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What a custom Nagios plugin must do
A Nagios plugin is not a daemon or a special binary format. It is an executable check that Nagios runs on a schedule. The check can inspect a local file, process, listening port, queue, API response, disk measurement, or any other condition that can be evaluated on the VPS. Nagios consumes two outputs from every run: the process exit status for the machine-readable state and at least one line on standard output for the operator-readable explanation. The official Plugin API states that scripts and executables must exit with one of the possible return values and return at least one line of text to STDOUT (Nagios Core Plugin API).
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Keep the first output line short enough for the Nagios interface, notifications, and a pager. Put the reason for the state first, then add performance data after a pipe character when the value should be graphed or retained for historical analysis. A plugin that silently exits, writes only to STDERR, or returns a state that does not match its message is difficult to operate and can produce misleading alerts.
The four standard return codes
| Exit code | State | Use it when |
|---|---|---|
| 0 | OK | The measured condition is within the acceptable range. |
| 1 | WARNING | A warning threshold has been reached, but the service is not yet in the critical condition. |
| 2 | CRITICAL | The critical threshold has been reached or the service is failing. |
| 3 | UNKNOWN | The plugin cannot determine a trustworthy state, for example because an argument is invalid or a required command failed. |
Define these values in the script instead of scattering literal numbers through conditionals. That makes later changes and reviews less error-prone.
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Plan the check before writing Bash
Choose one focused question for each plugin. Examples include whether a file is present, how old a queue is, whether a process is running, how much space a filesystem consumes, or whether an HTTP endpoint responds. A single-purpose check gives Nagios one clear state and one clear remediation path. If a script checks unrelated services and returns one combined result, an alert will not tell you which component needs attention.
Define the measured value and units
Write down exactly what the script measures and in which units. A percentage, seconds, bytes, count, and load value require different thresholds and performance-data units. Decide whether a larger value is worse or whether a smaller value is worse. Record the sampling method as well: a one-second process count, a file age in seconds, and a cumulative counter do not have interchangeable threshold meanings.
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- Return UNKNOWN when arguments are missing, malformed, or outside the range your check can interpret.
- Return CRITICAL when the service-specific failure itself is confirmed.
- Decide what a missing file, permission error, empty API response, or unavailable local command means, and document that policy in the output message.
- Set an internal timeout for commands that can block. A hung child process can consume a Nagios worker and prevent other checks from running.
Use a predictable interface
Accept warning, critical, and timeout options such as -w/--warning, -c/--critical, and -t/--timeout. Reject an omitted value, a non-numeric value where a number is required, and a critical threshold that contradicts the check’s documented ordering. Provide -h/--help so an administrator can discover the interface without opening the source.
Implement the Bash skeleton safely
Start with an explicit interpreter path. #!/usr/bin/env bash is portable when env can locate Bash; an absolute Bash path used by your VPS is even more deterministic. Define the four Nagios states, parse and validate arguments, run one measurement, classify it, print the result, and exit with the matching constant.
Use absolute paths for external programs. Nagios may run with a restricted or unexpected PATH, and relying on an administrator’s interactive shell can make a script work manually but fail under the scheduler. The plugin guidelines also warn against unsafe command lookup through PATH. Use paths such as /usr/bin/awk, /bin/cat, or the paths confirmed on your distribution; do not assume that every VPS places a utility in the same directory.
Minimal illustrative check
The following pattern reads the first field of /proc/loadavg, converts it to an integer, compares it with warning and critical values, and emits performance data. It is an illustrative pattern rather than a universal load policy: adapt the metric, units, thresholds, and command paths to your VPS.
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#!/usr/bin/env bash
OK=0; WARNING=1; CRITICAL=2; UNKNOWN=3
warning=80; critical=90
value=$(/usr/bin/awk '{print int($1)}' /proc/loadavg) || {
echo "UNKNOWN - cannot read load average"
exit "$UNKNOWN"
}
if (( value >= critical )); then
echo "CRITICAL - load value=${value} | load=${value};${warning};${critical};0"
exit "$CRITICAL"
elif (( value >= warning )); then
echo "WARNING - load value=${value} | load=${value};${warning};${critical};0"
exit "$WARNING"
else
echo "OK - load value=${value} | load=${value};${warning};${critical};0"
exit "$OK"
fiBecause int($1) truncates the load value, this example deliberately demonstrates the mechanics rather than a production-ready load monitor. A real implementation should choose a meaningful sampling and precision policy, use thresholds appropriate to the VPS, and explain the chosen policy in its help text.
Add option parsing and validation
For a reusable plugin, replace hard-coded thresholds with parsed arguments. A conventional Bash approach is to process short options with getopts and handle long options in a small case loop. Whichever method you choose, validate before running the check:
- Require both warning and critical values when the check needs them.
- Accept only the numeric format your comparison code handles; do not let an empty string become zero through implicit arithmetic conversion.
- Check that the timeout is a positive number and that it is applied to every potentially blocking operation.
- Print a usage line and exit UNKNOWN for malformed input, rather than silently selecting defaults.
Keep argument errors distinguishable from service failures. An operator who sees UNKNOWN - invalid critical threshold can correct configuration; an unexplained CRITICAL result may trigger unnecessary incident work.
Apply Nagios threshold ranges correctly
Nagios thresholds are ranges, not merely two numbers. The plugin guidelines define the syntax as an optional @ followed by a start and end separated by a colon. An alert occurs when the measured value falls outside the specified acceptable range; the @ prefix reverses that test so the alert occurs inside the range. A bare number is shorthand for a range whose lower bound is zero and whose upper bound is that number for the usual non-negative check.
| Form | Interpretation for a non-negative measurement | Typical use |
|---|---|---|
10 |
Alert above 10. | Warn or fail when a count exceeds a limit. |
10:20 |
Alert below 10 or above 20. | Keep a value within a band. |
10: |
Alert below 10; there is no upper bound. | Require a minimum level. |
~:10 |
Alert above 10; the lower bound is negative infinity. | Express an upper limit when negative values are possible. |
@10:20 |
Alert at values from 10 through 20, inclusive. | Flag an unsafe interval rather than values outside it. |
Use the same range parser for warning and critical values, then evaluate critical first. If both thresholds can match, returning CRITICAL before WARNING ensures the most severe state wins. Performance data should carry the exact warning and critical strings when possible so graphing and operators can see the limits that produced the state.
Return performance data for graphs and history
Append performance data to the first output line after |. The documented form is label=value[UOM];[warn];[crit];[min];[max]. Labels are separated by spaces, and units may include %, s, B, or c. For example:
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OK - queue depth 3 | queue=3c;10;20;0Use a stable label such as queue, latency, or disk_used; changing labels between runs creates fragmented graphs. Include minimum and maximum values only when they are meaningful. A percentage should carry %, a duration should carry s, and a byte measurement should carry B. Do not put diagnostic prose after the pipe where a monitoring system expects metric syntax.
Test the plugin outside Nagios first
Save the script with a descriptive name, for example check_queue_age.sh, and make it executable with chmod 755 check_queue_age.sh or a tighter mode that still permits the Nagios execution user to run it. Test all state paths from a shell before adding configuration.
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- Run the help option and confirm that missing or malformed arguments produce a useful usage message and UNKNOWN status.
- Run a normal case and inspect both the text and
echo $?for exit code 0. - Force the warning condition with a temporary test value and verify code 1, the word WARNING, and the expected performance data.
- Force the critical condition and verify code 2.
- Remove or deny access to an input and verify that the documented UNKNOWN or CRITICAL policy is followed.
- Run the command as the Nagios user, not only as root or your login account. This catches permission, working-directory, environment, and PATH assumptions.
- Run it repeatedly while observing elapsed time. A command that can hang needs an internal timeout before it is scheduled frequently.
Keep temporary files in a directory with appropriate ownership and permissions. Avoid predictable names in shared locations, clean up files on every exit path, and do not follow an unexpected symlink when writing state. These practices are part of the Nagios plugin development guidance alongside concise STDOUT, input validation, full command paths, and timeout handling (Nagios Plugin Development Guidelines).
Install a custom plugin on a Linux VPS
Nagios packages use distribution- and installation-dependent directories. Find the plugin directory from your Nagios configuration or package documentation rather than copying a path from another VPS. Common layouts include a directory under the Nagios installation prefix or a distribution-managed libexec directory, but neither should be treated as universal.
- Choose a root-owned directory already used for Nagios plugins, or create an installation-specific custom directory with controlled ownership.
- Copy the script there, preserve the shebang, and set executable permissions.
- Ensure the Nagios execution user can read the script and every input it needs. Grant only the minimum access required; do not make sensitive files world-readable merely to satisfy a check.
- Run the complete command manually as that user, including warning, critical, and timeout arguments.
- Record the final path and arguments before writing the Nagios command definition.
Do not rely on the script’s current working directory. Nagios can invoke it from a different directory, so use absolute paths for files, helper programs, and any configuration the check reads. If the check needs environment variables, define them explicitly in the Nagios execution context or pass configuration through arguments and a protected file.
Connect the script to Nagios configuration
A Nagios command definition gives the executable a stable name and maps positional macros to its arguments. The following is an installation-dependent example; adapt the plugin directory, macro names, and configuration file location to your package.
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define command{
command_name check_custom_load
command_line /path/to/plugins/check_custom_load.sh -w $ARG1$ -c $ARG2$
}Attach that command to a service definition and supply values for the macros:
define service{
host_name your-vps-host
service_description Custom load check
check_command check_custom_load!80!90
check_interval 5
retry_interval 1
}The interval values above are examples, not a recommendation for every VPS. Choose a schedule that matches the failure you are detecting and the script’s runtime. A check that launches an expensive API request should not run as frequently as a cheap local file test.
Validate before reloading
- Run Nagios’s configuration verification command with the configuration file used by your installation, commonly in the form
nagios -v /path/to/nagios.cfg. Use the binary and path supplied by your package if they differ. - Fix every syntax error, unknown command, missing macro, and permission warning before reloading.
- Reload the Nagios service using the service manager and command provided by your distribution.
- In the Nagios interface, schedule or wait for the service check and inspect the exact plugin output, status, and performance data.
If the configuration validates but the service is UNKNOWN, compare the command line shown by Nagios with the command that succeeded in your shell as the Nagios user. Differences in quoting, macro expansion, environment, and file permissions are common causes.
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Make checks reliable and secure
Protect command execution
Use fixed paths and avoid constructing shell commands from untrusted macro values. If an argument is intended to be a number, validate it as a number and use arithmetic comparison rather than interpolating it into an eval expression. Quote variable expansions when they represent paths or text. Never allow a host or service argument to become an arbitrary command fragment.
Bound every external operation
Network clients, filesystem operations on remote mounts, and subprocesses can block. Apply an internal timeout using the timeout utility available on your VPS or the relevant API’s own timeout option, then translate a timeout into the policy you documented. Ensure the timeout covers the actual child process and does not leave a background child running after the plugin exits.
Keep output deterministic
Send the one-line status to STDOUT. Send verbose debugging to a controlled log or enable it only through an explicit debug option; unsolicited lines can interfere with parsers and obscure the first status line. Avoid locale-dependent decimal separators and date formats when parsing command output. If a helper returns unexpected text, fail as UNKNOWN rather than classifying a guessed value.
Handle files and privileges carefully
Use private temporary directories or securely created temporary files, set restrictive permissions, and remove them on normal and error exits. Check ownership and mode before consuming a file that could be replaced by another user. The plugin should run with the least privilege needed for its measurement; escalating to root to read a value is usually a sign that permissions or the monitoring design need review.
Troubleshoot common failures
Nagios reports UNKNOWN immediately
- Run the exact command as the Nagios user and inspect the expanded arguments.
- Check that the script is executable and its interpreter exists at the shebang path.
- Confirm every helper uses an absolute path and that the Nagios user can read the target files.
- Look for an empty macro caused by a missing
!argumentin the service definition.
The text says OK but Nagios shows CRITICAL
Check the final exit statement and capture the status with echo $?. A command substitution or pipeline can mask a failing command unless its status is handled explicitly. Verify that the branch printing the message exits with the same state constant.
Thresholds never trigger
Print the parsed value temporarily, confirm its unit and precision, and test each boundary value. A value in bytes compared with a threshold intended for megabytes, or an integer truncated from a decimal, can make a correct comparison appear broken. Recheck whether the range is meant to alert inside or outside the interval and whether the @ prefix was intended.
Performance data is missing or ungraphable
Ensure the pipe is on the first output line, the metric label contains no spaces, and the value follows label=value[UOM];[warn];[crit];[min];[max]. Keep the label stable and use the correct unit. Diagnostic sentences belong before the pipe, not among metric fields.
When Bash is the right choice
Bash is convenient for small, local checks that compose standard Unix tools already present on a Linux VPS. It has little deployment overhead, is easy to inspect on the host, and works well for files, processes, simple counters, and command exit statuses. Its weak points are complex numeric parsing, structured API responses, concurrency, extensive error handling, and long-term testability. When a check needs robust JSON parsing, many branches, parallel operations, or substantial unit tests, another language may be easier to maintain and less ambiguous. The Nagios contract remains the same regardless of language: valid exit status, useful STDOUT, correctly defined thresholds, and bounded runtime.
Quick Recap
Pre-deployment checklist
- The script starts with the Bash interpreter used by the VPS.
- OK, WARNING, CRITICAL, and UNKNOWN are defined and returned consistently.
- Warning, critical, timeout, and help options are parsed and malformed values are rejected.
- Every external command and important file uses an installation-appropriate absolute path.
- The check measures one condition in documented units and applies the intended Nagios range semantics.
- The first STDOUT line is concise, and performance data follows the pipe in the documented format.
- Blocked operations have an internal timeout.
- The file, temporary-file handling, and privileges are safe for the Nagios execution user.
- All four state paths were tested manually as that user.
- Nagios configuration validation passed before reload, and the live service output was inspected afterward.
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