skip when utmp is missing

plugins/inputs/system/README.md

@@ -8,12 +8,13 @@ and number of users logged in. It is similar to the unix `uptime` command.
 ```toml
 # Read metrics about system load & uptime
 [[inputs.system]]
-  # no configuration
+  ## Print out when utmp is not available for n_users
+  # warn_on_missing = true
 ```
 #### Permissions:
 
 The `n_users` field requires read access to `/var/run/utmp`, and may require
-the `telegraf` user to be added to the `utmp` group on some systems.
+the `telegraf` user to be added to the `utmp` group on some systems. If this file does not exist `n_users` will be skipped.
 
 ### Metrics:
 

plugins/inputs/system/system.go

@@ -4,6 +4,7 @@ import (
 	"bufio"
 	"bytes"
 	"fmt"
+	"log"
 	"os"
 	"runtime"
 	"strings"
@@ -15,7 +16,9 @@ import (
 	"github.com/shirou/gopsutil/load"
 )
 
-type SystemStats struct{}
+type SystemStats struct{
+	WarnOnMissing bool
+}
 
 func (_ *SystemStats) Description() string {
 	return "Read metrics about system load & uptime"
@@ -25,10 +28,12 @@ func (_ *SystemStats) SampleConfig() string {
 	return `
   ## Uncomment to remove deprecated metrics.
   # fielddrop = ["uptime_format"]
+  ## Print out when utmp is not available for n_users
+  # warn_on_missing = true
 `
 }
 
-func (_ *SystemStats) Gather(acc telegraf.Accumulator) error {
+func (s *SystemStats) Gather(acc telegraf.Accumulator) error {
 	loadavg, err := load.Avg()
 	if err != nil && !strings.Contains(err.Error(), "not implemented") {
 		return err
@@ -44,7 +49,9 @@ func (_ *SystemStats) Gather(acc telegraf.Accumulator) error {
 	users, err := host.Users()
 	if err == nil {
 		fields["n_users"] = len(users)
-	} else if !os.IsPermission(err) {
+	} else if os.IsNotExist(err) && s.WarnOnMissing {
+		log.Printf("[inputs.system]: %s\n", err.Error())
+	} else if !os.IsPermission(err) && !os.IsNotExist(err) {
 		return err
 	}