Cron is a job scheduler that allows users to run commands periodically.
25
votes
12answers
2k views
Run unix command precisely at very short intervals WITHOUT accumulating time lag over time
Question
I'd like to be able to run a UNIX command precisely every second over a long time period.
I need a solution, which does not lag behind after a certain time, because of the time the command ...
20
votes
6answers
6k views
Correct locking in shell scripts?
Sometimes you have to make sure that only one instance of a shell script is running at the same time.
For example a cron job which is executed via crond that does not provide
locking on its own (e.g. ...
19
votes
7answers
17k views
Where are cron errors logged?
If I setup cron jobs incorrectly they appear to silently fail. Where should I look for an error log to understand what went wrong?
13
votes
5answers
461 views
How to Find a Fugitive Crontab
A few years ago I setup a cron job to automatically ping a URL every minute as part of a monitoring system (that's oversimplification, but it'll do for this question). Because I'm a horrible person, I ...
12
votes
3answers
2k views
Are multiple @daily crontab entries processed in order, serially?
I want two jobs to run sometime every day, serially, in exactly the order I specify. Will this crontab reliably do what I want?
@daily job1
@daily job2
I'm assuming they run one after the other, ...
11
votes
4answers
3k views
What is the 'working directory' when cron executes a job
I have a script that works when I run it from the command line, but when I schedule it with cron I get errors that it cannot find files or commands. My questions is twofold:
When I schedule a cron ...
11
votes
2answers
525 views
Origin of the word cron?
Trying to settle a friendly argument. Is "cron" an acronym for something?
Additionally, did it's current canonical name "cron" originate from something shortened for something like chronos or ...
10
votes
3answers
5k views
Does RHEL execute all cronjob files under /etc/cron.d/*?
This is a RedHat Enterprise Linux 5 system (RHEL). We manage this system using Cfengine.
We have several cronjobs which are running twice as often as usual. I checked the cronjobs under /etc/cron.d/ ...
9
votes
2answers
743 views
How can I delete all files in a folder that haven't been accessed in a certain amount of time?
I'd like to run a nightly cron job that deletes all the files in a folder that haven't been accessed in a week or more. What is the most efficient way to do this in bash?
9
votes
1answer
755 views
What does the following kernel message mean?
The following appears /var/log/messages, what that means ?
Feb 19 22:51:20 kernel: [ 187.819487] non-matching-uid symlink following attempted in sticky world-writable directory by sh (fsuid ...
8
votes
6answers
682 views
CronJob every 25 minutes
I want to know whether there is any easier way to run a job every 25 minutes. In cronjob, if you specify the minute parameter as */25, it'll run only on 25th and 50th minute of every hour
8
votes
6answers
15k views
Why did my crontab not trigger?
I used crontab -e to add the following line to my crontab:
* * * * * echo hi >> /home/myusername/test
Yet, I don't see that the test file is written to. Is this a permission problem, or is ...
8
votes
1answer
929 views
How can I execute `date` inside of a cron tab job?
I want to create a log file for a cron script that has the current hour in the log file name. This is the command I tried to use:
0 * * * * echo hello >> ~/cron-logs/hourly/test`date "+%d"`.log
...
8
votes
1answer
605 views
Suppress output unless non-zero exit code
What's the best way to suppress output (stdout and stderr) unless the program exits with a non-zero code? I'm thinking:
quiet_success()
{
file=/tmp/suppressed
if ! ( "$@" > "$file" ...
7
votes
2answers
220 views
Interactive shell with environment identical to cron
I am aware of a lot of pitfalls in the magic world of crontabs, but sometimes it would help troubleshooting a lot when you have some smart way to enter an interactive (bash) shell with exact identical ...
