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df vs du: When to Use Disk Free vs Disk Usage Tools

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Automating Disk Space Checks with df and Shell Scripts

Keeping disk usage under control prevents service failures, data loss, and degraded performance. This guide shows a practical, automatable approach using the df command and simple shell scripts to monitor disk space, send alerts, and take basic remedial actions.

What you’ll build

  • A portable shell script that:
    • Checks filesystem usage with df.
    • Alerts when usage exceeds configurable thresholds.
    • Optionally cleans temporary files or rotates logs.
    • Can be run manually or scheduled via cron/systemd timer.

Assumptions and defaults

  • POSIX‑compatible shell (bash/sh).
  • Linux systems where df supports human-readable and percent output.
  • Default threshold: 85% usage triggers alerts.
  • Alert method: console output and email via mailx (optional). Replace with your favorite notifier.

Script: disk-check.sh

bash
#!/usr/bin/env bash
# disk-check.sh — simple disk usage monitor
# Usage: disk-check.sh [threshold_percent] [email_address]
THRESHOLD=\({1</span><span class="token" style="color: rgb(57, 58, 52);">:-</span><span class="token" style="color: rgb(54, 172, 170);">85}</span><span> </span><span></span><span class="token assign-left" style="color: rgb(54, 172, 170);">ALERT_EMAIL</span><span class="token" style="color: rgb(57, 58, 52);">=</span><span class="token" style="color: rgb(54, 172, 170);">\){2:-}

# Filesystems to ignore (pattern list)
IGNORE_FS_REGEX=”^tmpfs|^devtmpfs|^overlay|^squashfs”

# Get df output (exclude header), show percent and mount
# Example line: /dev/sda150G  40G  10G  80% /
df -P -h | awk ‘NR>1{print \(1,\)5,\(6}'</span><span> </span><span class="token" style="color: rgb(57, 58, 52);">|</span><span> </span><span class="token" style="color: rgb(0, 0, 255);">while</span><span> </span><span class="token builtin" style="color: rgb(43, 145, 175);">read</span><span> -r fs pct </span><span class="token" style="color: rgb(57, 58, 52);">mount</span><span class="token" style="color: rgb(57, 58, 52);">;</span><span> </span><span class="token" style="color: rgb(0, 0, 255);">do</span><span> </span><span>  </span><span class="token" style="color: rgb(0, 128, 0); font-style: italic;"># skip pseudo filesystems</span><span> </span><span>  </span><span class="token" style="color: rgb(0, 0, 255);">if</span><span> </span><span class="token" style="color: rgb(57, 58, 52);">[</span><span class="token" style="color: rgb(57, 58, 52);">[</span><span> </span><span class="token" style="color: rgb(54, 172, 170);">\)fs =~ \(IGNORE_FS_REGEX</span><span> </span><span class="token" style="color: rgb(57, 58, 52);">]</span><span class="token" style="color: rgb(57, 58, 52);">]</span><span class="token" style="color: rgb(57, 58, 52);">;</span><span> </span><span class="token" style="color: rgb(0, 0, 255);">then</span><span> </span><span>    </span><span class="token builtin" style="color: rgb(43, 145, 175);">continue</span><span> </span><span>  </span><span class="token" style="color: rgb(0, 0, 255);">fi</span><span> </span> <span>  </span><span class="token" style="color: rgb(0, 128, 0); font-style: italic;"># strip % sign</span><span> </span><span>  </span><span class="token assign-left" style="color: rgb(54, 172, 170);">used</span><span class="token" style="color: rgb(57, 58, 52);">=</span><span class="token" style="color: rgb(54, 172, 170);">\){pct%%}
  if (( used >= THRESHOLD )); then
    msg=“ALERT: \(mount</span><span class="token" style="color: rgb(163, 21, 21);"> (</span><span class="token" style="color: rgb(54, 172, 170);">\)fs) is at \({used}</span><span class="token" style="color: rgb(163, 21, 21);">% (threshold </span><span class="token" style="color: rgb(54, 172, 170);">\){THRESHOLD}%)”
    echo “\(msg</span><span class="token" style="color: rgb(163, 21, 21);">"</span><span> </span><span>    </span><span class="token" style="color: rgb(0, 128, 0); font-style: italic;"># send email if configured</span><span> </span><span>    </span><span class="token" style="color: rgb(0, 0, 255);">if</span><span> </span><span class="token" style="color: rgb(57, 58, 52);">[</span><span> -n </span><span class="token" style="color: rgb(163, 21, 21);">"</span><span class="token" style="color: rgb(54, 172, 170);">\)ALERT_EMAIL” ]; then
      printf ”%s

Full df -h output:
“ ”\(msg</span><span class="token" style="color: rgb(163, 21, 21);">"</span><span> </span><span class="token" style="color: rgb(57, 58, 52);">|</span><span> </span><span class="token" style="color: rgb(57, 58, 52);">cat</span><span> - </span><span class="token" style="color: rgb(57, 58, 52);"><</span><span class="token" style="color: rgb(57, 58, 52);">(</span><span class="token" style="color: rgb(57, 58, 52);">df</span><span> -h</span><span class="token" style="color: rgb(57, 58, 52);">)</span><span> </span><span class="token" style="color: rgb(57, 58, 52);">|</span><span> mailx -s </span><span class="token" style="color: rgb(163, 21, 21);">"Disk space alert: </span><span class="token" style="color: rgb(54, 172, 170);">\)mount at \({used}</span><span class="token" style="color: rgb(163, 21, 21);">%"</span><span> </span><span class="token" style="color: rgb(163, 21, 21);">"</span><span class="token" style="color: rgb(54, 172, 170);">\)ALERTEMAIL“
    fi
    # optional automated action: rotate logs or clean cache
    # example: delete files older than 30 days in /var/log/myapp (customize)
    # find /var/log/myapp -type f -mtime +30 -delete
  fi
done

How it works (brief)

  • df -P -h prints portable, human-readable output.
  • awk extracts device, usage percent, and mount point.
  • Script ignores pseudo filesystems like tmpfs.
  • Alerts when usage >= threshold and optionally emails the admin.
  • Customize the automated action section for safe, environment-specific cleanup.

Scheduling with cron

Edit root’s crontab (or a service account) to run every 15 minutes:

  • Run crontab -e and add:
cron
*/15 * * * * /usr/local/bin/disk-check.sh 85 [email protected] >/dev/null 2>&1

Scheduling with systemd timer (alternative)

Create /etc/systemd/system/disk-check.service:

ini
[Unit]
Description=Disk space check

[Service]
Type=oneshot
ExecStart=/usr/local/bin/disk-check.sh 85 [email protected]

Create /etc/systemd/system/disk-check.timer:

ini
[Unit]
Description=Run disk-check every 15 minutes

[Timer]
OnBootSec=5min
OnUnitActiveSec=15min

[Install]
WantedBy=timers.target

Enable and start:

  • systemctl daemon-reload
  • systemctl enable –now disk-check.timer

Enhancements and safety tips

  • Use df -B1 for byte-precise checks in scripts requiring exact thresholds.
  • Add logging to syslog or a file for auditability.
  • Avoid destructive automated cleanup unless fully tested; prefer alerting first.
  • Consider using inotify-based or dedicated monitoring (Prometheus nodeexporter + alertmanager) for large environments.

Example: byte-precise check snippet

bash
# compare used bytes to threshold bytes for mount
avail_bytes=\((</span><span class="token" style="color: rgb(57, 58, 52);">df</span><span class="token" style="color: rgb(54, 172, 170);"> --output</span><span class="token" style="color: rgb(57, 58, 52);">=</span><span class="token" style="color: rgb(54, 172, 170);">avail -B1 </span><span class="token" style="color: rgb(163, 21, 21);">"</span><span class="token" style="color: rgb(163, 21, 21);">\)mount“ | tail -n1)
size_bytes=\((</span><span class="token" style="color: rgb(57, 58, 52);">df</span><span class="token" style="color: rgb(54, 172, 170);"> --output</span><span class="token" style="color: rgb(57, 58, 52);">=</span><span class="token" style="color: rgb(54, 172, 170);">size -B1 </span><span class="token" style="color: rgb(163, 21, 21);">"</span><span class="token" style="color: rgb(163, 21, 21);">\)mount” | tail -n1)
used_bytes=\(((</span><span class="token" style="color: rgb(54, 172, 170);"> size_bytes </span><span class="token" style="color: rgb(57, 58, 52);">-</span><span class="token" style="color: rgb(54, 172, 170);"> avail_bytes </span><span class="token" style="color: rgb(54, 172, 170);">))</span><span> </span><span></span><span class="token assign-left" style="color: rgb(54, 172, 170);">threshold_bytes</span><span class="token" style="color: rgb(57, 58, 52);">=</span><span class="token" style="color: rgb(54, 172, 170);">\)(( size_bytes * THRESHOLD / 100 ))
if (( used_bytes >= threshold_bytes )); then
  # alert
fi

Quick checklist before deploying

  • Test the script manually on non-production mounts.
  • Configure accurate ignore patterns for ephemeral filesystems.
  • Verify email delivery or other alert channels.
  • Start with notifications only; add automated cleanup after monitoring behavior for a period.

This script is a minimal, flexible starting point—customize mount ignores, alerting, and remediation steps to match your environment.

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