Archive by category "How to"

Delete files older than

find ./your_directory/ -mtime +30 -type f -delete

Install truecrypt on Linux CentOS

This is small guide on how to install truecrypt on Linux CentOS.

Truecrypt is probably the greatest encryption software that I’ve used to protect my files, unfortunately it has been discontinued by it’s developers for some very strange reasons.

However, that doesn’t mean that you cannot install it on your Linux machine.
So here goes the install process for truecrypt 7.1a, the console version of this crypto tool:

1.  Download the tar.gz archive from here or use wget from the command line, see below.

- wget

2. Extract the truecrypt archive:

tar -xvf ./ truecrypt-7.1a-linux-console-x86.tar.gz
cd ./truecrypt-7.1a-linux-console-x86

3. Give execution permissions to the installer script.

chmod 755 ./truecrypt-7.1a-linux-console-x86

4.  Install the requirements: and

yum install

5. Run the truecrypt installer:

[root@lnx truecrypt]# ./truecrypt-7.1a-setup-console-x86

– select option 1

6.  Create a new volume with:

/usr/bin/truecrypt -c

– follow the easy steps in the volume setup

7. Mount the container to a specified directory:

[root@nix truecrypt]# truecrypt -t -k "" --protect-hidden=no container1 /media/truecrypt1
Enter password for /home/user1/truecrypt/container1:
[root@nix truecrypt]# cd /media/truecrypt1
[root@nix truecrypt1]# ls
[root@nix truecrypt1]# pwd

8. Dismount a container:

truecrypt -d 

9.  (Optional) Check for some nice info

That’s it. Have fun hiding your stuff!

How to empty large log files

If you want to empty large log files (aka lots of GB) without deleting the file, here’s how you can do that:



nix# > mylargelog.log


nix# echo " " > mylargelog.log


Search in archives

When using the linux shell daily we encounter situations when we need to search specific strings in one or more archives. If you are wondering how to search in archives for different patterns or strings, this tutorial will show you how.

You might have an archived log file and you want to search for the word “error”, here’s how you can do it.

Presenting zcat:

zcat is a linux console utility that takes as input compressed data files and send to stdout the results. Used with advanced utilities like cut, grep or awk, zcat becomes a very powerful application that helps the linux system administrator to search through archived files.

Here’s an example.

[root@nyx /]# zcat httpd-log_20140821.gz | awk -F ";" '($6~"error")

[Wed Aug 27 11:08:27 2014] [error] [client] PHP Warning: date_default_timezone_get(): It is not safe to rely on the system's timezone settings.

The explaining:

zcat parses httpd-log_20140821.gz, outputs the lines that have the word “error” in the 6th column of the log file.

Another example:

zcat logs_2014082* | awk -F ";" '($1=="Transaction timed out") | sort -u

Alert: a Transaction timed out error was received at 2014-08-20 1:33

Alert: a Transaction timed out error was received at 2014-08-21 10:03

The explanation:

zcat parses the archived files hat start with ” logs_2014082″, searches if the 1st column contains “Transaction timed out” then sorts the output and removes duplicate lines.

Introducing zgrep.
Like the similar grep command, zgrep is a linux utility that was developed for the sole purpose of matching patters or strings inside an archived file.


zgrep error httpd.log.gz

[Sat Aug 23 06:12:20 2014] [error] [client] File does not exist: /www/html/
[Sat Aug 23 06:12:21 2014] [error] [client] File does not exist: /www/html/


The explanation: zgrep searches the httpd.log.gz file for the “error” word and sends the output to stdout.


– allows you to filter archived or plain text files one screen a a time. As it’s name says it does basically the same thing as more but it can search.

How to save iptables

In this article you will find out how to save iptables firewall rules in Linux CentOS, Redhat, Debian and Ubuntu.

Iptables  is a Linux application built for the purpose of allowing a system administrator to configure and maintain specific firewall tables/rules provided by the Linux kernel firewall module.
There are currently 3 different kernel modules build for the IPv4, IPv6 and ARP stack protocols (iptables is used for IPv4, ip6tables refferes to IPv6, arptables to Addres Resolution Protocol).

The most used and the one used in this tutorial is iptables. In order to execute iptables related commands you need root privileges or to be in the sudoers file in Debian/Ubuntu OSes.

So here’s how to save iptables and how to list existing ones:

The syntax used to check the iptables service status is:

service iptables status  ##CentOS, Redhat
sudo iptables -L -n -v   ##Debian, Ubuntu


To start iptables:

service iptables start ##CentOS, Redhat
sudo ufw enable ##Ubuntu


To list iptables rules:

iptables -L


Save iptables rules:

iptables-save  #Debian
sudo iptables-save  #Ubuntu
service iptables save #CentOS

On CentOS, for example, you would see an output like this:

iptables save





To restore iptables rules:

~ # iptables-restore < /etc/sysconfig/iptables
~ #

Autostart services in Linux

chkconfig is a simple command-line tool that helps a Linux administrator configure,  maintain,  autostart and manage the configuration of the symlinks located in /etc/rc[0-6].d path.

First of all let me show you which are the most used services in a Linux distributuion.

You can find out what services can be started in your server by typing:

chkconfig --list

The output should look something like this:

~ # chkconfig --list
acpid           0:off   1:off   2:on    3:on    4:on    5:on    6:off
cgconfig        0:off   1:off   2:off   3:off   4:off   5:off   6:off
cgred           0:off   1:off   2:off   3:off   4:off   5:off   6:off
cmdavd          0:off   1:off   2:off   3:off   4:off   5:off   6:off
cmdmgd          0:off   1:off   2:off   3:off   4:off   5:off   6:off
crond           0:off   1:off   2:on    3:on    4:on    5:on    6:off
htcacheclean    0:off   1:off   2:off   3:off   4:off   5:off   6:off
httpd           0:off   1:off   2:on    3:on    4:on    5:on    6:off
ip6tables       0:off   1:off   2:on    3:on    4:on    5:on    6:off
iptables        0:off   1:off   2:on    3:on    4:on    5:on    6:off
mailman         0:off   1:off   2:on    3:on    4:on    5:on    6:off
mdmonitor       0:off   1:off   2:on    3:on    4:on    5:on    6:off
mysqld          0:off   1:off   2:on    3:on    4:on    5:on    6:off
named           0:off   1:off   2:on    3:on    4:on    5:on    6:off
netconsole      0:off   1:off   2:off   3:off   4:off   5:off   6:off
netfs           0:off   1:off   2:off   3:on    4:on    5:on    6:off
network         0:off   1:off   2:on    3:on    4:on    5:on    6:off
ntpd            0:off   1:off   2:off   3:off   4:off   5:off   6:off
ntpdate         0:off   1:off   2:off   3:off   4:off   5:off   6:off
portreserve     0:off   1:off   2:on    3:on    4:on    5:on    6:off
psa             0:off   1:off   2:on    3:on    4:on    5:on    6:off
qmail           0:off   1:off   2:on    3:on    4:on    5:on    6:off
rdisc           0:off   1:off   2:off   3:off   4:off   5:off   6:off
restorecond     0:off   1:off   2:off   3:off   4:off   5:off   6:off
rsyslog         0:off   1:off   2:on    3:on    4:on    5:on    6:off
saslauthd       0:off   1:off   2:off   3:off   4:off   5:off   6:off
spamassassin    0:off   1:off   2:on    3:on    4:on    5:on    6:off
squid           0:off   1:off   2:off   3:off   4:off   5:off   6:off
sshd            0:off   1:off   2:on    3:on    4:on    5:on    6:off
xinetd          0:off   1:off   2:on    3:on    4:on    5:on    6:off

xinetd based services:
        chargen-dgram:  off
        chargen-stream: off
        daytime-dgram:  off
        daytime-stream: off
        discard-dgram:  off
        discard-stream: off
        echo-dgram:     off
        echo-stream:    off
        ftp_psa:        on
        poppassd_psa:   on
        rsync:          off
        smtp_psa:       on
        smtps_psa:      on
        submission_psa: on
        tcpmux-server:  off
        time-dgram:     off
        time-stream:    off

The left column contains the name of the process, to the right you have 7 columns, each one represents a Linux runlevel. Usually you will use runlevels 3,4,5. Runlevel 0 and 6 are related to shutdown (0) and reboot (6), so you should never use these.

The “on” and “off” reffer to the fact that a specific service will autostart (on) or will not autostart (off) after a reboot of the server.

The manual of chkconfig can be accesed via the command:

~ # chkconfig --help
chkconfig version - Copyright (C) 1997-2000 Red Hat, Inc.
This may be freely redistributed under the terms of the GNU Public License.usage:   chkconfig [--list] [--type ] [name]
         chkconfig --add 
         chkconfig --del 
         chkconfig --override 
         chkconfig [--level ] [--type ]  <on|off|reset|resetpriorities>
root@nyxware~ #

To autostart a service after each reboot you would use a command like this:

chkconfig --level 345 httpd on

or just

chkconfig httpd on

To stop a process from starting after each reboot enter the command:

chkconfig httpd off

or stop it from running at a specific runlevel:

# chkconfig --level 3 httpd off

Use grep to see the status of a specific service:

~ # chkconfig --list | grep ssh
sshd            0:off   1:off   2:on    3:on    4:on    5:on    6:off
~ #

  * replace httpd with your desired service name.

Show only specific columns in Linux log files

To troubleshoot specific application errors, a sysadmin needs to check his logs for warning, errors or other useful information.
However, most server logs are quite unreadable or contain too much information. This kind of situation is unacceptable and you need to extract only specific columns from that log.
Let’s say you are checking the Apache error log and you want to extract only a specific column. I’m trying to search the log for the word error, the display the IP’s that are getting that error and count how many times the error occurs per IP.
Here’s how you can do it.

First you need to find out where the Apache log file is. In my case it is /etc/httpd/logs/error_log (Redhat, CenOS). If you use Debian or Ubuntu the location will be /var/log/apache2/error.log.

Now that you know the log location, put the path in the command below:

cat /etc/httpd/logs/error_log | grep error | cut -d" " -f8 | uniq -c | sort

You should see something like this:

[root@nyx ~]# cat /etc/httpd/logs/error_log | grep error | cut -d" " -f8 | uniq -c | sort

So, the command syntax is pretty simple: I concatenate the log file, match the “error” word, then print the 8th column, remove duplicates, count the occurrence per IP and sort the list. Voila!

Now you know which IPs are receiving errors when they are visiting your site. You might want to do some checks and see what the problem is. Maybe those IPs are just running bots that are searching for vulnerabilities and it might be a good idea to block them in your firewall.

How to delete files in Linux

During everyday usage of the Linux operating system, you will encounter situations when you will need to delete one or more files from the file system.

The easiest way to delete a file from BASH is to issue the rm command.

In most Linux distributions the rm command is an alias for “rm -i”, so when you enter rm in the shell, the alias points to rm -i (the “i” stands for interactive).

[root@nyx backup]# rm wordpress.sql
rm: remove regular file `wordpress.sql'? y
[root@nyx backup]#

Delete the file without a confirmation:

[root@nyx backup]# rm -f wordpress.sqlwordpress.sql
[root@nyx backup]#

Delete all the files in the folder called “backup”.  -r stand for recursively, -f  is forcing the delete command without a confirmation. Be careful when you use “-r -f” !

[root@nyx backup]# rm -r -f ./backup/
[root@nyx backup]#





Find files older than x days and delete them:

find /home/backup/* -mtime +30 -exec rm {} \;  
## find files older than 30 days from the /home/backup directory and delete them