How do you set a DATE variable to use in a log for crontab output?
Solution 1
Cron is not a shell - it does not parse commands in the same way that a shell does. As such, your variable is assigned as if it was static text.
There are three solutions I know of to this problem:
Option 1: Use a shell script to generate your command, include whatever variables and logic you want - and call that shell script from cron.
* * * * * /path/to/myscript.sh
Where myscript.sh
:
DATEVAR=`date +20\%y\%m\%d_\%H\%M\%S`
echo $DATEVAR >> /tmp/crontab.log
Option 2: Include the date command directly in your command, and, since the entire command is passed to the shell, the date will be processed and replaced with an actual date.
* * * * * /bin/echo `date +20\%y\%m\%d_\%H\%M\%S` >> /tmp/crontab.log
Option 3: Set the string variable in cron, and pass that to your command to be processed (note - the percent signs do not need to be escaped, and the variable itself is wrapped in $() to execute it in a separate shell - backticks should work the same):
DATEVAR=date +20%y%m%d_%H%M%S
* * * * * /bin/echo $($DATEVAR) >> /tmp/crontab.log
(In all the cases above, you can, of course, use a variable for the log path, instead of 'hard coding' it.)
Solution 2
The "problem" is that cron
uses the percent sign as a special character. You have to quote it so it is ignored by cron
, but not by the shell. Like most of the folks who I saw that spent hours upon hours tinkering around with when and how and where to quote what, I went through that hack-and-slash as well.
Here's the solution I found that has worked on all flavors of *nix:
# Define the percent sign in a variable.
P=%
#
# Now use it in a $() subshell. Don't get freaked about the double quotes
# inside the $(), they are processed by the subshell.
57 2 * * * root /bin/echo "[CRON] at $(date "+${P}d-${P}b${P}Y")" 1>>/tmp/crontab.log 2>&1
As the old saying in the world of Perl: there is always more than one way to do it. I am not saying this is the only way or the best way -- what I am saying is that this works for crontab entries running on CentOS, Debian, Ubuntu, Mac, and AIX, so this is what am sticking with.
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qodeninja
I write qode mostly for myself... out of curiosity for solving problems, understanding how things work or making (sometimes unnecessarily) complex systems to only simplify them later (once I discover alternative strategies). For whatever reason, I like torturing myself with Regular Expressions, SED, Bash and JavaScript (Node), but have found a growing (painful) love with Python. Having said that, I enjoy scripting languages a lot more than compiled languages, and I've coded in almost all of the major modern ones except Ruby. I'm a secret Turing Machine/Computer Grammars/Regular Expressions nerd, and have written my own mini compilers and toy languages. I'm constantly writing command dispatchers that I later write scripting languages for; it's an addiction. There's plenty room for me to grow and learn still; and I appreciate the wisdom of grey beards and lady wizards even if I don't always follow their sage advice. FOSS is hella cool; cool projects are cool. Find me online if you have ideas. I'm a really bad programmer but I'll write a line or two for the betterization of the peoples. Edit: I recently discoved that VI is really just SED with wings. Still not using VI. Nano or bust.
Updated on September 18, 2022Comments
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qodeninja almost 2 years
I was playing aroudn with some variations of date like
DATE = $(date)
but that didnt work either
crontab -e
CRONLOG=/tmp/log/crontab.log DATEVAR=`date +20\%y\%m\%d_\%H\%M\%S` * * * * * echo $DATEVAR >> /tmp/log/crontab.log */2 * * * * echo "$DATEVAR hello" >> ${CRONLOG} */1 * * * * echo 'every minute' >> ${CRONLOG}
this just outputs the text as is...
I want to create a log entry in crontab.log with a timestamp on each update
How can I do this on CentOS 6?
UPDATE
DATEVAR=date +20%y%m%d_%H%M%S */1 * * * * /bin/echo [CRON] $($(DATEVAR)) >> /tmp/log/crontab.log
rendered only [CRON] and NOTHING when I tried it =/
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qodeninja over 12 yearsthanks for the great feedback +1, i tried to add it to the crontab but doing that didnt work =/ it never renders DATEVAR
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cyberx86 over 12 yearsI just did this on a RHEL/CentOS6 compatible system and it gave the expected result (i.e. added a new line to the file with the date). Which option (of the 3 above) did you try, and what was the result - be specific - and was there an error in /var/log/cron? (if you use the script option, a) don't forget to chmod +x and b) try it on its own first (i.e. not through cron))
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qodeninja over 12 yearsoption 3 =], I had tried to do that before but it didnt render. But now im noticing you did something a lil dif. why did you double up the $($(DATE))? see my update above
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cyberx86 over 12 yearsYou have a typo in the update you added - it is
$($DATEVAR)
not$($(DATEVAR))
. Think of it this way -$()
- that is, the outer bracket - launches a new shell to process whatever is in the brackets. In this case, the variable $DATEVAR is set to a string and is passed to the shell.$(DATEVAR)
is nothing - becauseDATEVAR
is nothing (i.e. is it missing a $ to identify it as a variable). You could also use backticks around$DATEVAR
to get the same result -
qodeninja over 12 yearsfair enough. updated it. lets try this bad boy! -- Hazaaah! it worked lol