chore(kb): improve cron-related articles

knowledge base/cron.md

@@ -1,21 +1,103 @@
 # Cron
 
+The `cron` command is a runner for something that needs to happen on a schedule.
+
+There are different implementations (Dillon's cron, Vixie's cron, `chrony`, …), and variations of it (e.g., `anacron`,
+systemd timers).
+
 1. [TL;DR](#tldr)
 1. [Further readings](#further-readings)
    1. [Sources](#sources)
 
 ## TL;DR
 
-Files in `/etc/cron.hourly` and similar must:
+`cron` searches its spool area (`/var/spool/cron/crontabs` on Linux systems, `/usr/lib/cron/tabs` on Mac OS X) for
+crontab files named after accounts in `/etc/passwd`, and loads those it founds into memory.
 
-- Be **scripts**.<br/>
-  Normal crontab files will error out.
-- Be **executable**.<br/>
-  If not executable, execution will silently fail.
-- Match the Debian cron script namespace (`^[a-zA-Z0-9_-]+$`).<br/>
-  Files **with** an extension **won't** work.
+> [!note]
+> Crontabs in the spool directory should **not** be accessed directly.<br/>
+> The [`crontab`][crontab] command should be used to access and update them instead.
+
+`cron` also reads `/etc/crontab`, which uses a slightly different format (must provide the username to run the task as
+in the task definition).<br/>
+Refer `crontab`'s manual page.
+
+On Linux, support for `/etc/cron.d` is included in the cron daemon itself, which handles this location as the
+system-wide crontab spool.<br/>
+This directory can contain any file defining tasks following the format used in `/etc/crontab`).
+
+Debian uses a default configuration for the cron system that is specific to the distribution.
+
+<details style='padding: 0 0 1rem 1rem'>
+
+Debian uses `/etc/crontab` to run crontabs under `/etc/cron.hourly`, `/etc/cron.daily`, `/etc/cron.weekly` and
+`/etc/cron.monthly`.
+
+Additionally, `cron` reads files in `/etc/cron.d`.<br/>
+Those are treated in the same way as `/etc/crontab`, and are expected to follow the special format of that file.
+However, they are **independent** from `/etc/crontab` and they **do not** inherit environment variable settings from
+it.
+
+`/etc/crontab` and the files in `/etc/cron.d` must:
+
+- Be owned by the `root` user.
+- **Not** be group- or other-writable.
+
+Files in the above directories are monitored for changes the same way `/etc/crontab` is.<br/>
+In contrast to the spool area, those files _can_ also be symlinks, as long as **both** the symlink **and** the file it
+points to are owned by the `root` user.<br/>
+Files under `/etc/cron.d` do **not** need to be executable, but files under `/etc/cron.{hourly,daily,weekly,monthly}`
+do, as they are run by `run-parts`. Refer `run-parts`' manual page.
+
+Files have to pass some sanity checks, including:
+
+- Be **executable**.
+- Be owned by the `root` user.
+- **Not** be writable by group or other.
+- Be named conforming to the filename requirements of `run-parts` (must match `^[a-zA-Z0-9\_\-]$`).<br/>
+  Any file which name does **not** conform to these requirements will **not** be executed by `run-parts`.
+
+</details>
+
+The `cron` system wakes up every minute, examines all stored crontabs, and checks each command to see if it should be
+executed in the current minute.
+
+When executing commands, any output is mailed _to_ the owner of the crontab (or to the user named in the `MAILTO`
+environment variable in the crontab, if such exists) _from_ the  owner  of  the crontab (or from the email address
+given in the `MAILFROM` environment variable in the crontab, if such exists).
+
+> [!note]
+> Children copies of `cron` running processes have their name coerced to uppercase.<br/>
+> Those will be visible in the output of the `syslog` and `ps` commands.
+
+Additionally, `cron` checks every minute if its spool directory's modtime (or the modtime for the `/etc/crontab` file)
+has changed. In this case, `cron` examines the modtime on all crontabs and reloads those which have changed.<br/>
+This _prevents_ `cron` from needing to be restarted whenever a crontab file is modified.
+
+> [!note]
+> The `crontab` command updates the modtime of the spool directory whenever it changes any crontab in it.
+
+Clock changes of more than 3 hours are considered to be corrections to the clock, and the new time is used
+immediately.<br/>
+When the system clock is changed by less than 3 hours (e.g., at the beginning and end of daylight savings time), jobs
+will be executed accordingly:
+
+- If the time moved forwards, jobs which would have run in the time that was skipped will be run soon after the time
+change.
+- if the time moved backwards by less than 3 hours, jobs _that fall into the repeated time_ will **not** be run again.
+
+Only jobs running at a particular time (not specified as `@hourly`, nor with `*` in the hour or minute specifier) are
+affected by time changes.<br/>
+Jobs specified with wildcards are run based on the new time immediately.
+
+`cron` logs its action to the `cron` syslog facility. Logging can be checked using the standard `syslogd` facility.
+
+Modern cron implementations have added the `@hourly`, `@daily`, `@weekly`, `@monthly`, and `@yearly` or `@annually`
+shorthands for common schedules.
 
 ```sh
+apt install 'cron'
+
 systemctl --enable --now 'crond.service'
 journalctl -xefu 'crond.service'
 
@@ -32,6 +114,9 @@ run-parts --report --test '/etc/cron.hourly'
 
 # Manually run crontab files in given directories.
 run-parts '/etc/cron.weekly'
+
+# Check scripts run.
+grep 'CRON' '/var/log/syslog'
 ```
 
 ## Further readings
@@ -42,6 +127,9 @@ run-parts '/etc/cron.weekly'
 
 - [Function of /etc/cron.hourly]
 - [Use anacron for a better crontab]
+- [Linux tips for using cron to schedule tasks]
+- [`/var/spool`: Application spool data][/var/spool: application spool data]
+- [Cron Jobs: The Complete Guide]
 
 <!--
   Reference
@@ -52,5 +140,8 @@ run-parts '/etc/cron.weekly'
 [crontab]: crontab.md
 
 <!-- Others -->
+[/var/spool: Application spool data]: https://refspecs.linuxfoundation.org/FHS_3.0/fhs/ch05s14.html
+[Cron Jobs: The Complete Guide]: https://cronitor.io/guides/cron-jobs
 [function of /etc/cron.hourly]: https://askubuntu.com/questions/7676/function-of-etc-cron-hourly#607974
+[Linux tips for using cron to schedule tasks]: https://opensource.com/article/21/11/cron-linux
 [use anacron for a better crontab]: https://opensource.com/article/21/2/linux-automation

knowledge base/crontab.md

@@ -41,6 +41,7 @@ sudo crontab -r -u 'nana'
 ## Further readings
 
 - [Cron]
+- [Crontab guru]
 
 ### Sources
 
@@ -56,3 +57,4 @@ sudo crontab -r -u 'nana'
 
 <!-- Others -->
 [cheat.sh]: https://cheat.sh/crontab
+[crontab guru]: https://crontab.guru/