├── README.md
└── systemd-timers-jan30-2022.md


/README.md:
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1 | systemd-timers-jan30-2022.md


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/systemd-timers-jan30-2022.md:
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  1 | # Systemd Timers
  2 | 
  3 | 
  4 | ## Basic problem
  5 | 
  6 | Cron is the 'good old standard' for periodic task management in Unix.
  7 | 
  8 | However, it is FAR from standard, can behave differently (or be configured differently) depending on service/config file/user.
  9 | 
 10 | How do you make a cron job? That depends on: cron, anacron, fcron, jobber, etc. My point is: this is not a steady, unified thing the way some Posix things or Unix traditions are, and it can be annoying depending on how complex your setup grows to be (logging, etc.).
 11 | 
 12 | So now I GENERALLY find myself using systemd timers, with good results.
 13 | 
 14 | 
 15 | ### Benefits:
 16 | 
 17 | They're systemd units, so they can do everything other systemd units can:
 18 | - have dependencies (other units)
 19 | - fine granularity for triggering (hw state changes, other events systemd lets you react to)
 20 | - easy management via systemctl enable/disable/start/stop
 21 | - Timer events can be scheduled based on finish times of executions some delays can be set between executions.
 22 | - OnFailure
 23 | - EnvironmentFile and other systemd unit goodies
 24 | - systemcall filters
 25 | - user/group ids
 26 | - membershipcontrols
 27 | - nice value
 28 | - OOM score
 29 | - IO scheduling class and priority
 30 | - CPU scheduling policy CPU
 31 | - affinity umask
 32 | - timer slacks
 33 | - secure bits
 34 | - network access
 35 | - etc. etc. etc.
 36 | 
 37 | 
 38 | ### Downsides
 39 | 
 40 | - can get a bit messy, lots of custom files instead of text in a single file.
 41 | 
 42 | 
 43 | ## Basic structure
 44 | 
 45 | Cron job line consists of:
 46 | 1. scheduling information
 47 | 2. action information.
 48 | 
 49 | Systemd timers work the same way, but split each concept into its own file.
 50 | 2 unit files: a service and a timer. One benefit you'll notice right away is that this lets us start/test these service units independently of their timers
 51 | 
 52 | ### Create demo service
 53 | 
 54 | `sudo vim /usr/local/bin/tutorialinux-systemd-timers.sh`
 55 | 
 56 | ```
 57 | #!/usr/bin/env bash
 58 | echo "I just ran on $(date)"
 59 | exit 0
 60 | ```
 61 | 
 62 | Make it executable:
 63 | `sudo chmod +x /usr/local/bin/tutorialinux-systemd-timers.sh`
 64 | 
 65 | 
 66 | ### Create systemd service unit file
 67 | 
 68 | `sudo vim /etc/systemd/system/tutorialinux.service`
 69 | 
 70 | 
 71 | ```
 72 | [Unit]
 73 | Description=Run the tutorialinux echo service
 74 | 
 75 | [Service]
 76 | ExecStart=/usr/local/bin/tutorialinux-systemd-timers.sh
 77 | ```
 78 | 
 79 | 
 80 | ### Create systemd timer unit file
 81 | 
 82 | `sudo vim /etc/systemd/system/tutorialinux.timer`
 83 | 
 84 | ```
 85 | [Unit]
 86 | Description=tutorialinux service timer
 87 | 
 88 | [Timer]
 89 | OnBootSec=0min
 90 | OnCalendar=*:*:0/30
 91 | Unit=tutorialinux.service
 92 | 
 93 | [Install]
 94 | WantedBy=multi-user.target
 95 | 
 96 | ```
 97 | 
 98 | ## Cool features: testing
 99 | 
100 | Test your time formats without saving/re-running your cronjob a bunch of times.
101 | 
102 | `systemd-analyze calendar "*:*:0/30"`
103 | 
104 | Based on: `man systemd.time`
105 | 
106 | 
107 | ### Turn it on!
108 | 
109 | Tell systemd about the new unit files, enable the timer,  and you're good to go!
110 | 
111 | `sudo systemctl daemon-reload`
112 | 
113 | `sudo systemctl start tutorialinux.timer`
114 | 
115 | If you want to keep this around after a reboot:
116 | 
117 | `sudo systemctl enable tutorialinux.timer`
118 | 
119 | 
120 | 
121 | 
122 | ## Logging
123 | 
124 | Just like any other systemd unit -- check the journal!
125 | 
126 | `journalctl -fu tutorialinux.service`
127 | 
128 | 
129 | ## Sources
130 | 
131 | - [Official Docs](https://www.freedesktop.org/software/systemd/man/systemd.timer.html)
132 | - [trstringer blog](https://trstringer.com/systemd-timer-vs-cronjob/)
133 | - [Timers vs Cronjobs - stackexchange](https://unix.stackexchange.com/questions/278564/cron-vs-systemd-timers)
134 | 
135 | 


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