Monday, July 13, 2020

Useful command-line date utilities

My earlier post, Fun with Date Arithmetic shows how to use the date command to compute a future or past date that is a certain number of days ahead or in the past. This post expands on how you can manipulate dates on the Linux command-line interface using the cal and dateutils programs. These commands can give you results that GUI calendars can't.


While the date command works with the basic unit of days, say display the day that is 3 days from today, the cal command manipulates months, say display the month that is 2 months away.

Running cal without any argument displays the calendar for the current month.

$ cal
     July 2020
Su Mo Tu We Th Fr Sa
          1  2  3  4
 5  6  7  8  9 10 11
12 13 14 15 16 17 18
19 20 21 22 23 24 25 
26 27 28 29 30 31

While GUI calendar programs are well capable of displaying single month calendars, they are no match to the cal command for simultaneously displaying multiple months.

To display the current and the following say 2 months, use the -A2 argument:

$ cal -A2
        July                 August              September
Su Mo Tu We Th Fr Sa  Su Mo Tu We Th Fr Sa  Su Mo Tu We Th Fr Sa
          1  2  3  4                     1         1  2  3  4  5 
 5  6  7  8  9 10 11   2  3  4  5  6  7  8   6  7  8  9 10 11 12
12 13 14 15 16 17 18   9 10 11 12 13 14 15  13 14 15 16 17 18 19 
19 20 21 22 23 24 25  16 17 18 19 20 21 22  20 21 22 23 24 25 26
26 27 28 29 30 31     23 24 25 26 27 28 29  27 28 29 30                             
                      30 31

To display past months is equally easy with the -B argument. In fact, you can combine -B and -A to, for example, display the previous and the next month as follows:

$ cal -A1 -B1
     June 2020             July 2020            August 2020
Su Mo Tu We Th Fr Sa  Su Mo Tu We Th Fr Sa  Su Mo Tu We Th Fr Sa
    1  2  3  4  5  6            1  2  3  4                     1
 7  8  9 10 11 12 13   5  6  7  8  9 10 11   2  3  4  5  6  7  8
14 15 16 17 18 19 20  12 13 14 15 16 17 18   9 10 11 12 13 14 15
21 22 23 24 25 26 27  19 20 21 22 23 24 25  16 17 18 19 20 21 22
28 29 30              26 27 28 29 30 31     23 24 25 26 27 28 29
                                            30 31

cal has a special shortcut for displaying the above combination, namely, the current month with the immediate before and after month:

$ cal -3


While date is a nifty program to do date arithemetic, dateutils is a more versatile collection of tools for date manipulation.

To install the dateutils program on Debian,

$ sudo apt install dateutils


Use the dateutils.dadd sub-command to do date arithmetic. The program takes 2 input: a date and a duration either before or after the said date, specified in years, months, weeks, or days. For instance, to output the date that is 1 week and 2 days in the future from today, say 2020-07-10:

$ dateutils.dadd today +1w2d

You can replace the special keyword today with a specific date. For instance, to compute the date that is 1 year, 2 months and 3 days in the past from 2020-07-10:

$ dateutils.dadd 2020-07-10 -1y2m3d


Given 2 dates, dateutils.ddiff computes the duration between them. The default output unit is the number of days. You can customize the output units by specifying a format using the -f argument.

$ dateutils.ddiff 2020-07-02 2020-08-21 -f "%y years %m months %w weeks %d days"
0 years 1 months 2 weeks 5 days

Related posts:

. Fun with Date Arithmetic

Monday, July 6, 2020

Use Certbot to renew Let's Encrypt TLS certificates

You created a new website or perhaps even configured a SMTP mail server. You patted yourself on the back because you had not forgotten about securing the web and mail services. Specifically, you set up a free TLS certificate from Let's Encrypt. So, it is time to put your feet up and admire the good work you had done, right?

Not yet. The TLS certificate from Let's Encrypt would expire every 90 days, and is renewable only after 60 days. Doesn't this scream for automation?

Assuming that you have shell access to the host server, this blog post explains how to use Certbot to automate the renewal of Let's Encrypt certificates, and points out some gotchas to avoid.


The recommended way to deploy Let's Encrypt certificates on a Linux system is to use the certbot tool. This tutorial assumes that you have successfully used certbot to obtain and install a Let's Encrypt certificate.

The Let's Encrypt ecosystem with certbot is designed with automation in mind. When you install certbot on various Linux distributions such as Debian, Ubuntu, Fedora, CentOS, etc, the mechanism for certificate renewal is already put in place. What you need to do is make sure that the timer for certbot is enabled.

$ systemctl status certbot.timer
● certbot.timer - Run certbot twice daily
   Loaded: loaded (/lib/systemd/system/certbot.timer; enabled; vendor preset: enabled)
   Active: active (waiting) since Sat 2020-06-27 10:20:51 PDT; 3 days ago
  Trigger: Wed 2020-07-01 12:14:52 PDT; 2h 1min left

To enable the timer,

$ sudo systemctl enable certbot.timer 

Certbot is scheduled to automatically run twice daily to check if certificate renewal is needed. A certificate is only renewed, however, if expiry is impending—within 30 days before expiration.

For your peace of mind, you can verify the current status of your TLS certificates using the web tools SSL Test and

Gotcha # 1: Refreshing certificates

Renewing a certificate before it expires is only half the battle. The other half is to get the web server (and email server if applicable) to use the new certificate. The obsolete version is still in use until the webserver (and email server) is reloaded.

Certbot provides a hook interface to run scripts before or after a certificate is renewed. I used the Deploy hooks and the Pre-hooks to automate the reloading of web and email server programs.

Deploy hooks

A Deploy hook is run after the successful renewal of a certificate. Deploy hooks are placed in the /etc/letsencrypt/renewal-hooks/deploy directory.

If there is more than 1 script in the directory, the scripts are executed in alphabetical order based on their filenames.

I created 2 Deploy scripts, and to reload NGINX and postfix respectively. Both scripts should be executable(file permissions set to 770).

A typical script to reload NGINX is as follows.

$ sudo cat /etc/letsencrypt/renewal-hooks/deploy/
#! /bin/sh
set -e
/etc/init.d/nginx configtest
/etc/init.d/nginx reload 

To switch to the new certificate for postfix, both postfix and dovecot need to be reloaded.

$ sudo cat /etc/letsencrypt/renewal-hooks/deploy/
#! /bin/sh
set -e
/etc/init.d/postfix reload
/etc/init.d/dovecot reload


Pre-hooks are scripts to be run when a certificate is due for renewal, i.e., within 30 days prior to expiration, and before the renewal is actually performed. Those scripts are placed in the /etc/letsencrypt/renewal-hooks/pre/ directory.

I created a Pre-hook script named to email me when a certificate is due for renewal.

$ sudo cat /etc/letsencrypt/renewal-hooks/pre/ 
#! /bin/sh
set -e
echo 'Message' | /usr/bin/mail -s 'Subject' 

Manual execution

After auto-renewal is set up, there is little need to manually renew a certificate. But, you have the option to do so. To manually renew:

$ sudo certbot -q renew

The -q option suppresses all output except errors.

Recall that a certificate will only be renewed if it is within 30 days to expiry. You can override this restriction by specifying the --force-renewal option. Use it with caution however(see gotcha # 2).

$ sudo certbot -q --force-renewal renew

Gotcha # 2: Rate limit

Renewal is subject to a Duplicate Certificate limit of 5 per week. Please read the rate limits documentation to be acquainted with what counts toward this limit.

If the rate limit for a certificate is exceeded, renewal is temporarily suspended until the rate limit resets—on a sliding basis—after a week. For instance, if you renew a certificate 3 times on Monday and twice more on Friday, renewal is suspended until the following Monday.

Thursday, July 2, 2020

How to generate and read QR code on Linux

QR code, short for Quick Response code, was initially created to improve on bar codes used in inventory management. Nowadays, QR codes are ubiquitous, on posters, billboards, web pages, etc. This post will illustrate how to generate and read QR codes using the Linux command line interface (CLI).

The programs you will need to generate and read QR codes are qrencode and zbarimg respectively. (If you want to work with a GUI tool, there is QtQR.) To install the 2 programs on Debian:

$ sudo apt install qrencode zbar-tools 


A QR code is a matrix of square dots (or 'modules' in QRspeak). QR codes have as many as 40 versions of increasing data capacity. Version 1's dimension is 21 × 21 modules, and each higher version adds 4 modules per side ending with version 40 with 177 × 177 modules.

The exact maximum data capacity of a version depends on several factors, including the type of characters stored, e.g., numeric vs alphanumeric, and the level of error correction desired. At Medium error correcting capability, version 1 can store up to 20 alphanumeric characters; version 40, 3,391.

Fortunately, as we'll see next, the qrencode utility specifies good defaults, and hides much of the gory details from you.

QR code generation

In its simplest form, qrencode takes the input string to be encoded and outputs the PNG graphic to a file. The following command encodes the URL for this website.

$ qrencode -o webURL.png  ''

You can specify different parameters to fine-tune the QR code. Use the -l parameter to change the error correction level from the default L for Lowest to M for Medium, Q for Quite High, or H for Highest. In addition, you can explicitly specify the version to use, the size of the module and the margin, etc. The following example generates a version two QR code for the same website at the Highest error correcting level.

$ qrencode -o webURL.png -l H -v 2 ''

Besides the URL, marketers typically encode information such as phone numbers and email addresses.

$ qrencode  -o webPhone.png  '(604)555-1234'
$ qrencode  -o webEmail.png ''

Many QR code scanners will automatically open the associated app upon scanning a QR code of a special format, e.g., a browser for URLs, email client for email addresses, and phone app for telephone numbers.

QR code scanning

The Linux program zbarimg decodes the QR code stored in a file. To invoke, simply provide the input filename which contains the QR code.

$ zbarimg webURL.png
scanned 1 barcode symbols from 1 images in 0 seconds

If you specify the -d parameter, zbarimg will display the QR code in addition to the decoded information.

The default camera app of recent Android or iOS phones can also function as QR code scanner. To scan, run the camera app and point it towards the QR code.

Tuesday, June 23, 2020

Adding Google Analytics tracking code to a WordPress website

My previous post revealed how to piggyback a new WordPress website on an existing WordPress instance using what is known in WordPress-speak as multisite. In this post, I'll walk through how to embed the Google Analytics tracking code, aka the Global Site Tag, in the new website.

Website administrators want to know who, when, how, and what users do on their websites. Google Analytics can provide that information if the proper tracking code is found on the web pages.

There is more than 1 way to insert the tracking code, including using the Google Tag Manager or the WordPress plugin MonsterInsights. This post details a manual method of directly embedding the tracking code in a WordPress theme, assuming that you have already set up a Google Analytics account for the target website. In addition, I assume that you have created and activated a child theme for your website. If you are hosting multiple websites within the same WordPress instance, I assume you have a separate child theme for each site.

  1. Login to Google Analytics, and select the account corresponding to the target website.
  2. Click Admin on the LHS menu bar.
  3. Click Tracking Info and then Tracking Code in the middle column.
  4. Copy the Global Site Tag script to be pasted next in the WordPress theme.
  5. SSH into web host, and copy the header.php file from the parent theme to the children theme.
  6. $ cp /var/www/  /var/www/
  7. Paste the tracking code script.
    The script should be inserted in the header.php file in your child theme directory, say /var/www/, just before the call to wp_head() at the end of the header specification.
  8. Navigate back to Tracking Info/Tracking Code on Google Analytics and click Send Test Traffic.
    A new session of your website pops up in the browser.
  9. Navigate to Reports section on the LHS menu bar, click Realtime, then Overview.
    You should see the just opened session being counted in the number of active users on site.

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Sunday, June 14, 2020

How to migrate single site WordPress to multisite

The scenario

I had an existing WordPress website, say, that was hosted on a DigitalOcean VPS running LEMP(Linux, NGINX, MySQL, PHP). The website supported HTTPS using a TLS certificate issued by Let's Encrypt.

I wanted to start a new WordPress website, say Barring a miracle, would initially have minimal traffic.

I decided that the new site would run on a virtual host on the same VPS, using WordPress's multisite feature.

WordPress multisite

Hosting multiple websites/domains on the same VPS can be a double-edged sword. Suffice to say, the advantage is economy of scale, and the disadvantage, putting all one's eggs in one basket.

The multisite model in WordPress can be summarized as '1 instance, 1 database'. The multiple websites share the same WordPress DocumentRoot directory (/var/www/example1) and the same WordPress mySQL database.

Within the single database, site-specific information is stored in tables identified by the blog IDs. For instance, the wp_posts table for retains the same name in multisite. However, the corresponding table for is named wp_2_posts (the 2 in the name refers to the official blog ID).

Multisite introduces a new level of complexity in administration. Seeing multisite in action is the best way to know what you are getting into before actually migrating your production website.

Trialing the migration

Setting up a separate VPS with the same configuration as the production system is the best option for conducting a trial migration. Notwithstanding, I opted for a poor man's platform to test the migration, my home workstation.

I won't be able to completely replicate at home the production environment. Most notably, no HTTPS for the home machine because there won't be TLS certificates.

To reflect the change from HTTPS to HTTP, I modify 2 WordPress administrative options, siteurl and home. Run the following SQL commands under mySQL:

update wp_options set option_value='' where option_name = 'siteurl';
update wp_options set option_value='' where option_name = 'home';

Note that instead of reusing the same names for and, I renamed them to say and respectively. In addition, I configure local DNS on my home workstation to map and to localhost's IP address. The reason is that I can access both the production and the trial websites at the same time. Add the following lines to the /etc/hosts file:

The rest of this post will detail the steps to convert WordPress from hosting a single site to hosting multiple sites.

Configuring system

  1. Configure DNS.

    Register with a domain name registry and add the appropriate DNS records.

  2. Obtain Let’s Encrypt TLS certificate for second domain.

    I assume certbot is already installed, certbot.timer enabled, and port 443 open.

    Although it is possible to bundle multiple domains, and in a single certificate, it is recommended that you create separate certificates for unique domain names.

    $ sudo certbot certonly --webroot -n --agree-tos -m -w /var/www/ -d 

    -m: the email address for the certificate contact.

    -w: the DocumentRoot for which is the same as

    -d: the domain.

  3. Install certificate.

    Link the private key and the certificate generated by Let’s Encrypt to their respective expected TLS locations.

    $ sudo ln -s /etc/letsencrypt/live/ /etc/ssl/private/key2.pem 
    $ sudo ln -s /etc/letsencrypt/live/ /etc/ssl/certs/cert2.pem

    Note that the names key2.pem and cert2.pem must be different from their counterparts for Make a note of their names as you will need them later.

  4. Configure website.

    Create /etc/nginx/sites-available/ by copying and making the necessary changes.

    My skeleton file looks like the following. The highlighted lines are relevant to the migration per se.

    server {
        listen 80;
        return 301 https://$host$request_uri;
    server { 
        listen 443 ssl; 
        ssl_certificate     /etc/ssl/certs/cert2.pem;
        ssl_certificate_key /etc/ssl/private/key2.pem;
        root /var/www/; 
        server_name *;
        index index.html index.php;
        location / {
            try_files $uri $uri/ /index.php?q=$uri&$args;
        location ~ \.php$ {
          fastcgi_pass unix:/var/run/php/; 
        location ~* /(?:uploads|files)/.*\.php$ {
          deny all;
        location = /robots.txt {
          allow all;
          log_not_found off;
          access_log off;
        add_header X-XSS-Protection "1; mode=block";
        add_header X-Content-Type-Options nosniff;
        add_header X-Robots-Tag none;
        add_header X-Download-Options noopen;
        add_header X-Permitted-Cross-Domain-Policies none;
        add_header Strict-Transport-Security "max-age=15768000; includeSubDomains; preload";
        add_header Referrer-Policy no-referrer;
        add_header X-Frame-Options "SAMEORIGIN";
  5. Notes:

    • The location of the certificate (cert2.pem) and key (key2.pem) need to be specified.
    • The DocumentRoot location is the same as
    • The relevant server names are specified for this website(
    • PHP handling is listening to the same socket as (/var/run/php/
  6. Enable website.
    $ sudo ln -s /etc/nginx/sites-available/ /etc/nginx/sites-enabled/
  7. Reload NGINX.

    Test the syntax of the file changes before actually reloading the configuration files.

    $ sudo nginx -t
    $ sudo systemctl reload nginx

Configuring WordPress

  1. Install wp_cli.

    Although one can handcraft the necessary lines in the WordPress configuration file (/var/www/, I’d recommend using the command-line program [wp-cli]( To install, run this command sequence:

    $ curl -O
    $ chmod +x wp-cli.phar
    $ sudo mv wp-cli.phar /usr/local/bin/wp
  2. Convert to multisite.
    $ wp core multisite-convert --subdomains --path=/var/www/

    The above command can be run while the website is up because it only statically inserts the following lines into wp-config.php.

    define( 'WP_ALLOW_MULTISITE', true );
    define( 'MULTISITE', true );
    define( 'SUBDOMAIN_INSTALL', true );
    $base = '/';
    define( 'DOMAIN_CURRENT_SITE', '' );
    define( 'PATH_CURRENT_SITE', '/' );
    define( 'SITE_ID_CURRENT_SITE', 1 );
    define( 'BLOG_ID_CURRENT_SITE', 1 );
  3. Patch ‘blocked cookie’ bug.

    Unless the bug is patched, login to your new website ( is prevented. The error message from Firefox is ‘Cookies are blocked or not supported by your browser. You must enable cookies to use WordPress.’

    To patch, edit /var/www/, and insert the following define statement anywhere above the “That’s all” comment line.

    define('COOKIE_DOMAIN', false);
    /* That's all, stop editing! Happy publishing. */
  4. Restart the PHP-FPM and NGINX daemons.
    $ sudo systemctl restart php7.3-fpm
    $ sudo systemctl restart nginx

Creating new site

  1. Login to the original WordPress website using the URL(

    The ID to use to login is the same admin ID for In multisite, this admin ID is promoted to super-admin status, capable of administering all domains within the network.

  2. Click My Sites and select Network Admin and then Sites.
  3. Click Add New.
  4. Enter the required data, and click Add Site.
  5. The Site Address is where one is supposed to enter the URL except it expects a subdomain, such as example2 which it will then concatenate with the primary domain to become, not what I really wanted as in So for now, I simply play along by entering example2, and I will change it later. If you know a better way, please let me know in a comment.

    The Admin Email can be that of an existing user, say the super-admin, or a new user.

  6. Again, click My Sites and select Network Admin and then Sites.
  7. Hover over and click Edit.
  8. Enter the correct Site Address,, and click Save Changes.

Now, the new website is created and ready for you to edit. Browse to and login as the super-admin user.

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