Posts Tagged ‘cron job’

Tools to scan a Linux / Unix Web server for Malware and Rootkits / Lynis and ISPProtect – clean Joomla / WordPress and other CMS for malware and malicious scripts and trojan codes

Monday, March 14th, 2016


If you have been hacked or have been suspicious that someone has broken up in some of the shared web hosting servers you happent o manage you already probably have tried the server with rkhuter, chroot and unhide tools which gives a general guidance where a server has been compromised

However with the evolution of hacking tools out there and the boom of Web security XSS / CSS / Database injections and PHP scripts vulnerability catching an intruder especially spammers has been becoming more and more hard to achieve.

Just lately a mail server of mine's load avarage increased about 10 times, and the CPU's and HDD I/O load jump over the sky.
I started evaluating the situation to find out what exactly went wrong with the machine, starting with a hardware analysis tools and a physical check up whether all was fine with the hardware Disks / Ram etc. just to find out the machine's hardware was working perfect.
I've also thoroughfully investigated on Logs of Apache, MySQL, TinyProxy and Tor server and bind DNS and DJBDns  which were happily living there for quite some time but didn't found anything strange.

Not on a last place I investigated TOP processes (with top command) and iostat  and realized the CPU high burst lays in exessive Input / Output of Hard Drive. Checking the Qmail Mail server logs and the queue with qmail-qstat was a real surprise for me as on the queue there were about 9800 emails hanging unsent, most of which were obviously a spam, so I realized someone was heavily spamming through the server and started more thoroughfully investigating ending up to a WordPress Blog temp folder (writtable by all system users) which was existing under a Joomla directory infrastructure, so I guess someone got hacked through the Joomla and uploaded the malicious php spammer script to the WordPress blog. I've instantly stopped and first chmod 000 to stop being execuded and after examing deleted view73.php, javascript92.php and index8239.php which were full of PHP values with binary encoded values and one was full of encoded strings which after being decoding were actually the recepient's spammed emails.
BTW, the view*.php javascript*.php and index*.php files were owned by www-data (the user with which Apache was owned), so obviously someone got hacked through some vulnerable joomla or wordpress script (as joomla there was quite obscure version 1.5 – where currently Joomla is at version branch 3.5), hence my guess is the spamming script was uploaded through Joomla XSS vulnerability).

As I was unsure wheteher the scripts were not also mirrored under other subdirectories of Joomla or WP Blog I had to scan further to check whether there are no other scripts infected with malware or trojan spammer codes, webshells, rootkits etc.
And after some investigation, I've actually caught the 3 scripts being mirrored under other webside folders with other numbering on filename view34.php javascript72.php, index8123.php  etc..

I've used 2 tools to scan and catch malware the trojan scripts and make sure no common rootkit is installed on the server.

1. Lynis (to check for rootkits)
2. ISPProtect (Proprietary but superb Website malware scanner with a free trial)

1. Lynis – Universal security auditing tool and rootkit scanner

Lynis is actually the well known rkhunter, I've used earlier to check servers BSD and Linux servers for rootkits.
To have up-to-date version of Lynis, I've installed it from source:

cd /tmp
tar xvfz lynis-2.1.1.tar.gz
mv lynis /usr/local/
ln -s /usr/local/lynis/lynis /usr/local/bin/lynis


Then to scan the server for rootkits, first I had to update its malware definition database with:

lynis update info

Then to actually scan the system:

lynis audit system

Plenty of things will be scanned but you will be asked on a multiple times whether you would like to conduct different kind fo system services and log files, loadable kernel module rootkits and  common places to check for installed rootkits or server placed backdoors. That's pretty annoying as you will have to press Enter on a multiple times.


Once scan is over you will get a System Scan Summary like in below screenshot:


Lynis suggests also a very good things that might be tampered to make the system more secure, so using some of its output when I have time I'll work out on hardening all servers.

To prevent further incidents and keep an eye on servers I've deployed Lynis scan via cron job once a month on all servers, I've placed under a root cronjob on every first dae of month in following command:



server:~# crontab -u root -e
0 3 1 * * /usr/local/bin/lynis –quick 2>&1 | mail -s "lynis output of my server"


2. ISPProtect – Website malware scanner

ISPProtect is a malware scanner for web servers, I've used it to scan all installed  CMS systems like WordPress, Joomla, Drupal etc.
ISPProtect is great for PHP / Pyhon / Perl and other CMS based frameworks.
ISPProtect contains 3 scanning engines: a signature based malware scanner, a heuristic malware scanner, and a scanner to show the installation directories of outdated CMS systems.
Unfortunately it is not free software, but I personally used the FREE TRIAL option  which can be used without registration to test it or clean an infected system.
I first webserver first locally for the infected site and then globally for all the other shared hosting websites.

As I wanted to check also rest of hosted websites, I've run ISPProtect over the all bunch of installed websites.
Pre-requirement of ISPProtect is to have a working PHP Cli and Clamav Anti-Virus installed on the server thus on RHEL (RPM) based servers make sure you have it installed if not:

server:~# yum -y install php

server:~# yum -y install clamav

Debian based Linux servers web hosting  admins that doesn't have php-cli installed should run:

server:~# apt-get install php5-cli

server:~# apt-get install clamav

Installing ISPProtect from source is with:

mkdir -p /usr/local/ispprotect
chown -R root:root /usr/local/ispprotect
chmod -R 750 /usr/local/ispprotect
cd /usr/local/ispprotect
tar xzf ispp_scan.tar.gz
rm -f ispp_scan.tar.gz
ln -s /usr/local/ispprotect/ispp_scan /usr/local/bin/ispp_scan


To initiate scan with ISPProtect just invoke it:

server:~# /usr/local/bin/ispp_scan



I've used it as a trial

Please enter scan key:  trial
Please enter path to scan: /var/www

You will be shown the scan progress, be patient because on a multiple shared hosting servers with few hundred of websites.
The tool will take really, really long so you might need to leave it for 1 hr or even more depending on how many source files / CSS / Javascript etc. needs to be scanned.

Once scan is completed scan and infections found logs will be stored under /usr/local/ispprotect, under separate files for different Website Engines and CMSes:

After the scan is completed, you will find the results also in the following files:

Malware => /usr/local/ispprotect/found_malware_20161401174626.txt
Wordpress => /usr/local/ispprotect/software_wordpress_20161401174626.txt
Joomla => /usr/local/ispprotect/software_joomla_20161401174626.txt
Drupal => /usr/local/ispprotect/software_drupal_20161401174626.txt
Mediawiki => /usr/local/ispprotect/software_mediawiki_20161401174626.txt
Contao => /usr/local/ispprotect/software_contao_20161401174626.txt
Magentocommerce => /usr/local/ispprotect/software_magentocommerce_20161401174626.txt
Woltlab Burning Board => /usr/local/ispprotect/software_woltlab_burning_board_20161401174626.txt
Cms Made Simple => /usr/local/ispprotect/software_cms_made_simple_20161401174626.txt
Phpmyadmin => /usr/local/ispprotect/software_phpmyadmin_20161401174626.txt
Typo3 => /usr/local/ispprotect/software_typo3_20161401174626.txt
Roundcube => /usr/local/ispprotect/software_roundcube_20161401174626.txt

ISPProtect is really good in results is definitely the best malicious scripts / trojan / trojan / webshell / backdoor / spammer (hacking) scripts tool available so if your company could afford it you better buy a license and settle a periodic cron job scan of all your servers, like lets say:


server:~# crontab -u root -e
0 3  1 * *   /usr/local/ispprotect/ispp_scan –update && /usr/local/ispprotect/ispp_scan –path=/var/www – –non-interactive –scan-key=AAA-BBB-CCC-DDD

Unfortunately ispprotect is quite expensive so I guess most small and middle sized shared hosting companies will be unable to afford it.
But even for a one time run this tools worths the try and will save you an hours if not days of system investigations.
I'll be glad to hear from readers if aware of any available free software alternatives to ISPProtect. The only one I am aware is Linux Malware Detect (LMD).
I've used LMD in the past but as of time of writting this article it doesn't seems working any more so I guess the tool is currently unsupported / obsolete.


No space left on device with free disk space / Why no space left on device while there is plenty of disk space on drive – Running out of Inodes

Tuesday, November 17th, 2015



On one of the servers, I'm administrating the websites started showing some Mysql database table corrup errors like:


Table './database_name/site_news_list_com' is marked as crashed and last (automatic?) repair failed

The server is using Oracle MySQL server community stable edition on Debian GNU / Linux 6.0, so I first thought during work the server crashed either due to some bug issue in MySQL or it crashed due to some PHP cron job that did something messy. Thus to solve the crashed tables, tried using mysqlcheck tool which helped pretty fine, at many times whether there were database / table corruptions. I've run the following set of mysqlcheck commands with root (superuser) in a bash shell after logging in through SSH:


server:~# /usr/bin/mysqlcheck –defaults-extra-file=/etc/mysql/debian.cnf \–check –all-databases -u root -p`grep -i password /root/.my.cnf |sed -e 's#password=##g'`>> /var/log/cronwork.log
server:~# /usr/bin/mysqlcheck –defaults-extra-file=/etc/mysql/debian.cnf –analyze –all-databases -u root -p`grep -i password /root/.my.cnf |sed -e 's#password=##g'`>> /var/log/cronwork.log
server:~# /usr/bin/mysqlcheck –defaults-extra-file=/etc/mysql/debian.cnf \–auto-repair –optimize –all-databases -u root -p`grep -i password /root/.my.cnf |sed -e 's#password=##g'`>> /var/log/cronwork.log
server:~# /usr/bin/mysqlcheck –defaults-extra-file=/etc/mysql/debian.cnf \–optimize –all-databases -u root -p`grep -i password /root/.my.cnf |sed -e 's#password=##g'`>> /var/log/cronwork.log

In order for above commands to work, I've created the /root/.my.cnf containing my root (mysql CLI) mysql username and password, e.g. file has content like below:




Btw a good note here is its generally a good idea (if you want to have consistent mysql databases) to automatically execute via a cron job 2 times a month, I've in root cronjob the following:


crontab -u root -l |grep -i mysqlcheck
04 06 5,10,15,20,25,1 * * /usr/bin/mysqlcheck –defaults-extra-file=/etc/mysql/debian.cnf \–check –all-databases –silent -u root -p`grep -i password /root/.my.cnf |sed -e 's#password=##g'`>> /var/log/cronwork.log 07 06 5,10,15,20,25,1 * * /usr/bin/mysqlcheck –defaults-extra-file=/etc/mysql/debian.cnf –analyze –all-databases –silent -u root -p`grep -i password /root/.my.cnf |sed -e 's#password=##g'`>> /var/log/cronwork.log 12 06 5,10,15,20,25,1 * * /usr/bin/mysqlcheck –defaults-extra-file=/etc/mysql/debian.cnf \–auto-repair –optimize –all-databases –silent -u root -p`grep -i password /root/.my.cnf |sed -e 's#password=##g'`>> /var/log/cronwork.log 17 06 5,10,15,20,25,1 * * /usr/bin/mysqlcheck –defaults-extra-file=/etc/mysql/debian.cnf \–optimize –all-databases –silent -u root -p`grep -i password /root/.my.cnf |sed -e 's#password=##g'`>> /var/log/cronwork.log

Strangely I got a lot of errors that some .MYI / .MYD .frm temp files, necessery for the mysql tables recovery can't be written inside /home/mysql/database_name

That was pretty weird and I thought there might be some issues with permissions, causing the inability to write, due to some bug or something so I went straight and checked /home/mysql/database_name permissions, e.g.::


server:/home/mysql/database_name# ls -ld soccerfame
drwx—— 2 mysql mysql 36864 Nov 17 12:00 soccerfame
server:/home/mysql/database_name# ls -al1|head -n 10
total 1979012
drwx—— 2 mysql mysql 36864 Nov 17 12:00 .
drwx—— 36 mysql mysql 4096 Nov 17 11:12 ..
-rw-rw—- 1 mysql mysql 8712 Nov 17 10:26 1_campaigns_diez.frm
-rw-rw—- 1 mysql mysql 14672 Jul 8 18:57 1_campaigns_diez.MYD
-rw-rw—- 1 mysql mysql 1024 Nov 17 11:38 1_campaigns_diez.MYI
-rw-rw—- 1 mysql mysql 8938 Nov 17 10:26 1_campaigns.frm
-rw-rw—- 1 mysql mysql 8738 Nov 17 10:26 1_campaigns_logs.frm
-rw-rw—- 1 mysql mysql 883404 Nov 16 22:01 1_campaigns_logs.MYD
-rw-rw—- 1 mysql mysql 330752 Nov 17 11:38 1_campaigns_logs.MYI

As seen from above output, all was perfect with permissions, so it should have been something else, so I decided to try to create a random file with touch command inside /home/mysql/database_name directory:


touch /home/mysql/database_name/somefile-to-test-writtability.txt touch: cannot touch ‘/scr1/data/somefile-to-test-writtability.txt‘: No space left on device

Then logically I thought the /home/mysql/ mounted ext4 partition got filled, because of crashed SQL database or a bug thus, checked with disk free command df whether there is enough space on server:

server:~# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/md1 20G 7.6G 11G 42% /
udev 10M 0 10M 0% /dev
tmpfs 13G 1.3G 12G 10% /run
tmpfs 32G 0 32G 0% /dev/shm
tmpfs 5.0M 0 5.0M 0% /run/lock
tmpfs 32G 0 32G 0% /sys/fs/cgroup
/dev/md2 256G 134G 110G 55% /home

Well that's weird? Obviously only 55% of available disk space is used and available 134G which was more than enough so I got totally puzzled why, files can't be written.

Then very logically, I thought it might be that /home directory has remounted as read only, because the SSD memory disk on server is failing and checked for errors in dmesg, i.e.:


server:~# dmesg|grep -i error

Also checked how exactly was partition mounted, to check whether it is (RO) read-only:


server:~# mount -l|grep -i /home
/dev/md2 on /home type ext4 (rw,relatime,discard,data=ordered)

Now everything become even more weirder, as obviously the disk continued to be claiming no space left on device, while in reality there was plenty of disk space.

Then after running a quick research on the internet for the no space left on device with free disk space, I've come across this great thread which let me realize the partition run out of inodes and that's why no new file inodes could be assigned and therefore, the linux kernel is refusing to write the file on ext4 partition.

For those who haven't heard of Linux Partition Inodes here is link to Wikipedia and a quick quote:


In a Unix-style file system, the inode is a data structure used to represent a filesystem object, which can be one of various things including a file or a directory. Each inode stores the attributes and disk block location(s) of the filesystem object's data.[1] Filesystem object attributes may include manipulation metadata (e.g. change,[2] access, modify time), as well as owner and permission data (e.g. group-id, user-id, permissions).[3]
Directories are lists of names assigned to inodes. The directory contains an entry for itself, its parent, and each of its children.

Once I understood it is the inodes, I checked how many of them are occupied with cmd:


server:~# df -i /home
Filesystem Inodes IUsed IFree IUse% Mounted on
/dev/md2 17006592 17006592 0 100% /home

You see, there were 0 (zero) free file inodes on server and that was the reason for no space left on device while there was actually free disk space

To clean up (free) some inodes on partition, first thing I did is to delete all old logs which were inside /home and files I positively know not to be necessery, then to find which directories allocating most innodes used:


server:~# find . -xdev -type f | cut -d "/" -f 2 | sort | uniq -c | sort -n

If you're on a regular old fashined IDE Hard Drive and not SSD or you have too much files inside this command will take really long …:

Therefore a better solution might be to frist:

a) Try to find root folders with large inodes count:

for i in /home/*; do echo $i; find $i |wc -l; done
Try to find specific folders:

You should get output like:



b) Then once you know the directory allocating most inodes, run the command again to see the sub-directories with most files (eating) partition innodes:


for i in /home/webservice/*; do echo $i; find $i |wc -l; done


One usual large folder which could free you some nodes is the linux source headers, but in my case it was simply a lot of tiny old logs being logged on the system for few years in the past without cleaning:

After deleting the log dirs and cache folder in my case /home/new_website/{log,cache}:

server:~# rm -rf /home/new_website/log/*
server:~# rm -rf /home/new_website/cache/*



a) Then, stopping Apache webserver to check prevent Apache to use MySQl databases while running database repair and restaring MySQL:

server:~# /etc/init.d/apache2 stop Restarting MySQL server
server:~# /etc/init.d/mysql restart

b) And re-issuing MySQL Check / Repair / Optimize database commands:


mysqlcheck –defaults-extra-file=/etc/mysql/debian.cnf \–check –all-databases -u root -p`grep -i password /root/.my.cnf |sed -e 's#password=##g'`>> /var/log/cronwork.log

mysqlcheck –defaults-extra-file=/etc/mysql/debian.cnf –analyze –all-databases -u root -p`grep -i password /root/.my.cnf |sed -e 's#password=##g'`>> /var/log/cronwork.log

mysqlcheck –defaults-extra-file=/etc/mysql/debian.cnf \–auto-repair –optimize –all-databases -u root -p`grep -i password /root/.my.cnf |sed -e 's#password=##g'`>> /var/log/cronwork.log

mysqlcheck –defaults-extra-file=/etc/mysql/debian.cnf \–optimize –all-databases -u root -p`grep -i password /root/.my.cnf |sed -e 's#password=##g'`>> /var/log/cronwork.log

c) And finally starting the Apache Webserver again:

server:~# /etc/init.d/apache2 start

Some innodse got freed up:

server:~# df -i /home Filesystem Inodes IUsed IFree IUse% Mounted on
/dev/md2 17006592 16797196 209396 99% /home

And hooray by God's Grace and with help of prayers of The most Holy Theotokos (Virgin) Mary, websites started again !

Updating Flash Player on Debian GNU / Linux / Keeping Flash player up-to-date with update-flashplugin-nonfree

Saturday, November 10th, 2012


Update flash player on Debian GNU / Linux update-flashplugin-nonfree macromedia flash logo

Assuming you have previously installed and running Adobe Flash Player – package flashplugin-nonfree i.e.:

debian:~# dpkg -l |grep -i flashplugin-nonfree
ii flashplugin-nonfree 1:2.8.3 Adobe Flash Player - browser plugin

and you want to Update flash player to the latest provided version for Linux, there is an update script part of flashplugin-nonfree, package /usr/sbin/update-flashplugin-nonfree. The script updates flash player to latest Linux version avaiable fetching the version from macromedia's website in a .tar.gz and untarring it substituting the old flash library.

To update your Debian FlashPlayer, launch as superuser:

debian:~# update-flashplugin-nonfree --install
--2012-11-10 00:51:48-- Resolving Connecting to||:80... connected. HTTP request sent, awaiting response... 200 OK Length: 7228964 (6.9M) [application/x-gzip] Saving to: “./install_flash_player_11_linux_x86_64.tar.gz”

0K .......... .......... .......... ..........
.......... 0% 69.5K 1m41s 50K .......... .......... .......... ..........
.......... 1% 91.1K 88s 100K .......... .......... .......... ..........
.......... 2% 70.8K 91s ........

After a while (usually up to a minute), update will be completed. Restart your browser of use IceWeasel, Epiphany, Opera, Chrome etc. and test it with About Flash Player Page and / or youtube. You should be with latest Flash Linux version now.

It might be a good idea to automate future flash player updates via a cron job, I think launching the update script every two weeks is a good timing;

To do so add to root user cron like so:

10,27 * * * * /usr/sbin/update-flashplugin-nonfree –install -q 2>&1 >/dev/null

If you still haven't configured your pulseaudio to play multiple sound streams do that too.

I've seen also on Debian's Wiki FlashPlayer page, mentioning that on some systems after update to Flash Player 11 there might be laggy performance issues, due to disabled hardware acceleration in Flash Player > v. 10. If that's the case with you you might also need to put a mss.cfg like this one to /etc/adobe/mss.cfg

# wget -q
# mv adobe-flash-player-config-for-hardware-acceleration-mms.cfg /etc/adobe/mms.cfg

Finally if you experience, some flash video lagging issues, you could try experimenting with OverrideGPUValidation=true flash setting which in some cases improves Linux flash video performance

Firefox users might be also interested to check out – the URL provides information on essential Firefox video plugins and whether plugins installed are up2date or prone to remote web exploitation vulnerability.

Auto restart Apache on High server load (bash shell script) – Fixing Apache server temporal overload issues

Saturday, March 24th, 2012


I've written a tiny script to check and restart, Apache if the server encounters, extremely high load avarage like for instance more than (>25). Below is an example of a server reaching a very high load avarage:;

server~:# uptime
13:46:59 up 2 days, 18:54, 1 user, load average: 58.09, 59.08, 60.05
load average: 0.09, 0.08, 0.08

Sometimes high load avarage is not a problem, as the server might have a very powerful hardware. A high load numbers is not always an indicator for a serious problems. Some 16 CPU dual core (2.18 Ghz) machine with 16GB of ram could probably work normally with a high load avarage like in the example. Anyhow as most servers are not so powerful having such a high load avarage, makes the machine hardly do its job routine.

In my specific, case one of our Debian Linux servers is periodically reaching to a very high load level numbers. When this happens the Apache webserver is often incapable to serve its incoming requests and starts lagging for clients. The only work-around is to stop the Apache server for a couple of seconds (10 or 20 seconds) and then start it again once the load avarage has dropped to less than "3".

If this temporary fix is not applied on time, the server load gets increased exponentially until all the server services (ssh, ftp … whatever) stop responding normally to requests and the server completely hangs …

Often this server overloads, are occuring at night time so I'm not logged in on the server and one such unexpected overload makes the server unreachable for hours.
To get around the sudden high periodic load avarage server increase, I've written a tiny bash script to monitor, the server load avarage and initiate an Apache server stop and start with a few seconds delay in between.

# script to check server for extremely high load and restart Apache if the condition is matched
check=`cat /proc/loadavg | sed 's/\./ /' | awk '{print $1}'`
# define max load avarage when script is triggered
# log file
# location of inidex.php to overwrite with temporary message
# location to Apache init script
site_maintenance_msg="Site Maintenance in progress - We will be back online in a minute";
if [ $check -gt "$max_load" ]; then>
#25 is load average on 5 minutes
cp -rpf $index_php_loc $index_php_loc.bak_ap
echo "$site_maintenance_msg" > $index_php_loc
sleep 15;
if [ $check -gt "$max_load" ]; then
$apache_init stop
sleep 5;
$apache_init restart
echo "$(date) : Apache Restart due to excessive load | $check |" >> $high_load_log;
cp -rpf $index_php_loc.bak_ap $index_php_loc

The idea of the script is partially based on a forum thread – Auto Restart Apache on High Load is a link to my script

The script is written in a way that it makes two "if" condition check ups, to assure 100% there is a constant high load avarage and not just a temporal 5 seconds load avarage jump. Once the first if is matched, the script first tries to reduce the server load by overwritting a the index.php, index.html script of the website with a one stating the server is ongoing a maintenance operations.
Temporary stopping the index page, often reduces the load in 10 seconds of time, so the second if case is not necessery at all. Sometimes, however this first "if" condition cannot decrease enough the load and the server load continues to stay too high, then the script second if comes to play and makes apache to be completely stopped via Apache init script do 2 secs delay and launch the apache server again.

The script also logs about, the load avarage encountered, while the server was overloaded and Apache webserver was restarted, so later I can check what time the server overload occured.
To make the script periodically run, I've scheduled the script to launch every 5 minutes as a cron job with the following cron:

# restart Apache if load is higher than 25
*/5 * * * * /usr/sbin/ >/dev/null 2>&1

I have also another system which is running FreeBSD 7_2, which is having the same overload server problems as with the Linux host.
Copying the auto restart apache on high load script on FreeBSD didn't work out of the box. So I rewrote a little chunk of the script to make it running on the FreeBSD host. Hence, if you would like to auto restart Apache or any other service on FreeBSD server get /usr/sbin/ my script and set it on cron on your BSD.

This script is just a temporary work around, however as its obvious that the frequency of the high overload will be rising with time and we will need to buy new server hardware to solve permanently the issues, anyways, until this happens the script does a great job 🙂

I'm aware there is also alternative way to auto restart Apache webserver on high server loads through using monit utility for monitoring services on a Unix system. However as I didn't wanted to bother to run extra services in the background I decided to rather use the up presented script.

Interesting info to know is Apache module mod_overload exists – which can be used for checking load average. Using this module once load avarage is over a certain number apache can stop in its preforked processes current serving request, I've never tested it myself so I don't know how usable it is. As of time of writting it is in early stage version 0.2.2
If someone, have tried it and is happy with it on a busy hosting servers, please share with me if it is stable enough?

How to add cron jobs from command line or bash scripts / Add crontab jobs in a script

Saturday, July 9th, 2011

I’m currently writting a script which is supposed to be adding new crontab jobs and do a bunch of other mambo jambo.

By so far I’ve been aware of only one way to add a cronjob non-interactively like so:

                 linux:~# echo '*/5 * * * * /root/' | crontab -

Though using the | crontab – would work it has one major pitfall, I did completely forgot | crontab – OVERWRITES CURRENT CRONTAB! with the crontab passed by with the echo command.
One must be extremely careful if he decides to use the above example as you might loose your crontab definitions permanently!

Thanksfully it seems there is another way to add crontabs non interactively via a script, as I couldn’t find any good blog which explained something different from the classical example with pipe to crontab –, I dropped by in the good old to consult the bash gurus there 😉

So I entered irc and asked the question how can I add a crontab via bash shell script without overwritting my old existing crontab definitions less than a minute later one guy with a nickname geirha was kind enough to explain me how to get around the annoying overwridding.

The solution to the ovewrite was expected, first you use crontab to dump current crontab lines to a file and then you append the new cron job as a new record in the file and finally you ask the crontab program to read and insert the crontab definitions from the newly created files.
So here is the exact code one could run inside a script to include new crontab jobs, next to the already present ones:

linux:~# crontab -l > file; echo '*/5 * * * * /root/ >/dev/null 2>&1' >> file; crontab file

The above definition as you could read would make the new record of */5 * * * * /root/ >/dev/null be added next to the existing crontab scheduled jobs.

Now I’ll continue with my scripting, in the mean time I hope this will be of use to someone out there 😉