Reasons why crontab does not work

Question

Many a time and oft, crontab scripts are not executed as expected. There are numerous reasons for that, for example: wrong crontab notation, permissions, environment variables and many more.

This community wiki aims to aggregate the top reasons for crontab scripts not being executed as expected. Write each reason in a separate answer.

Please include one reason per answer - details about why it's not executed - and fix(es) for that one reason.

Please write only cron-specific issues, e.g. commands that execute as expected from the shell but execute erroneously by cron.

Answer 1

Different environment

Cron passes a minimal set of environment variables to your jobs. To see the difference, add a dummy job like this:

* * * * * env > /tmp/env.output

Wait for /tmp/env.output to be created, then remove the job again. Now compare the contents of /tmp/env.output with the output of env run in your regular terminal.

A common "gotcha" here is the PATH environment variable being different. Maybe your cron script uses the command somecommand found in /opt/someApp/bin, which you've added to PATH in /etc/environment? cron does not read that file, so runnning somecommand from your script will fail when run with cron, but work when run in a terminal.

To get around that, just set your own PATH variable at the top of the script. E.g.

#!/bin/bash
PATH=/opt/someApp/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin

# rest of script follows

Some prefer to just use absolute paths to all the commands instead. I recommend against that. Consider what happens if you want to run your script on a different system, and on that system, the command is in /opt/someAppv2.2/bin instead. You'd have to go through the whole script replacing /opt/someApp/bin with /opt/someAppv2.2/bin instead of just doing a small edit on the first line of the script.

You can also set the PATH variable in the crontab file, which will apply to all cron jobs. E.g.

PATH=/opt/someApp/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin

15 1 * * * backupscript --incremental /home /root

Answer 2

My top gotcha: If you forget to add a newline at the end of the crontab file. In other words, the crontab file should end with an empty line.

Below is the relevant section in the man pages for this issue (man crontab then skip to the end):

   Although cron requires that each entry in a crontab end  in  a  newline
   character,  neither the crontab command nor the cron daemon will detect
   this error. Instead, the crontab will appear to load normally. However,
   the  command  will  never  run.  The best choice is to ensure that your
   crontab has a blank line at the end.

   4th Berkeley Distribution      29 December 1993               CRONTAB(1)

Answer 3

Cron daemon is not running. I really screwed up with this some months ago.

Type:

pgrep cron 

If you see no number, then cron is not running. sudo /etc/init.d/cron start can be used to start cron.

EDIT: Rather than invoking init scripts through /etc/init.d, use the service utility, e.g.

sudo service cron start

Answer 4

In many environments cron executes commands using sh, while many people assume it will use bash.

Suggestions to test or fix this for a failing command:

• Try running the command in sh to see if it works
• Wrap the command in a bash subshell to make sure it gets run in bash:
  bash -c "mybashcommand"
• Tell cron to run all commands in bash by setting the shell at the top of your crontab:
  SHELL=/bin/bash
• If the command is a script, make sure the script contains a shebang:
  #!/bin/bash

Answer 5

Permissions problems are quite common, I'm afraid.

Note that a common workaround is to execute everything using root's crontab, which sometimes is a Really Bad Idea. Setting proper permissions is definitely a largely overlooked issue.

Answer 6

Absolute path should be used for scripts:

For example, /bin/grep should be used instead of grep:

# m h  dom mon dow   command
0 0 *  *  *  /bin/grep ERROR /home/adam/run.log &> /tmp/errors

Instead of:

# m h  dom mon dow   command
0 0 *  *  *  grep ERROR /home/adam/run.log &> /tmp/errors

This is especially tricky, because the same command will work when executed from shell. The reason is that cron does not have the same PATH environment variable as the user.

Answer 7

I had some issues with the time zones. Cron was running with the fresh installation time zone. The solution was to restart cron:

sudo service cron restart

Answer 8

The script filenames in cron.daily/, cron.hourly/, etc., should not contain dot(.), otherwise run-parts will skip them.

See run-parts(8):

   If neither the --lsbsysinit option nor the --regex option is given then
   the names must consist entirely of upper and lower case  letters,  dig‐
   its, underscores, and hyphens.

   If  the  --lsbsysinit  option  is given, then the names must not end in
   .dpkg-old  or .dpkg-dist or .dpkg-new or .dpkg-tmp, and must belong  to
   one  or more of the following namespaces: the LANANA-assigned namespace
   (^[a-z0-9]+$);   the   LSB   hierarchical   and   reserved   namespaces
   (^_?([a-z0-9_.]+-)+[a-z0-9]+$);  and  the  Debian cron script namespace
   (^[a-zA-Z0-9_-]+$).

So, if you have a cron script backup.sh, analyze-logs.pl in cron.daily/ directory, you'd best to remove the extension names.

Answer 9

Cron is calling a script which is not executable.

By running chmod +x /path/to/scrip the script becomes executable and should resolve this issue.

Answer 10

It is also possible that the user's password has expired. Even root's password can expire. You can tail -f /var/cron/log and you will see cron fail with password expired. You can set the password to never expire by doing this: passwd -x -1 <username>

Answer 11

If your crontab command has a % symbol in it, cron tries to interpret it. So if you were using any command with a % in it (such as a format specification to the date command) you will need to escape it.

That and other good gotchas here:
http://www.pantz.org/software/cron/croninfo.html

Answer 12

Script is location-sensitive. This is related to always using absolute paths in a script, but not quite the same. Your cron job may need to cd to a specific directory before running, e.g. a rake task on a Rails application may need to be in the application root for Rake to find the correct task, not to mention the appropriate database configuration, etc.

So a crontab entry of

23 3 * * * /usr/bin/rake db:session_purge RAILS_ENV=production

would be better as

23 3 * * * cd /var/www/production/current && /usr/bin/rake db:session_purge RAILS_ENV=production

Or, to keep the crontab entry simpler and less brittle:

23 3 * * * /home/<user>/scripts/session-purge.sh

with the following code in /home/<user>/scripts/session-purge.sh:

cd /var/www/production/current
/usr/bin/rake db:session_purge RAILS_ENV=production

Answer 13

Most frequent reason I have seen cron fail in an incorrectly stated schedule. It takes practice to specify a job scheduled for 11:15 pm as 30 23 * * * instead of * * 11 15 * or 11 15 * * *. Day of week for jobs after midnight also gets confused M-F is 2-6 after midnight not 1-5. Specific dates are usually a problem as we rarely use them * * 3 1 * is not March 3rd.

If your work with different platforms using unsupported options such as 2/3 in time specifications can also cause failures. This is a very useful option but not universally available. I have also run across issues will lists like 1-5 or 1,3,5.

Using unqualified paths have also caused problems. The default path is usually /bin:/usr/bin so only standard commands will run. These directories usually don't have the desired command. This also affects scripts using non standard commands. Other environment variables can also be missing.

Clobbering an existing crontab entirely has caused me problems. I now load from a file copy. This can be recovered from the existing crontab using crontab -l if it gets clobbered. I keep the copy of crontab in ~/bin. It is commented throughout and ends with the line # EOF. This is reloaded daily from a crontab entry like:

#!/usr/bin/crontab
# Reload this crontab
#
54 12    *   *   *   ${HOME}/bin/crontab

The reload command above relies on an executable crontab with a bang path running crontab. Some systems require the running crontab in the command and specifying the file. If the directory is network shared, then I often use crontab.$(hostname) as the name of the file. This will eventually correct cases where the wrong crontab is loaded on the wrong server.

Using the file provides a backup of what the crontab should be, and allows temporary edits (the only time I use crontab -e) to be backed out automatically. There are headers available which help with getting the scheduling parameters right. I have added them when inexperience users would be editing a crontab.

Rarely, I have run into commands that require user input. These fail under crontab, although some will work with input redirection.

Answer 14

Crontab specs which worked in the past can break when moved from one crontab file to another. Sometimes the reason is that you've moved the spec from a system crontab file to a user crontab file or vice-versa.

The cron job specification format differs between users' crontab files (/var/spool/cron/username) and the system crontabs (/etc/crontab and the the files in /etc/cron.d).

The system crontabs have an extra field 'user' right before the command-to-run.

This will cause errors stating things like george; command not found when you move a command out of /etc/crontab or a file in /etc/cron.d into a user's crontab file.

Conversely, cron will deliver errors like /usr/bin/restartxyz is not a valid username or similar when the reverse occurs.

Answer 15

cron script is invoking a command with --verbose option

I had a cron script fail on me because I was in autopilot while typing the script and I included the --verbose option:

#!/bin/bash
some commands
tar cvfz /my/archive/file.tar.gz /my/shared/directory
come more commands

The script ran fine when executing from shell, but failed when running from crontab because the verbose output goes to stdout when run from shell, but nowhere when run from crontab. Easy fix to remove the 'v':

#!/bin/bash
some commands
tar cfz /my/archive/file.tar.gz /my/shared/directory
some more commands

Answer 16

If your cronjob invokes GUI-apps, you need to tell them what DISPLAY they should use.

Example: Firefox launch with cron.

Your script should contain export DISPLAY=:0 somewhere.

Answer 17

If you are accessing an account via SSH keys it is possible to login to the account but not notice that the password on the account is locked (e.g. due to expiring or invalid password attempts)

If the system is using PAM and the account is locked, this can stop its cronjob from running. (I've tested this on Solaris, but not on Ubuntu)

You may find messages like this in /var/adm/messages:

Oct 24 07:51:00 mybox cron[29024]: [ID 731128 auth.notice] pam_unix_account: cron attempting to validate locked account myuser from local host
Oct 24 07:52:00 mybox cron[29063]: [ID 731128 auth.notice] pam_unix_account: cron attempting to validate locked account myuser from local host
Oct 24 07:53:00 mybox cron[29098]: [ID 731128 auth.notice] pam_unix_account: cron attempting to validate locked account myuser from local host
Oct 24 07:54:00 mybox cron[29527]: [ID 731128 auth.notice] pam_unix_account: cron attempting to validate locked account myuser from local host

All you should need to do is run:

# passwd -u <USERNAME>

as root to unlock the account, and the crontab should work again.

Answer 18

Line written in a way crontab doesn't understand. It needs to be correctly written. Here's CrontabHowTo.

Answer 19

Cron daemon could be running, but not actually working. Try restarting cron:

sudo /etc/init.d/cron restart

Answer 20

Writing to cron via "crontab -e" with the username argument in a line. I've seen examples of users (or sysadmins) writing their shell scripts and not understanding why they don't automate. The "user" argument exists in /etc/crontab, but not the user-defined files. so, for example, your personal file would be something like:

# m h dom mon dow command

* * */2  *   *  /some/shell/script

whereas /etc/crontab would be:

# m h dom mon dow user   command

* * */2  *   *  jdoe   /some/shell/script

So, why would you do the latter? Well, depending on how you want to set your permissions, this can become very convoluted. I've written scripts to automate tasks for users who don't understand the intricacies, or don't want to bother with the drudgery. By setting permissions to --x------, I can make the script executable without them being able to read (and perhaps accidentally change) it. However, I might want to run this command with several others from one file (thus making it easier to maintain) but make sure file output is assigned the right owner. Doing so (at least in Ubuntu 10.10) breaks on both the inability to read the file as well as execute, plus the afore-mentioned issue with putting periods in /etc/crontab (which, funnily enough, causes no error when going through crontab -e).

As an example, I've seen instances of sudo crontab -e used to run a script with root permissions, with a corresponding chown username file_output in the shell script. Sloppy, but it works. IMHO, The more graceful option is to put it in /etc/crontab with username declared and proper permissions, so file_output goes to the right place and owner.

Answer 21

Building off what Aaron Peart mentioned about verbose mode, sometimes scripts not in verbose mode will initialize but not finish if the default behavior of an included command is to output a line or more to the screen once the proc starts. For example, I wrote a backup script for our intranet which used curl, a utility that downloads or uploads files to remote servers, and is quite handy if you can only access said remote files through HTTP. Using 'curl http://something.com/somefile.xls' was causing a script I wrote to hang and never complete because it spits out a newline followed by a progress line. I had to use the silent flag (-s) to tell it not to output any information, and write in my own code to handle if the file failed to download.

Answer 22

I was writing an install shell script that creates another script to purge old transaction data from a database. As a part of the task it had to configure daily cron job to run at an arbitrary time, when the database load was low.

I created a file mycronjob with cron schedule, username & the command and copied it to the /etc/cron.d directory. My two gotchas:

1. mycronjob file had to be owned by root to run
2. I had to set permissions of the file to 644 - 664 would not run.

Answer 23

To make sure you don't have any syntax problems, run crontab /etc/crontab once. (Just to check for syntax problems)

For example sometimes, some invisible unicode characters (specially when you copy/paste from somewhere), are making syntax problem.

Answer 24

If you edited your crontab file using a windows editor (via samba or something) and it's replaced the newlines with \n\r or just \r, cron won't run.

Also, if you're using /etc/cron.d/* and one of those files has a \r in it, cron will move through the files and stop when it hits a bad file. Not sure if that's the problem?

Use:

od -c * | grep \r

Answer 25

This error message should be mentioned

sudo service cron restart
grep -i cron /var/log/syslog|tail -2
2013-02-05T03:47:49.283841+01:00 ubuntu cron[49906]: (user) INSECURE MODE (mode 0600 expected) (crontabs/user)

sudo chmod 600 /var/spool/cron/crontabs/user        # correct permission
sudo touch /var/spool/cron/crontabs                 # signal crond to reload the file

Answer 26

I've never understood why crond does not have a at mode. When using at, it creates a wrapper scripts that saves your whole environment, path, etc so when atd starts the job, it runs just like in your terminal