Archive for the ‘coding’ Category

Nagios NRPE wrapper to encode meta-characters

April 23, 2017 Leave a comment

The Nagios Remote Plugin Executor allows remote execution of Nagios check commands, which is a powerful tool for monitoring the health of the machines and services on your network.

If your remote commands take command line parameters, you might run into trouble regarding special characters, typically required in regular expressions and other values you might want to send across. To actually see the error on the target machine, you need to Create a Log File for NRPE.

If your command fails due to this kind of error, then read on:

Apr 6 18:06:58 somehost nrpe[somepid]: Error: Request contained illegal metachars!
Apr 6 18:06:58 somehost nrpe[somepid]: Client request was invalid, bailing out...

NRPE inspects the arguments for characters that have special meaning for typical Unix shells to prevent shell command injection. If it encounters any charcaters deemed unsafe the command execution is rejected and you will see the error message above.

Unfortunately NRPE does not provide a way to safely encode and decode the parameter values.

To work around this, I created a wrapper script for the check_nrpe command, let’s call it I chose url-encoding for its simplicity and familiarity. The goal is to be able to create Nagios command definitions like this:

$USER1$/custom_scripts/ -H $HOSTADDRESS$ -c my_remote_command -a '$ARG1$' '$ARG2$'

And the remote command would bedefined in /etc/nagios/nrpe.cfg or under in a custom file under /etc/nrpe.d/, depending on your Linux distribtion:

command[my_remote_command]=/path_to_my_custom_nrpe_plugins/ '$ARG1$' '$ARG2$'

Here is a possible implementation of

#! /bin/bash


urlencode() {

    local length="${#1}"
    for (( i = 0; i < length; i++ )); do
        local c="${1:i:1}"
        case ${c} in
            [a-zA-Z0-9.~_-]) printf "$c" ;;
            *) printf '%%%02X' "'$c" ;;


for x in "$@"; do
  if [ ${x} = '-a' ]; then
    if [ ${args} = 'yes' ]; then
       x=$(urlencode ${x})
  command="${command} ${x}"

# execute the check_nrpe with the encoded args:

In the remote shell script like, you would then decode the arguments like this:

function urldecode() {
  local url_encoded="${1//+/ }"
  printf '%b' "${url_encoded//%/\\x}"
arg1="$(urldecode $1)"
arg2="$(urldecode $2)"

And then use the decoded arguments as before.

Categories: bash, coding Tags: ,

Summing up numeric values using bash

March 22, 2017 Leave a comment

For example to add up the disk usage at several disjoint locations:

me@box$ numfmt --to=iec $[$(du -s /home/*/docs | cut -f 1 | paste -sd+)]

The $[..] is for arithmetic evaluation in bash.
Alternatively pipe to the bc command.

Categories: bash, coding

Simplest Dependency Injection

January 17, 2017 Leave a comment


Note to self: If you create a Java application and think you need DI but not AOP, then do not automatically reach for containers like Spring, CDI, Guice. Just do all construction and wiring in the standard main method.


Schedule wireless availability using ddwrt

January 7, 2017 Leave a comment

Sometimes I have problems ending my internet use in time to get enough quality sleep.

So I decided to set up a schedule that automatically disables our wireless home network during certain night hours for “nights before work/school day” and slightly longer hours for “night before weekend day”. Luckily this is quite easily done with the ddwrt firmware that I run on my router.

I couldn’t use the “Radio Time Restrictions” feature under Wireless – Advanced Settings, because it only supports one uniform schedule for all days of the week. Instead I used ddwrt’s cron support and the wl command.

I configured the following entries under Administration – Management – Cron. This turns on the wireless every morning at 6am, shuts it down at 10:30pm on Sunday to Thursday and shuts it down at 11:45pm on Friday and Saturday:

00 06 * * *   root wl radio on
30 22 * * 0-4 root wl radio off
45 23 * * 5-6 root wl radio off
Categories: coding Tags: , , ,

Convert mpc to mp3 on Linux

January 1, 2017 Leave a comment

You need the lame and mpcdec commands. On Debian, mpcdec is in the musepack-tools package:

sudo apt-get install lame musepack-tools

Then to convert all mpc files in the current directory to matchingly named mp3 files:

for x in *.mpc; do mpcdec "${x}" - | lame -r - "${x%.mpc}.mp3"; done
Categories: bash, coding, debian, linux, music

bash : Loop over lines in file with user prompt

December 8, 2016 Leave a comment

I used the following to loop over the lines in a file, while prompting the user for a key press on each iteration:

while read -u 3 line ; do 
  #clear the screen
  printf "\033c" 
  echo "$line"; echo

  # do something with the $line here

  read -n 1 -s -p "[Press any key to continue]"
done 3< "some-file.txt"

The reading of the lines is done via file descriptor 3 to avoid interference with the reading of the user’s key presses.

Categories: bash, coding Tags: ,

Slightly better looking ?

October 24, 2016 Leave a comment

My resume website now has a new scaling background image and uses the Open Sans web font. I hope it looks nice. Any feedback is welcome.

Categories: coding, css, Tags: ,