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
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
|
---
title: "Cronjobs"
date: 2020-07-01
tags: ["server"]
---
Cron is a service that lets you run scheduled tasks on a computer. These tasks
are called **cronjobs.** If you have already followed the initial course you
will have already used cron when you set up Certbot, but we'll explain how they work generally here.
## What tasks would I want to schedule?
You can schedule anything! Some examples of what you might have done
already include:
- `updatedb` to update your `locate` database to let you quickly search for files
- `certbot` to update renewing of your https certs
Some tasks that you might _want_ to schedule may include:
- Package updates - if you really just want to leave your server alone
you can automate updating packages on your server
- Backups - you may want to backup certain files every day and some
every week, this is possible with cron
And many more, anything you can do can be turned into a cronjob.
## Basic Cronjobs
This the preferred method for personal tasks and scripts; it\'s also the
easiest to get started with. Run the command `crontab -e` to access your
user\'s crontab
Once you have figured out the command you want to run you need to figure
out how often you want to run it and when. I am going to schedule my
system updates once a week at 3:30 AM on Mondays.
We now have to convert this time (Every Monday at 3:30 AM) into a cron
time. Cron uses a simple but effective way of scheduling when to run
things.
Crontab expressions look like this `* * * * * command-to-run` The five
elements before the command tell when the command is supposed to be run
automatically.
So for our Monday at 3:30 AM job we would do the following:
```txt
.---------------- minute (0 - 59)
| .------------- hour (0 - 23)
| | .---------- day of month (1 - 31)
| | | .------- month (1 - 12)
| | | | .---- day of week (0 - 6)
| | | | |
* * * * *
30 3 * * 1 apt -y update && apt -y upgrade
```
### Some notes
- On the day of the week option, Sunday is 0 and counting up from
there, Saturday will be 6.
- `*` designates \"everything\". Our command above has a `*` in the
day of month and month columns. This means it will run regardless of
the day of the month or month.
- The hour option uses 24 hour time. 3 = 3AM, while we'd use 15 for 3PM.
### More examples
Let\'s add another job, our backup job (for the purpose of this our
backup command is just called `backup`). We want to run `backup` every
evening at 11PM. Once we work out the timings for this we can add this new job
to the same file as the above by running `crontab -e` This would mean our
full crontab would look like this:
```txt
0 23 * * * backup
```
### Consecutive times
Suppose we want a command to run every weekday. We know we can put `1`
(Monday), but we can also use `1-5` to signify from day 1 (Monday) to
day 5 (Friday).
```txt
0 6 * * 1-5 echo "Wakey, wakey, wagie!" >> /home/wagie/alarm
```
The above `echo` command runs every Monday through Friday at 6:00AM.
### Non-consecutive times
We can also randomly specify non-consecutive arguments with a comma.
Suppose you have a script you want to run at the midday of the 1st,
15th, and 20th day of every month. You can specify that by putting
`1,15,20` for the day of the month argument:
```txt
0 12 1,15,20 * * /usr/bin/pay_bills_script
```
### \"Every X minutes/days/months\"
We can also easily run a command every several minutes or months,
without specifying the specific times:
```txt
*/15 * * * * updatedb
```
This cronjob will run the `updatedb` command every 15 minutes.
### Beware of this Rookie Mistake Though\...
Suppose you want to run a script once every other month. You might be
_tempted_ write this:
```txt
* * * */2 *
```
That might _feel right_, but this script _will be running once every
minute during that every other month_. You should specify the first two
arguments, because with `*` it will be running every minute and hour!
```txt
0 0 1 */2 *
```
This makes the command run _only_ at 0:00 (12:00AM) on the first day of
every two months, which is what we really want.
Consult the website [crontab.guru](https://crontab.guru) for an
intuitive and interactive tester of cronjobs.
## User vs. Root Cronjobs
It is important to note that user accounts all have different cronjobs.
If you have a user account `chad` and edit his crontab with
`crontab -e`, the commands you add will be run as the `chad` user, not
`root` or anyone else.
Bear in mind that if you need root access to run a particular command,
you will usually want to add it as root.
## System-wide cron directories
`crontab -e` is the typical interface for adding cronjobs, but it\'s
important to at least know that system-wide jobs are often stored in the
file directory. Some programs which need cronjobs will automatically
install them in the following way.
Run the command `ls /etc/cron*` and you should see a list of directories and
their contents. The list of directories should look something like the below:
- /etc/cron.d _This is a crontab like the ones that you create with_
`crontab -e`
- /etc/cron.hourly
- /etc/cron.daily
- /etc/cron.weekly
- /etc/cron.monthly
The directories cron.{hourly,daily,weekly,monthly} are where you can put
**scripts** to run at those times. You don\'t put normal cron entries
here. I prefer to use these directories for system wide jobs that don\'t
relate to an individual user.
## Contribution
- Mark McNally \-- [website](https://mark.mcnally.je),
[Youtube](https://www.youtube.com/channel/UCMiInY8BhSUtCarO6uu6i_g)
- Edits and examples by Luke
|
