Why crontab scripts are not working?

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 ignores PATH from that file, so runnning somecommand from your script will fail when run with cron, but work when run in a terminal. It's worth noting that variables from /etc/environment will be passed on to cron jobs, just not the variables cron specifically sets itself, such as PATH.

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

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 (i.e. cron's main PID), 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

EDIT: Also you could use systemctl in modern Linux, e.g.

sudo systemctl start cron

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:

  sh -c "mycommand"
  

• 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

Cron is calling a script that is not executable.

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

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.