Useful Bash scripts for network checks

The pattern behind every check

Network checks look different on the surface, but they share one shape. You keep a list of targets in a text file, loop over it, run a test against each one, write down the result and finish with a summary. Keeping targets in a file means a colleague can add a new switch without editing code.

Only run these checks against networks and devices you manage or have written permission to test. Even a simple ping sweep or port check can trigger alerts, and scanning other people's systems may break the law.

Reachability sweep with a CSV report

Create hosts.txt with one IP address or hostname per line, then run this script.

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

TARGETS="${1:-hosts.txt}"
OUT="reach_$(date +%Y%m%d_%H%M).csv"
fail=0

echo "timestamp,host,status" > "$OUT"
while read -r host; do
  [[ -z "$host" || "$host" == \#* ]] && continue
  if ping -c 2 -W 2 "$host" > /dev/null 2>&1; then
    status="up"
  else
    status="down"; fail=$((fail + 1))
  fi
  echo "$(date '+%F %T'),$host,$status" >> "$OUT"
done < "$TARGETS"

echo "Done: $fail host(s) down. Results in $OUT"
exit $(( fail > 0 ? 1 : 0 ))

Notice that set -e is left out here on purpose: a failing ping is expected, so the script handles it in the if instead of stopping. Blank lines and lines starting with # are skipped so you can comment the host file.

Ports, DNS and web checks

The commands below are the building blocks. Drop them into the same loop pattern.

# Is TCP 22 open on a switch? (-z: just test, -w: timeout seconds)
nc -z -w 3 192.168.10.2 22 && echo "ssh open"

# Pure Bash TCP test when nc is not installed
timeout 3 bash -c '</dev/tcp/192.168.10.2/443' && echo "443 open"

# Does the name resolve to the expected address?
expected="192.168.10.50"
actual=$(dig +short intranet.example.com A | head -n1)
[[ "$actual" == "$expected" ]] || echo "DNS mismatch: got '$actual'"

# HTTP status code of a health endpoint
code=$(curl -s -o /dev/null -w '%{http_code}' --max-time 5 https://intranet.example.com/health)
[[ "$code" == "200" ]] || echo "web check failed: $code"

Making checks useful at work

1. Keep one target file per site or role, for example core.txt and cctv.txt.
2. Use short timeouts so one dead host does not stall the run.
3. Write a dated CSV so you can compare today with last week in a spreadsheet.
4. Return a non-zero exit code when something fails so a scheduler or monitoring tool can alert.
5. Schedule the script with cron and keep it in Git so changes are tracked.

A common mistake is to treat a failed ping as proof that a device is down. Many firewalls, servers and cloud hosts block ICMP (Internet Control Message Protocol) by design, so a host can be perfectly healthy while ignoring ping. Where that is the case, test the service the users actually need, such as TCP 443 on a web server or TCP 22 on a switch management address. The reverse also happens: a device can answer ping while its main service has crashed, which is why an HTTP status check is more meaningful than reachability for applications. Match each check to the question you are really asking.

Tip: these scripts complement, not replace, a proper monitoring system. They are perfect for change windows: run the sweep before and after a change and compare the two CSV files.