Проверить cron на ошибки

The syntax for the crontab entry looks correct. Indeed, if you edit your crontab using «crontab -e» (as you should), you’ll get an error if you specify a syntactically invalid crontab entry anyway.

1. Firstly, does /path_to_my_php_script/info.php run correctly from the command-line?

2. If so, does it also run correctly like this?:

   /bin/sh -c "(export PATH=/usr/bin:/bin; /path_to_my_php_script/info.php </dev/null)"
   

3. If that works, does it work like this?

   /bin/sh -c "(export PATH=/usr/bin:/bin; /path_to_my_php_script/info.php </dev/null >/dev/null 2>&1)"
   

Step (3) is similar to how cron will run your program (as documented in «man 5 cron».

The most likely problem you’re having is that the PATH cron is using to run your program is too restrictive. Therefore, you may wish to add something like the following to the top of your crontab entry (you’ll need to add in whatever directories your script will need):

PATH=~/bin:/usr/bin/:/bin

Also note that cron will by default use /bin/sh, not bash. If you need bash, also add this to the start of your crontab file:

SHELL=/bin/bash

Note that both those changes will affect all the crontab entries. If you just want to modify these values for your info.php program, you could do something like this:

*/2 * * * * /bin/bash -c ". ~/.bashrc; /path_to_my_php_script/info.php"

It’s also worth mentioning that on a system configured for «mail» (in other words a system which has an MTA configured [sendmail/postfix/etc]), all output from crontab programs is sent to you via email automatically. A default Ubuntu desktop system won’t have local mail configured, but if you’re working on a server you can just type «mail» in a terminal to see all those cron mails. This also applies to the «at» command.

cron is a job scheduler for Linux and Unix-like operating systems. It comes pre-installed on all Linux distributions and is most suitable for automating repetitive tasks.

For cron to function properly, you must periodically check that the utility is running fine on your system. You can do that using four different ways, including checking the cron service status, examining cron logs, running a test cron job, and listing down running processes on your system.

What Is cron?

The silent job handler, cron, automates and schedules system tasks. Users who configure and maintain software environments make use of cron to schedule jobs such as commands or shell scripts—also called cron jobs—to run periodically at fixed times or intervals.

cron automates system maintenance or administration tasks that you might need to carry out frequently.

How to Check if cron Is Working on Linux

Here are some of the ways you can check if cron is working properly:

Method 1: Check the cron Service Status on Linux

One way to check whether cron is working is by checking the status of the cron service by running a basic Linux command. Open the Linux terminal by pressing Ctrl + Alt + T and use the systemctl command to check the status of cron:

 sudo systemctl status cron 

If you see the following output, this means cron is active and running fine on your system.

But what if cron is in an inactive state as shown below?

You can start the service like this:

 sudo service cron start 

After starting the cron service, enable it so that it starts every time the system reboots:

 sudo service cron enable 

Method 2: Check cron Logs on Linux

Another way to check if cron is working properly is by examining the log files. cron logs are stored in the /var/log/syslog directory on Linux.

syslog is a protocol that Linux systems use to centralize event data logs. Logs are then accessed to perform audits, monitoring, troubleshooting, reporting, and other necessary IT operational tasks.

You can see the cron job logs in the syslog file by executing the following command:

 grep CRON /var/log/syslog 

You will see a similar output indicating that cron is running fine and its logs are being stored in the log file:

As the syslog folder contains other system logs along with the cron logs, it can be a little tricky to examine only the cron-related logs in the file. You can resolve this by creating a separate log file that only stores cron entries that appear in the syslog file.

To do that, open the following file using the nano editor:

 nano /etc/rsyslog.d/50-default.conf 

Locate the line that starts with:

 #cron.* 

Uncomment this line by removing the hash (#) sign. Then, save and exit the file by pressing Ctrl + X, then press Y and hit Enter.

Restart the rsyslog service by issuing this command:

 sudo service rsyslog restart 

You will now receive logs in the cron.log file located in the /var/log directory.

If your terminal fails to fetch any logs, that could mean cron is not running. In that case, confirm if cron is working or not using other methods.

Method 3: Running a cron Job on Linux

You can also check if cron is working by simply creating a test cron job and executing it. If the job succeeds in performing the task, this means it’s functioning fine.

You first need to create a Bash script. Bash scripting allows you to automate everyday tasks on Linux.

Let’s create a Bash script that will print “my cronjob is working!” into a TXT file. To create the file, first, find out your present working directory using this command:

 pwd 

Create a file in the current directory using the touch command:

 touch file.txt 

After this, create a Bash file using nano:

 nano script.sh 

Add the following contents to the file:

 #! /usr/bin/bash
echo "my cronjob is working!" >> /home/username/file.txt 

Make sure to provide the correct location of the text file that you created. Save and exit the file by pressing Ctrl + X, then Y, and hit Enter.

Give execute permissions to the Bash script:

 chmod +x script.sh 

Now create a cron job to execute the script. To do that, open the crontab file with:

 crontab -e 

Enter the following line at the end of the file.

 * * * * * /path/to/script.sh 

The five asterisks match the time of execution, in which the first asterisk represents the minutes, the second one represents hours, the third asterisk means the day, the fourth one indicates the month, and the last asterisk indicates the year.

The name of the executable file and its path is also mentioned in the file.

After saving and closing the file, you will see a “crontab: installing new crontab” message on the terminal.

To check if the cron job worked or not, go to the directory of file.txt and print its contents on the terminal using cat:

 cat file.txt 

This indicates that cron is working fine.

Method 4: Check Running Processes on Linux

Another way you can check if the cron daemon is working is by listing down the running processes on your system. You can achieve this using the ps command. The cron daemon will show up in the output as crond.

 ps -ef | grep crond 

This confirms that the cron process is running on your Linux system.

Automate and Schedule System Tasks With cron

cron is one of the most vital utilities on Linux that allows the system to perform efficiently. Sometimes you have to check whether cron is working while troubleshooting system issues.

You can achieve this by either checking cron logs or seeing the running processes on your system. You can also check the cron status using the systemctl utility. Besides that, running a test cron job can also tell you if the service is running or not.

The smooth working of cron allows you to automate and schedule everyday system jobs. Most of the cron jobs are automatically created by installed applications. Automating and scheduling tasks improves the overall system performance and keeps it healthy.

The cron scheduler has been bundled with Unix and Linux operating systems for decades. It’s widely used, and usually very reliable. However, when something goes wrong with cron, it can be difficult to debug.

This guide will walk you through locating cron jobs on your server, validating their schedules, and debugging common runtime problems.

Why cron jobs fail

Cron jobs can fail for a variety of reasons, but at Cronitor we have observed common failure patterns that can provide a useful starting point for your debugging process. In the rest of this guide, we will dive into these common failures with suggestions and solutions.

If you’re adding a new cron job and it is not working…

• Schedule errors are easy to make

  Cron job schedule expressions are quirky and difficult to write. If your job didn’t run when you expected it to, the easiest thing to rule out is a mistake with the cron expression.

• Cron has subtle environment differences

  A common experience is to have a job that works flawlessly when run at the command line but fails whenever it’s run by cron. When this happens, eliminate these common failures:

  1. The command has an unresovable relative path like ../scripts. (Try an absolute path)
  2. The job uses environment variables. (Cron does not load .bashrc and similar files)
  3. The command uses advanced bash features (cron uses /bin/sh by default)

  Tip:Our free software, CronitorCLI, includes a shell command to test run any job the way cron does.

• There is a problem with permissions

  Invalid permissions can cause your cron jobs to fail in at least 3 ways. This guide will cover them all.

If your cron job stopped working suddenly…

• System resources have been depleted

  The most stable cron job is no match for a disk that is full or an OS that can’t spawn new threads. Check all the usual graphs to rule this out.

• You’ve reached an inflection point

  Cron jobs are often used for batch processing and other data-intensive tasks that can reveal the constraints of your stack. Jobs often work fine until your data size grows to a point where queries start timing-out or file transfers are too slow.

• Infrastructure drift occurs

  When app configuration and code changes are deployed it can be easy to overlook the cron jobs on each server. This causes infrastructure drift where hosts are retired or credentials change that break the forgotten cron jobs.

• Jobs have begun to overlap themselves

  Cron is a very simple scheduler that starts a job at the scheduled time, even if the previous invocation is still running. A small slow-down can lead to a pile-up of overlapped jobs sucking up available resources.

• A bug has been introduced

  Sometimes a failure has nothing to do with cron. It can be difficult to thoroughly test cron jobs in a development environment and a bug might exist only in production.

How to fix a cron job not running on schedule

You may find that you have scheduled a cron job that is just not running when expected. Follow these steps to locate your scheduled job and diagnose the problem.

1. Locate the scheduled job

Tip: If you know where your job is scheduled, skip this step

Cron jobs are run by a system daemon called crond that watches several locations for scheduled jobs. The first step to understanding why your job didn’t start when expected is to find where your job is scheduled.

Search manually for cron jobs on your server

• Check your user crontab with crontab -l

  dev01: ~ $ crontab -l
  # Edit this file to introduce tasks to be run by cron.
  # m h  dom mon dow   command
  5 4 * * *      /var/cronitor/bin/database-backup.sh
  

• Jobs are commonly created by adding a crontab file in /etc/cron.d/
• System-level cron jobs can also be added as a line in /etc/crontab
• Sometimes for easy scheduling, jobs are added to /etc/cron.hourly/, /etc/cron.daily/, /etc/cron.weekly/ or /etc/cron.monthly/
• It’s possible that the job was created in the crontab of another user. Go through each user’s crontab using crontab -u username -l

Or, scan for cron jobs automatically with CronitorCLI

• Install CronitorCLI for free

  Step 1 of 2: Enter your email address for your free CronitorCLI download:

  Download CronitorCLI

  Step 2 of 2: Download CronitorCLI directly from your server:

  • Linux
  • MacOS

  Paste each instruction into a terminal and execute.

  wget https://cronitor.io/dl/linux_amd64.tar.gz
  sudo tar xvf linux_amd64.tar.gz -C /usr/local/bin/
  sudo cronitor configure --api-key {{ api_key }} #optional
  

  Paste each instruction into a terminal and execute.

  curl -sOL https://cronitor.io/dl/darwin_amd64.tar.gz
  sudo tar xvf darwin_amd64.tar.gz -C /usr/local/bin
  sudo cronitor configure --api-key {{ api_key }} #optional
  

• Run cronitor list to scan your system for cron jobs:

  

If you can’t find your job but believe it was previously scheduled double check that you are on the correct server.

2. Validate your job schedule

Once you have found your job, verify that it’s scheduled correctly. Cron schedules are commonly used because they are expressive and powerful, but like regular expressions can sometimes be difficult to read. We suggest using Crontab Guru to validate your schedule.

* Paste the schedule expression from your crontab into the text field on Crontab Guru * Verify that the plaintext translation of your schedule is correct, and that the next scheduled execution times match your expectations * Check that the effective server timezone matches your expectation. In addition to checking system time using date, check your crontab file for TZ or CRON_TZ timezone declarations that would override system settings. For example, CRON_TZ=America/New_York

3. Check your permissions

Invalid permissions can cause your cron jobs to fail in at least 3 ways:

1. Jobs added as files in a /etc/cron.*/ directory must be owned by root. Files owned by other users will be ignored and you may see a message similar to WRONG FILE OWNER in your syslog. 2. The command must be executable by the user that cron is running your job as. For example if your ubuntu user crontab invokes a script like database-backup.sh, ubuntu must have permission to execute the script. The most direct way is to ensure that the ubuntu user owns the file and then ensure execute permissions are available using chmod +x database-backup.sh. 3. The user account must be allowed to use cron. First, if a /etc/cron.allow file exists, the user must be listed. Separately, the user cannot be in a /etc/cron.deny list.

Related to the permissions problem, ensure that if your command string contains a % that it is escaped with a backslash.

4. Check that your cron job is running by finding the attempted execution in syslog

When cron attempts to run a command, it logs it in syslog. By grepping syslog for the name of the command you found in a crontab file you can validate that your job is scheduled correctly and cron is running.

• Begin by grepping for the command in /var/log/syslog (You will probably need root or sudo access.)

  dev01: ~ $ grep database-backup.sh /var/log/syslog
  Aug  5 4:05:01 dev01 CRON[2128]: (ubuntu) CMD (/var/cronitor/bin/database-backup.sh)
  

• If you can’t find your command in the syslog it could be that the log has been rotated or cleared since your job ran. If possible, rule that out by updating the job to run every minute by changing its schedule to * * * * *.
• If your command doesn’t appear as an entry in syslog within 2 minutes the problem could be with the underlying cron daemon known as crond. Rule this out quickly by verifying that cron is running by looking up its process ID. If cron is not running no process ID will be returned.

  dev01: ~ $ pgrep cron
  323
  

• If you’ve located your job in a crontab file but persistently cannot find it referenced in syslog, double check that crond has correctly loaded your crontab file. The easiest way to do this is to force a reparse of your crontab by running EDITOR=true crontab -e from your command prompt. If everything is up to date you will see a message like No modification made. Any other message indicates that your crontab file was not reloaded after a previous update but has now been updated. This will also ensure that your crontab file is free of syntax errors.

If you can see in syslog that your job was scheduled and attempted to run correctly but still did not produce the expected result you can assume there is a problem with the command you are trying to run.

How to debug unexpected cron job failures

Your cron jobs, like every other part of your system, will eventually fail. This section will walk you through testing your job and spotting common failures.

1. Test run your command like cron does

When cron runs your command the environment is different from your normal command prompt in subtle but important ways. The first step to troubleshooting is to simulate the cron environment and run your command in an interactive shell.

Run any command like cron does with CronitorCLI

• Install CronitorCLI for free

  Step 1 of 2: Enter your email address for your free CronitorCLI download:

  Download CronitorCLI

  Step 2 of 2: Download CronitorCLI directly from your server:

  • Linux
  • MacOS

  Paste each instruction into a terminal and execute.

  wget https://cronitor.io/dl/linux_amd64.tar.gz
  sudo tar xvf linux_amd64.tar.gz -C /usr/local/bin/
  sudo cronitor configure --api-key {{ api_key }} #optional
  

  Paste each instruction into a terminal and execute.

  curl -sOL https://cronitor.io/dl/darwin_amd64.tar.gz
  sudo tar xvf darwin_amd64.tar.gz -C /usr/local/bin
  sudo cronitor configure --api-key {{ api_key }} #optional
  

• To force a scheduled cron job to run immediately, use cronitor select to scan your system and present a list of jobs to choose from.

• To simulate running any command the way cron does, use cronitor shell:

  

Or, manually test run a command like cron does

• If you are parsing a file in /etc/cron.d/ or /etc/crontab each line is allowed to have an effective «run as» username after the schedule and before the command itself. If this applies to your job, or if your job is in another user’s crontab, begin by opening a bash prompt as that user sudo -u username bash
• By default, cron will run your command using /bin/sh, not the bash or zsh prompt you are familiar with. Double check your crontab file for an optional SHELL=/bin/bash declaration. If using the default /bin/sh shell, certain features that work in bash like [[command]] syntax will cause syntax errors under cron.
• Unlike your interactive shell, cron doesn’t load your bashrc or bash_profile so any environment variables defined there are unavailable in your cron jobs. This is true even if you have a SHELL=/bin/bash declaration. To simulate this, create a command prompt with a clean environment.

  dev01: ~ $ env -i /bin/sh
  

• By default, cron will run commands with your home directory as the current working directory. To ensure you are running the command like cron does, run cd ~ at your prompt.
• Paste the command to run (everything after the schedule or declared username) into the command prompt. If crontab is unable to run your command, this should fail too and will hopefully contain a useful error message. Common errors include invalid permissions, command not found, and command line syntax errors.
• If no useful error message is available, double check any application logs your job is expected to produce, and ensure that you are not redirecting log and error messages. In linux, command >> /path/to/file will redirect console log messages to the specified file and command >> /path/to/file 2>&1 will redirect both the console log and error messages. Determine if your command has a verbose output or debug log flag that can be added to see additional details at runtime.
• Ideally, if your job is failing under cron it will fail here too and you will see a useful error message, giving you a chance to modify and debug as needed. Common errors include file not found, misconfigured permissions, and commandline syntax errors.

2. Check for overlapping jobs

At Cronitor our data shows that runtime durations increase over time for a large percentage of cron jobs. As your dataset and user base grows it’s normal to find yourself in a situation where cron starts an instance of your job before the previous one has finished. Depending on the nature of your job this might not be a problem, but several undesired side effects are possible:

• Unexpected server or database load could impact other users

• Locking of shared resources could cause deadlocks and prevent your jobs from ever completing successfully

• Creation of an unanticipated race condition that might result in records being processed multiple times, amplifying server load and possibly impacting customers

  To verify if any instances of a job are running on your server presently, grep your process list. In this example, 3 job invocations are running simultaneously:

  dev01: ~ $ ps aux | grep database-backup.sh
  ubuntu           1343   0.0  0.1  2585948  12004   ??  S    31Jul18   1:04.15 /var/cronitor/bin/database-backup.sh
  ubuntu           3659   0.0  0.1  2544664    952   ??  S     1Aug18   0:34.35 /var/cronitor/bin/database-backup.sh
  ubuntu           7309   0.0  0.1  2544664   8012   ??  S     2Aug18   0:18.01 /var/cronitor/bin/database-backup.sh
  

To quickly recover from this, first kill the overlapping jobs and then watch closely when your command is next scheduled to run. It’s possible that a one time failure cascaded into several overlapping instances. If it becomes clear that the job often takes longer than the interval between job invocations you may need to take additional steps, e.g.:

• Increase the duration between invocations of your job. For example if your job runs every minute now, consider running it every other minute.
• Use a tool like flock to ensure that only a single instance of your command is running at any given time. Using flock is easy. After installing from apt-get or yum you only need to prefix the command in your crontab:

  dev01: ~ $ crontab -l
  # Edit this file to introduce tasks to be run by cron.
  # m h  dom mon dow   command
  * * * * *      flock -w 0 /var/cronitor/bin/database-backup.sh
  

Read more about flock from the man page.

What to do if nothing else works

Here are a few things you can try if you’ve followed this guide and find that your job works flawlessly when run from the cron-like command prompt but fails to complete successful under crontab.

• First get the most basic cron job working with a command like date >> /tmp/cronlog. This command will simply echo the execution time to the log file each time it runs. Schedule this to run every minute and tail the logfile for results.

• If your basic command works, replace it with your command. As a sanity check, verify if it works.

• If your command works by invoking a runtime like python some-command.py perform a few checks to determine that the runtime version and environment is correct. Each language runtime has quirks that can cause unexpected behavior under crontab.

  • For python you might find that your web app is using a virtual environment you need to invoke in your crontab.
  • For node a common problem is falling back to a much older version bundled with the distribution.
  • When using php you might run into the issue that custom settings or extensions that work in your web app are not working under cron or commandline because a different php.ini is loaded.

  If you are using a runtime to invoke your command double check that it’s pointing to the expected version from within the cron environment.

• If nothing else works, restart the cron daemon and hope it helps, stranger things have happened. The way to do this varies from distro to distro so it’s best to ask google.

Где хранится информация о запуске заданий по расписанию? Для проверки используем Ubuntu Linux 12.04/14.04 LTS сервер. Как проверит, запущен ли cron и правильно ли он работает?

Как проверить запущен ли cron (планировщик заданий)? Используйте команду pgrep  или ps

Результат

Так же вы можете проверить файл /var/log/syslog используя команду grep что бы получить информации о работе планировщика cron

Результат:

Где хранятся журналы работы cron на Ubuntu Linux?

Журналы хранятся в файле /var/log/cron.log. Вы можете настроить его следующим образом. Редактировать /etc/rsyslog.d/50-default.conf файл с помощью текстового редактора, например, vi или nano:

или

Найдите строку

Необходимо её раскомментировать, и перезапустить оба сервиса:

Теперь все сообщения об ошибках в работе планировщика можно будет просматривать в файле

Воспользуйтесь различными командами для поиска нужной информации в логе

 Предоставляем уcлуги администрирования серверов Linux Ububtu

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Logs probably go to syslog, which varies depending on what syslog daemon is involved and how that is configured to log, start with

grep -r cron /etc/*syslog*

to find where and what is going on on the system, or per derobert under systemd the relevant command is

journalctl -b 0 _SYSTEMD_UNIT=cron.service

Adding a test cron job that touches a file (ideally not in /tmp, unless the vendor makes that per-user private, for security reasons) should also confirm whether cron is working or not, just be sure to eventually remove the test cron job before it fills up a partition or something silly.

Other usability and security pointers: some cron daemons can run scripts directly, in which case you can just copy the script into /etc/cron.daily, though this may not suit something that you do not want to run (with everything else!) at exactly the top of the hour. root running a script under a user’s home directory could be very bad, as a compromise of that user account could then be leveraged to root access, or the script could needlessly fail if the home directory is on NFS or encrypted; move the script elsewhere to avoid this (/root/bin, or somewhere under /usr/local or /opt depending on the local filesystem preferences).

Even more pointers fall into the realm of shell script debugging, mostly to note that cron is not the shell; use env or set to see what is set under cron, check that PATH is correct, and etc. (One old and horrible linux kernel bug involved java daemons crashing but only if they were run from cron; that was fun to debug…)