diff --git a/knowledge base/cron.md b/knowledge base/cron.md index 8ef9e8c..dabcc3d 100644 --- a/knowledge base/cron.md +++ b/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**.
- Normal crontab files will error out. -- Be **executable**.
- If not executable, execution will silently fail. -- Match the Debian cron script namespace (`^[a-zA-Z0-9_-]+$`).
- Files **with** an extension **won't** work. +> [!note] +> Crontabs in the spool directory should **not** be accessed directly.
+> The [`crontab`][crontab] command should be used to access and update them instead. + +`cron` also reads `/etc/crontab`, which uses a slightly different format (must provide the username to run the task as +in the task definition).
+Refer `crontab`'s manual page. + +On Linux, support for `/etc/cron.d` is included in the cron daemon itself, which handles this location as the +system-wide crontab spool.
+This directory can contain any file defining tasks following the format used in `/etc/crontab`). + +Debian uses a default configuration for the cron system that is specific to the distribution. + +
+ +Debian uses `/etc/crontab` to run crontabs under `/etc/cron.hourly`, `/etc/cron.daily`, `/etc/cron.weekly` and +`/etc/cron.monthly`. + +Additionally, `cron` reads files in `/etc/cron.d`.
+Those are treated in the same way as `/etc/crontab`, and are expected to follow the special format of that file. +However, they are **independent** from `/etc/crontab` and they **do not** inherit environment variable settings from +it. + +`/etc/crontab` and the files in `/etc/cron.d` must: + +- Be owned by the `root` user. +- **Not** be group- or other-writable. + +Files in the above directories are monitored for changes the same way `/etc/crontab` is.
+In contrast to the spool area, those files _can_ also be symlinks, as long as **both** the symlink **and** the file it +points to are owned by the `root` user.
+Files under `/etc/cron.d` do **not** need to be executable, but files under `/etc/cron.{hourly,daily,weekly,monthly}` +do, as they are run by `run-parts`. Refer `run-parts`' manual page. + +Files have to pass some sanity checks, including: + +- Be **executable**. +- Be owned by the `root` user. +- **Not** be writable by group or other. +- Be named conforming to the filename requirements of `run-parts` (must match `^[a-zA-Z0-9\_\-]$`).
+ Any file which name does **not** conform to these requirements will **not** be executed by `run-parts`. + +
+ +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.
+> 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.
+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.
+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.
+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] +[/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 diff --git a/knowledge base/crontab.md b/knowledge base/crontab.md index c249d52..cc470b7 100644 --- a/knowledge base/crontab.md +++ b/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' [cheat.sh]: https://cheat.sh/crontab +[crontab guru]: https://crontab.guru/