Sometimes it’s necessary to install cronjobs out of scripts. It’s not a good idea to directly write to /var/spool/cron/crontabs. Various things might happen like permission problems or simply problems with the format. It’s better to make this in two steps.
First create a crontab-file, like /tmp/mytmpcronfile0001:
*/5 * * * * echo hello world >> /tmp/hello.txt
30 22 * * * echo foo bar >> /tmp/hello.txt
Now we can install this crontab for the current user by calling crontab:
crontab /tmp/mytmpcronfile0001
To list all cronjobs we can use:
crontab -l
User root is also able to install cronjobs for other users:
crontab -u someuser /tmp/mytmpcronfile0001
Let’s do this in Ruby
The following code demonstrates how we can do this in Ruby. There is no error handling in this code, so be aware of this if you want to use it.
#!/usr/bin/env ruby
class Crontab
attr_accessor :hour
attr_accessor :min
attr_accessor :mday
attr_accessor :month
attr_accessor :wday
attr_accessor :command
def initialize
@hour = '*'
@min = '*'
@mday = '*'
@month = '*'
@wday = '*'
@command = "/bin/false"
end
end
class Crony
attr_accessor :cronfile
def initialize(file)
@cronfile = file
end
def readfile
ret = Array.new
if not File.exist?(@cronfile)
raise "File doesn't exist"
end
file = File.open(@cronfile,"r")
file.each do |line|
if line !~ /^\s*#/
h = parse(line)
ret.push(h)
end
end
file.close
return ret
end
def writefile(hashy)
file = File.open(@cronfile,"w")
hashy.each do |cron|
file.write("#{cron.min} #{cron.hour} #{cron.mday} #{cron.month} #{cron.wday} #{cron.command}\n")
end
file.close
command = "crontab #{@cronfile}"
`#{command}`
end
def parse(line)
c = Crontab.new
if line =~ /(\*|\d+)\s(\*|\d+)\s(\*|\d+)\s(\*|\d+)\s(\*|\d+)\s(.+)$/
c.min = $1
c.hour = $2
c.mday = $3
c.month = $4
c.wday = $5
c.command = $6
else
raise "Invalid cron-entry"
end
return c
end
end
crony = Crony.new("/tmp/some_cronjob")
arr = Array.new
day = Crontab.new
day.hour= "09"
day.min="30"
day.command='echo hello day >> /tmp/hello.txt'
arr.push(day)
night = Crontab.new
night.hour= "22"
night.min="45"
night.command='echo hello night > hello.txt'
arr.push(night)
crony.writefile(arr)
