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Difference between hard link and symbolic link

I thought hard link is alias of a file and symbolic link is another file pointing to the linked file. So, I thought using symbolic link may be safer than using hard link. I thought “rm -f symlink” just delete

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How to setup apache proxy server?

When talking about apache proxy, we usually mean apache forward proxy. There is another kind of apache proxy: apache reverse proxy. The difference between apache forward proxy and apache reverse proxy is that apache forward server accepts connections from clients

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apachectl fullstatus

I never used “apachectl fullstatus” before. To get httpd status, I used “service httpd status” or “service httpd status -l” or “systemctl status httpd”. But now, I want to get more information about apache. You may hear about apache2ctl, but

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ERROR: cannot verify curl.se’s certificate, issued by “/C=US/O=Let’s Encrypt/CN=R3”:Issued certificate has expired.

When you wget some url such as:

, you may get this error: ERROR: cannot verify curl.se’s certificate, issued by “/C=US/O=Let’s Encrypt/CN=R3”:Issued certificate has expired. This is because the server of this url uses Let’s Encrypt issued certificate, while

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mysql change charset

Years ago, you may consider converting the charset of your mysql database from latin1 to utf8 as utf8 was prevailing and the default charset of mysql database is latin1. Today, you might want to change charset from utf8 to utf8mb4

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How to use yum in CentOS6.7 or earlier versions?

If you are using CentOS6.7 or earlier versions, you will sadly find you cannot use yum any longer. If you try to install a software using “yum install xxxx”, you will encounter the following errors: http://mirror.centos.org/centos/6/extras/i386/repodata/repomd.xml: [Errno 14] PYCURL ERROR

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How to use Mailx to read emails?

I found when talking about mailx, many tutorials teach you how to use it to send emails. This is ridiculous as the first thing you need to know about an email client is how to use it to read emails.

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Default environment for cron job

The cronjob scripts are run in different environment than in command line, which causes a lot of trouble for cronjob developers. We list here the default environment for cronjob:

You can see there are much fewer variables compared to

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How to install letsencrypt certificate?

Letsencrypt certificates are free to obtain. If you go to letsencrypt’s official website to learn how to get their certificates, you will be told to install a bunch of software such as snapd in order to get the certificate files

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How to enable/disable ipv6 on Windows 10?

You may find your ipv6 is disabled. Open network & internet settings, click “change adapter options”, double-click an adapter, you will see “IPV6 connectivity: No network access”. It is very weird because you have ticked the “Internet Protocol Version 6

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