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A custom Nagios plugin is an executable check: it evaluates one condition, prints a concise result to standard output, and exits with Nagios’s status code. On a Linux VPS, Bash is a practical choice for checks that combine local files, processes, sockets, command output, or simple API calls. This guide shows how to design the script, implement warning and critical thresholds, emit performance data, install it safely, connect it to a service definition, and troubleshoot the common failure modes.
The Nagios plugin contract
Nagios Core does not require a plugin to be written in a particular language. It requires two things: a status exit code and at least one line of text on standard output. The official Nagios Core Plugin API describes this contract. Nagios uses the exit code as the machine-readable state and displays the first output line as the explanation for operators.
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| Exit code | State | Use it when |
|---|---|---|
| 0 | OK | The measured condition is inside the acceptable range. |
| 1 | WARNING | A warning threshold has been crossed, but the service is not yet considered a failure. |
| 2 | CRITICAL | A critical threshold has been crossed or the service cannot perform its required check. |
| 3 | UNKNOWN | Arguments are invalid, data cannot be interpreted, or the plugin cannot determine a reliable state. |
Always make the status and message agree. Returning 0 while printing “CRITICAL” creates contradictory information for the scheduler, notifications, and dashboards. Conversely, a useful message with no output at all leaves an operator without context even when the exit code is correct.
Plan the check before writing Bash
Keep one plugin focused on one service condition. Define the value being measured, its unit, the source of the value, acceptable limits, and the behavior when the source is unavailable. For example, a root-filesystem check can measure used space as a percentage, warn at 80, and become critical at 90. A process check might count matching processes; a queue check might read a local command or API; a certificate check might measure days until expiry.
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- Input: command-line options such as
-wfor warning,-cfor critical,-tfor timeout, and a target path or service name. - Measurement: one command or a small, deterministic pipeline that produces a value.
- Evaluation: threshold comparisons with clearly documented units and boundary behavior.
- Output: a short first line for the Nagios UI, optionally followed by performance data after a pipe character.
- Failure policy: a predictable choice between UNKNOWN and CRITICAL when collection fails or times out.
Write down whether equality triggers a state. In the example below, a value equal to the critical limit is CRITICAL, and a value equal to the warning limit is WARNING only when it has not already met the critical limit.
A complete Bash example: root filesystem usage
The following illustrative plugin checks the filesystem containing a supplied mount point. It accepts integer percentage thresholds, uses absolute paths for external commands, applies an internal timeout, prints performance data, and exits with the matching Nagios code. It is an example pattern rather than a tested, universal script; verify command locations and adapt the collection logic to your VPS distribution.
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OK=0
WARNING=1
CRITICAL=2
UNKNOWN=3
usage() {
echo "Usage: $0 -w PERCENT -c PERCENT [-t SECONDS] [-m MOUNTPOINT]"
}
warning=""
critical=""
timeout_seconds=10
mountpoint="/"
while getopts ":w:c:t:m:h" opt; do
case "$opt" in
w) warning="$OPTARG" ;;
c) critical="$OPTARG" ;;
t) timeout_seconds="$OPTARG" ;;
m) mountpoint="$OPTARG" ;;
h) usage; exit "$OK" ;;
:) echo "UNKNOWN - option -$OPTARG requires a value"; exit "$UNKNOWN" ;;
?) echo "UNKNOWN - invalid option: -$OPTARG"; usage; exit "$UNKNOWN" ;;
esac
done
if [[ -z "$warning" || -z "$critical" ]]; then
echo "UNKNOWN - both warning and critical thresholds are required"
exit "$UNKNOWN"
fi
if [[ ! "$warning" =~ ^[0-9]+$ || ! "$critical" =~ ^[0-9]+$ || ! "$timeout_seconds" =~ ^[1-9][0-9]*$ ]]; then
echo "UNKNOWN - thresholds and timeout must be positive integers"
exit "$UNKNOWN"
fi
if (( warning >= critical || warning > 100 || critical > 100 )); then
echo "UNKNOWN - use 0-100 with warning lower than critical"
exit "$UNKNOWN"
fi
if [[ ! -d "$mountpoint" ]]; then
echo "UNKNOWN - mount point does not exist: $mountpoint"
exit "$UNKNOWN"
fi
# Verify these paths with command -v on the target VPS before deployment.
df_output=$(/usr/bin/timeout "${timeout_seconds}s" /bin/df -P "$mountpoint" 2>/dev/null) || {
echo "UNKNOWN - df failed or exceeded ${timeout_seconds}s"
exit "$UNKNOWN"
}
used_percent=$(printf '%sn' "$df_output" | /usr/bin/awk 'NR == 2 { gsub(/%/, "", $5); print $5 }')
if [[ ! "$used_percent" =~ ^[0-9]+$ ]]; then
echo "UNKNOWN - could not parse filesystem usage"
exit "$UNKNOWN"
fi
if (( used_percent >= critical )); then
echo "CRITICAL - ${mountpoint} is ${used_percent}% used | usage=${used_percent}%;${warning};${critical};0;100"
exit "$CRITICAL"
elif (( used_percent >= warning )); then
echo "WARNING - ${mountpoint} is ${used_percent}% used | usage=${used_percent}%;${warning};${critical};0;100"
exit "$WARNING"
else
echo "OK - ${mountpoint} is ${used_percent}% used | usage=${used_percent}%;${warning};${critical};0;100"
exit "$OK"
fiThe script deliberately validates all user-supplied values before invoking a command. The /usr/bin/timeout wrapper prevents a blocked filesystem operation from holding a Nagios worker indefinitely. If your system stores df, awk, or timeout elsewhere, replace those paths after checking them on the VPS; do not rely on the scheduler’s inherited PATH.
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Understanding warning and critical thresholds
The example uses a simple upper-bound policy: values at or above the warning limit produce WARNING, and values at or above the critical limit produce CRITICAL. That is appropriate for usage, latency, queue depth, and similar “higher is worse” measurements. For “lower is worse” metrics such as free space or available workers, reverse the comparison or measure the inverse quantity.
Nagios plugins also support a range notation documented in the Nagios Plugins Development Guidelines. Common forms include:
| Range | Meaning | Typical interpretation |
|---|---|---|
10 |
Alert outside 0 through 10. | Warn when a value is greater than 10. |
10: |
Alert below 10. | Useful when a minimum acceptable value is required. |
~:10 |
Alert above 10 while allowing any value below 0. | Useful for metrics that can legitimately be negative. |
10:20 |
Alert outside 10 through 20. | Require a value to stay within a bounded interval. |
@10:20 |
Alert inside 10 through 20. | Use when the middle of the range is the failure condition. |
Range syntax is a contract between the plugin and its users. Document the unit and whether the range is inclusive, then test boundary values such as exactly 10 and exactly 20. If you need arbitrary ranges, decimals, negative values, or different warning and critical semantics, implement and test a dedicated range parser rather than silently treating every argument as an integer upper limit.
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Plugin output and performance data
The first output line should identify the state, the target, and the measured value without a long diagnostic. The guidelines require a status message on standard output and define performance data after a pipe character. The documented format is label=value[UOM];[warn];[crit];[min];[max].
labelidentifies the metric; use a stable, graph-friendly name without spaces.valueis the measured number.UOMmay be omitted or may identify units such as%,s,B, orc.warnandcritrepeat the thresholds used by the check when that makes graphs self-describing.minandmaxare optional bounds for the metric.
For the filesystem plugin, usage=87%;80;90;0;100 tells a graphing consumer that the value is a percentage with warning at 80, critical at 90, and a natural range from 0 to 100. Keep labels stable over time; changing a label creates a new series in many monitoring systems. If you emit multiple metrics, separate them with spaces after the pipe and keep the human message before the pipe concise.
Make Bash plugins safe and reliable
Use absolute command paths
Nagios may run as a restricted service account with a minimal or unexpected environment. An attacker who can influence PATH could cause an unqualified command name to resolve to a malicious executable. Use absolute paths for utilities, verify them during installation, and avoid evaluating arguments as shell code.
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Validate every argument
Reject missing options, non-numeric limits, impossible relationships, unknown flags, and targets that do not exist. Quote variables whenever they are expanded. Do not concatenate untrusted input into an eval expression, a shell command string, or a temporary filename.
Bound every potentially blocking operation
A plugin that waits forever can consume a worker slot and delay unrelated checks. Apply an internal timeout to network requests, DNS lookups, API calls, and commands that read remote mounts. Decide in advance whether timeout means UNKNOWN (the state cannot be determined) or CRITICAL (the monitored service is considered failed), and use that policy consistently.
Handle temporary files carefully
Prefer pipelines and command substitution that do not require a temporary file. If a file is unavoidable, create it in a directory with appropriate permissions, use a unique name, clean it with a trap, and protect against symlink replacement. Never write predictable filenames in a world-writable directory.
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Keep output and side effects limited
Plugins should inspect a service, not repair it. Avoid restarting daemons, deleting data, changing permissions, or making configuration changes from a routine check. Send diagnostics to standard output in the expected format; unexpected logging to standard output can corrupt the status line.
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Install the plugin on a Linux VPS
Plugin directories and configuration locations depend on the Linux distribution and how Nagios was packaged. Common layouts include /usr/lib/nagios/plugins for distribution packages and /usr/local/nagios/libexec for source installations. Treat these as examples: identify the directory configured as Nagios’s plugin path on your VPS before copying the file.
- Find the active plugin directory. Inspect the Nagios configuration for the
$USER1$macro or the package documentation. The command definition normally uses that macro as the plugin directory. - Copy and protect the script. For example, after selecting the correct directory, run
sudo install -o root -g nagios -m 0755 check_root_fs /usr/lib/nagios/plugins/check_root_fs. Replace the directory and group with the values used by your installation. - Run it manually as the Nagios account. Use
sudo -u nagios /usr/lib/nagios/plugins/check_root_fs -w 80 -c 90 -m /. Confirm the message, performance data, and numeric exit code withecho $?. - Define a command object. Add a command definition to the configuration file included by your installation:
define command {
command_name check_root_fs_custom
command_line $USER1$/check_root_fs -w $ARG1$ -c $ARG2$ -m $ARG3$
}- Attach the command to a service. A service definition can pass the thresholds and mount point as arguments:
define service {
host_name my-vps
service_description Root filesystem usage
check_command check_root_fs_custom!80!90!/
check_interval 5
retry_interval 1
max_check_attempts 3
notification_interval 60
}- Validate before reloading. Run the validator appropriate to your installation, such as
sudo nagios -v /etc/nagios4/nagios.cfgfor a distribution layout or the Nagios binary’s-voption for a source installation. Fix every error reported before applying a reload. - Reload and schedule an immediate check. Use the service manager command supplied by your package, commonly
sudo systemctl reload nagiosor a distribution-specific service name. In the Nagios interface, schedule an immediate check and confirm that the service output matches the manual run.
Do not assume that the web interface, daemon user, configuration filename, or service unit has the same name on every VPS. Packaging choices change those details; the command definition and service attachment are the portable concepts.
Test the plugin before enabling notifications
- Run the script with valid warning and critical values and verify all three normal states by temporarily changing the thresholds or test fixture.
- Test missing options, an invalid number, an inverted warning/critical pair, an unknown option, and a nonexistent mount point. Each should return UNKNOWN with a useful explanation.
- Run as the actual Nagios account, not only as root. Check read permissions, executable permissions, environment variables, and access to any sockets or files.
- Force a timeout with a deliberately slow test command and confirm that the plugin exits within the configured limit.
- Check that performance data remains parseable when the value is 0, 100, or at either threshold.
- Use the Nagios configuration validator after every command or service definition change, then inspect the scheduled check’s stored output.
Troubleshoot common failures
| Symptom | Likely cause | Correction |
|---|---|---|
| “No output” or an empty service result | The script exits before printing, writes diagnostics only to stderr, or is not executable. | Run it as the Nagios user, print a status line on every path, and verify mode 0755 and the shebang. |
| UNKNOWN: command not found | The daemon’s PATH does not contain a utility used by the script. |
Use an absolute path and verify it on the VPS. |
| Works in a shell but fails in Nagios | Different user permissions, working directory, environment, or mount visibility. | Reproduce with sudo -u nagios and use absolute paths and explicit file locations. |
| Every result is CRITICAL | Threshold units do not match the measured value, or a percentage sign was not removed. | Print the parsed value temporarily, document the unit, and compare like-for-like numbers. |
| Checks hang or arrive late | A network or filesystem command has no internal timeout. | Wrap the operation with a timeout and return the documented UNKNOWN or CRITICAL state. |
| Configuration reload fails | A typo in a command, host, service, macro, or include file. | Run the Nagios validator and correct the first reported error before checking later messages. |
| Graphs show separate or missing series | Performance-data labels changed, units changed, or output contains malformed separators. | Keep labels stable and follow the label=value[UOM];[warn];[crit];[min];[max] format. |
When Bash is the right implementation
| Requirement | Bash is a good fit when | Consider another language when |
|---|---|---|
| Data source | The check reads local files, process listings, sockets, or standard Unix command output. | You need a complex client library, deeply nested JSON, or several concurrent requests. |
| Numeric logic | Values are simple integers or straightforward comparisons. | You need precise decimal arithmetic, date calculations, or a complete range grammar. |
| Portability | Bash and the required utilities are already installed on the VPS. | The check must run across systems with very different command implementations. |
| Error handling | A few explicit failure paths are easy to validate. | You need typed exceptions, structured retries, extensive parsing, or a large test suite. |
| Maintenance | The script remains short, single-purpose, and reviewed like configuration. | It has grown into a multi-module application with shared state or concurrency. |
Bash’s advantage is availability and easy composition of existing tools, not sophisticated application structure. Once quoting, parsing, retries, and concurrent calls dominate the code, a language with stronger libraries and testing support generally reduces operational risk.
Operational checklist
- Use
#!/usr/bin/env bashor the absolute Bash path installed on the VPS. - Define and use the four standard constants: OK 0, WARNING 1, CRITICAL 2, UNKNOWN 3.
- Validate options, thresholds, units, and target paths before collecting data.
- Call external programs by absolute path and quote variable expansions.
- Print one concise status line to standard output on every exit path.
- Place performance data after
|using stable labels and documented units. - Enforce an internal timeout for every operation that can block.
- Install with least-privilege ownership and execute it as the Nagios account.
- Validate Nagios configuration before reload, then schedule and inspect a real check.
- Keep the plugin observational; do not hide remediation actions inside a health check.
Final implementation perspective
A dependable custom plugin is small, explicit, and predictable: it measures one condition, applies thresholds whose units are clear, emits parseable output, and terminates within a known time. Bash is well suited to that shape on a Linux VPS. The quality of the result depends less on clever shell syntax than on honoring Nagios’s exit-code and output contract, validating inputs, using safe command paths, and testing under the same account and environment that executes scheduled checks.
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