LINUX

Apache webserver (httpd) service installation on CentOS 7 Linux

First set up the httpd (web) service

$ yum install httpd

If httpd service is already installed, update the service

$ yum update httpd

Now, start the web server

$ service httpd start

Redirecting to /bin/systemctl start httpd.service

Check the status of the web server

$ service httpd status

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Setup Virtualbox Additions on Centos 7 to share files

First step is to install dependencies

$ sudo yum groupinstall “Development Tools”

$ sudo yum install kernel-devel

Mount the Virtualbox Additions CD ISO. You should download the virtualbox additions iso from http://download.virtualbox.org/virtualbox/ depending on your version of Virtual Box.

$ sudo mkdir /media/cdrom/

$ sudo mount /dev/cdrom /media/cdrom/

$ cd /media/cdrom

$ sudo ./VBoxLinuxAdditions.run

Before this step, create a shared folder in your host system. Say for example, if you created a shared folder named “CentosShare” in the host system you will follow the steps given below to access the share from within Centos.

$ mkdir ~/share

$ mount -t vboxs CentosShare ~/share

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How to set apache server error status codes for beginners

Edit Apache configuration file for your site setting. It may be httpd.conf or a virtual host file that ends with .conf specific to your site.

Add the below line with the host file and give the correct path for the error message page.

ErrorDocument 404 /404.html

If you do not want to create a custom page to showcase the error message, you can simply add the error message in the configuration file

ErrorDocument 404 “404 – Page not found.

Error status codes on the client side

400 – Bad Request
401 – Unauthorized
402 – Payment Required
403 – Forbidden
404 – Not Found
405 – Method Not Allowed
406 – Not Acceptable
407 – Proxy Authentication Required
408 – Request Timeout
409 – Conflict
410 – Gone
411 – Length Required
412 – Precondition Failed
413 – Request Entity Too Large
414 – Request-URI Too Long
415 – Unsupported Media Type
416 – Requested Range Not Satisfiable
417 – Expectation Failed

Error status codes shown from server

500 – Internal Server Error
501 – Not Implemented
502 – Bad Gateway
503 – Service Unavailable
504 – Gateway Timeout
505 – HTTP Version Not Supported

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Prominent apache status code and its meaning for beginners

Informational

100 – Continue
101 – Switching Protocols

Successful

200 – OK
201 – Created
202 – Accepted
203 – Non-Authoritative Information
204 – No Content
205 – Reset Content
206 – Partial Content

Redirection

300 – Multiple Choices
301 – Moved Permanently
302 – Found
303 – See Other
304 – Not Modified
305 – Use Proxy
307 – Temporary Redirect
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PHP SYSTEM WARNING: ‘date(): It is not safe to rely on the system’s timezone settings

Edit /etc/php.ini and add the timezone setting within the file. For a list of timezone settings, visit http://php.net/manual/en/timezones.php


[Date] ; Defines the default timezone used by the date functions
; http://php.net/date.timezone
date.timezone = America/New_York

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How to avoid search engines from crawling your website?

Your answer is to create a robots.txt file in the root of your web directory and to have the code setting given below in the file.

User-agent: *
Disallow: /

You can read more about Robots exclusion protocol, here

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How to use different Python version with virtual environments?

Use the flag -p with virtualenv command to specify the python version that you would want to use

$ virtualenv -p /usr/bin/python/2.7

For windows environment, use

c:\> virtualenv –python=c:\Python27\python.exe myenv

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Python: How to setup virtual environment?

Different projects in python may require different modules and its dependencies. Also, there may be a necessity that certain projects be run on newer/older python releases hence introducing version conflicts. Virtual environment is a tool that helps you manage these scenarios.

To install virtualenv, use pip

$ pip install virtualenv

Now, create a virtual environment “myenv”

$ virtualenv myenv

To use the virtual environment, key in

$ source myenv/bin/activate

(if in windows, key in “myenv\Scripts\activate“)

The name of your virtual environment will appear on the left of the prompt

(myenv) …$

To exit out of the virtual environment, key in

$ deactivate

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Sudo, Visudo -bash: visudo: command not found

Visudo is needed to make modifications to the sudoers file. In a fresh install of CentOS “sudoers” file is not normally found. To install visudo issue the command

$ yum -y install sudo

As a sys admin, you need to be careful when performing tasks with root privileges. To be cautious, you can create a new user and assign him with root privileges using “visudo”

$ /usr/sbin/visudo

# User privilege specification
root ALL=(ALL) ALL
test ALL=(ALL) ALL

In the above file, test is a new user created with root privileges. In order for the “test” user to have root privileges, command “sudo” should be used. It prevents the user from issuing any commands that can cause system wide havoc and it also logs the command inside “/var/log/secure” for review

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PostgreSQL: Create database, connect and list (command line)

Changing host configuration
$ vi /var/lib/pgsql/8.4/data/pg_hba.conf

Add postgresql to startup process
$ chkconfig postgresql-8.4 on

Start postgresql service
$ service postgresql-8.4 start

Access postgresql
$ su – postgresql
$ psql

Create database
postgres-# create database test

List databases
postgres-# \list

Exit postgresql
postgres-# \q (or \quit)

Connect to database
postgres-# \c test (\connect test)

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