Posts Tagged ‘interval’

How to set a crontab to execute commands on a seconds time interval on GNU / Linux and FreeBSD

Sunday, October 30th, 2011

crontab-execute-cron-jobs-every-second-on-linux-cron-logo
Have you ever been in need to execute some commands scheduled via a crontab, every let’s say 5 seconds?, naturally this is not possible with crontab, however adding a small shell script to loop and execute a command or commands every 5 seconds and setting it up to execute once in a minute through crontab makes this possible.
Here is an example shell script that does execute commands every 5 seconds:

#!/bin/bash
command1_to_exec='/bin/ls';
command2_to_exec='/bin/pwd';
for i in $(echo 1 2 3 4 5 6 7 8 9 10 11); do
sleep 5;
$command1_to_exec; $command2_to_exec;
done

This script will issue a sleep every 5 seconds and execute the two commands defined as $command1_to_exec and $command2_to_exec

Copy paste the script to a file or fetch exec_every_5_secs_cmds.sh from here

The script can easily be modified to execute on any seconds interval delay, the record to put on cron to use with this script should look something like:

# echo '* * * * * /path/to/exec_every_5_secs_cmds.sh' | crontab -

Where of course /path/to/exec_every_5_secs_cmds.sh needs to be modified to a proper script name and path location.

Another way to do the on a number of seconds program / command schedule without using cron at all is setting up an endless loop to run/refresh via /etc/inittab with a number of predefined commands inside. An example endless loop script to run via inittab would look something like:

while [ 1 ]; do
/bin/ls
sleep 5;
done

To run the above sample never ending script using inittab, one needs to add to the end of inittab, some line like:

mine:234:respawn:/path/to/script_name.sh

A quick way to add the line from consone would be with echo:

echo 'mine:234:respawn:/path/to/script' >> /etc/inittab

Of course the proper paths, should be put in:

Then to load up the newly added inittab line, inittab needs to be reloaded with cmd:

# init q

I've also red, some other methods suggested to run programs on a periodic seconds basis using just cron, what I found in stackoverflow.com's  as a thread proposed as a solution is:

* * * * * /foo/bar/your_script
* * * * * sleep 15; /foo/bar/your_script
* * * * * sleep 30; /foo/bar/your_script
* * * * * sleep 45; /foo/bar/your_script

One guy, even suggested a shorted way with cron:

0/15 * * * * * /path/to/my/script

Possible way to increase Linux TCP/IP port thoroughput via sysctl kernel variable

Thursday, August 25th, 2011

Sysctl is a great way to optimize Linux. sysctl has a dozens of values which could drastically improve server networking and overall performance.

One of the many heplful variables to optimize the way the Linuz kernel works on busy servers is net.ipv4.ip_local_port_range .

The default sysctl setting for net.ipv4.ip_local_port_range on Debian, Ubuntu Fedora, RHEL, CentOS is:

net.ipv4.ip_local_port_range = 32768 65536

This means that the kernel and the corresponding server running services instructing the Linuz kernel open new port sockets can only open local ports in the range of 32768 – 65536 .
On a regular Desktop GNU/Linux machine or a not high iron server this settins is perfectly fine, however on a high scale servers the local port range in the interval of 32768-65536 might be insufficient at times, especially if there are programs which require binding of many local ports.

Therefore on a high load servers, generally it’s a good to raise the port range to be assigned by kernel to 8912 – 65536 , to do so the setting has to be changed like shown below:

linux:~# sysctl -w net.ipv4.ip_local_port_range = 8192 65536
...

If changing this setting on the server doesn’t show any negative impact on performance in few hours time or a day or even better decreases the server average load, it’s a good idea that it be added to sysctl.conf to load up the setting on next kernel boot.

linux:~# echo 'net.ipv4.ip_local_port_range' >> /etc/sysctl.conf

Enjoy 😉