Jtdcftul

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)

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

The script filenames in cron.d/, 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.

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
  

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

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.

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

It is also possible that the user's password has expired. Even root's password can expire. You can tail -f /var/log/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>

In some systems (Debian, Ubuntu) logging for cron is not enabled by default. In /etc/rsyslog.conf or /etc/rsyslog.d/50-default.conf the line:

# cron.*                          /var/log/cron.log

should be edited (sudo nano /etc/rsyslog.conf) uncommented to:

cron.*                          /var/log/cron.log

After that, you need to restart rsyslog via

/etc/init.d/rsyslog restart

or

service rsyslog restart 

Source: Enable crontab logging in Debian Linux

In some systems (Ubuntu) separate logging file for cron is not enabled by default, but cron related logs are appearing in syslog file. One may use

cat /var/log/syslog | grep cron -i

to view cron-related messages.

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.

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.

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.

Insecure cron table permission

A cron table is rejected if its permission is insecure

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)

The problem is solved with

# correct permission

sudo chmod 600 /var/spool/cron/crontabs/user

# signal crond to reload the file

sudo touch /var/spool/cron/crontabs

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

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 or /var/spool/cron/crontabs/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.

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

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.

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.

If you have a command like this:

* * * * * /path/to/script >> /tmp/output

and it doesn't work and you can't see any output, it may not necessarily mean cron isn't working. The script could be broken and the output going to stderr which doesn't get passed to /tmp/output. Check this isn't the case, by capturing this output as well:

* * * * * /path/to/script >> /tmp/output 2>&1

to see if this helps you catch your issue.

=== Docker alert ===

If you're using docker,

I think it is proper to add that I couldn't manage to make cron to run in the background.

To run a cron job inside the container, I used supervisor and ran cron -f, together with the other process.

Edit: Another issue - I also didn't manage to get it work when running the container with HOST networking. See this issue here also: https://github.com/phusion/baseimage-docker/issues/144

In my case cron and crontab had different owners.

NOT working I had this:

User@Uva ~ $ ps -ef | grep cron | grep -v grep

User    2940    7284 pty1     19:58:41 /usr/bin/crontab

SYSTEM   11292     636 ?        22:14:15 /usr/sbin/cro 

Basically I had to run cron-config and answer the questions correctly. There is a point where I was required to enter my Win7 user password for my 'User' account. From reading I did, it looks like this is a potential security issue but I am the only administrator on a single home network so I decided it was OK.

Here is the command sequence that got me going:

User@Uva ~ $ cron-config

The cron daemon can run as a service or as a job. The latter is not recommended.

Cron is already installed as a service under account LocalSystem.

Do you want to remove or reinstall it? (yes/no) yes

OK. The cron service was removed.



Do you want to install the cron daemon as a service? (yes/no) yes

Enter the value of CYGWIN for the daemon: [ ] ntsec



You must decide under what account the cron daemon will run.

If you are the only user on this machine, the daemon can run as yourself.

   This gives access to all network drives but only allows you as user.

To run multiple users, cron must change user context without knowing

  the passwords. There are three methods to do that, as explained in

  http://cygwin.com/cygwin-ug-net/ntsec.html#ntsec-nopasswd1

If all the cron users have executed "passwd -R" (see man passwd),

  which provides access to network drives, or if you are using the

  cyglsa package, then cron should run under the local system account.

Otherwise you need to have or to create a privileged account.

  This script will help you do so.

Do you want the cron daemon to run as yourself? (yes/no) no



Were the passwords of all cron users saved with "passwd -R", or

are you using the cyglsa package ? (yes/no) no



Finding or creating a privileged user.

The following accounts were found: 'cyg_server' .

This script plans to use account cyg_server.

Do you want to use another privileged account name? (yes/no) yes

Enter the other name: User



Reenter: User





Account User already exists. Checking its privileges.

INFO: User is a valid privileged account.

INFO: The cygwin user name for account User is User.



Please enter the password for user 'User':

Reenter:

Running cron_diagnose ...

... no problem found.



Do you want to start the cron daemon as a service now? (yes/no) yes

OK. The cron daemon is now running.



In case of problem, examine the log file for cron,

/var/log/cron.log, and the Windows event log (using /usr/bin/cronevents)

for information about the problem cron is having.



Examine also any cron.log file in the HOME directory

(or the file specified in MAILTO) and cron related files in /tmp.



If you cannot fix the problem, then report it to cygwin@cygwin.com.

Please run the script /usr/bin/cronbug and ATTACH its output

(the file cronbug.txt) to your e-mail.



WARNING: PATH may be set differently under cron than in interactive shells.

         Names such as "find" and "date" may refer to Windows programs.





User@Uva ~ $ ps -ef | grep cron | grep -v grep

    User    2944   11780 ?        03:31:10 /usr/sbin/cron

    User    2940    7284 pty1     19:58:41 /usr/bin/crontab



User@Uva ~ $

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.

A permission problem will appear in /var/log/syslog as something similar to:

Apr 24 18:30:01 ip-11-22-33-44 cron[40980]: (*system*) INSECURE MODE (group/other writable) (/etc/crontab)

Apr 24 18:30:01 ip-11-22-33-44 cron[40980]: (*system*) INSECURE MODE (group/other writable) (/etc/cron.d/user)

The first line refers to /etc/crontab file and the later to a file I placed under /etc/cront.d.

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

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

sudo /etc/init.d/cron restart

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.

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.

Although you can define environment variables in your crontable, you're not in a shell script. So constructions like the following won't work:

SOME_DIR=/var/log

MY_LOG_FILE=${SOME_LOG}/some_file.log



BIN_DIR=/usr/local/bin

MY_EXE=${BIN_DIR}/some_executable_file



0 10 * * * ${MY_EXE} some_param >> ${MY_LOG_FILE}

This is because variables are not interpreted in the crontable: all values are taken litterally. And this is the same if you omit the brackets.
So your commands won't run, and your log files won't be written...

Instead you must define all your environment variables straight:

SOME_DIR=/var/log

MY_LOG_FILE=/var/log/some_file.log



BIN_DIR=/usr/local/bin

MY_EXE=/usr/local/bin/some_executable_file



0 10 * * * ${MY_EXE} some_param >> ${MY_LOG_FILE}

When a task is run within cron, stdin is closed. Programs that act differently based on whether stdin is available or not will behave differently between the shell session and in cron.

An example is the program goaccess for analysing web server log files. This does NOT work in cron:

goaccess -a -f /var/log/nginx/access.log > output.html

and goaccess shows the help page instead of creating the report. In the shell this can be reproduced with

goaccess -a -f /var/log/nginx/access.log > output.html < /dev/null

The fix for goaccess is to make it read the log from stdin instead of reading from the file, so the solution is to change the crontab entry to

cat /var/log/nginx/access.log | goaccess -a > output.html

On my RHEL7 servers, root cron jobs would run, but user jobs would not. I found that without a home directory, the jobs won't run (but you will see good errors in /var/log/cron). When I created the home directory, the problem was solved.

If you edited your crontab file using a windows editor (via samba or something) and it's replaced the newlines with nr 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 /etc/cron.d/* | grep r