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.