cron environment differences, and how you prevent overlapping jobs. Be ready to walk through a small script and explain failure modes.