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Cron job script for running the Linux/Unix Nextcloud command line client
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nextcloud-sync-cron

Script to run the Linux/Unix Nextcloud command line client in a cron job.

Synopsis

nextcloud-sync-cron.sh [--logdir dir] [--verbose] [--help] [--version] configfile

Description

For running the Nextcloud command line client program, nextcloudcmd, in a cron job.

The standard Nextcloud command line client performs one sync run and exits. Therefore, to keep a local directory synchronised with a Nextcloud repository, it needs to be run everytime files have changed or regularly to scan for any changes. Running it directly from a cron job is possible, but can lead to problems. This script addresses those problems.

Features are:

  • Prevents multiple copies of the script from running at the same time. This is needed to prevent cron starting another run before a previous run has finished.

  • Blocking and/or delaying retries when errors are encountered. This prevents excessive retries before problems are fixed. For example, retrying with the wrong password can blacklist the user account and make the problem harder to fix.

  • Account details are stored in a separate configuration file instead of embedded inside the crontab.

Command line options

  • --logdir dir directory for log files.

  • --verbose show error messages on stderr.

  • --version shows version information.

  • --help shows a brief help message.

  • configfile file containing the configuration information.

Configuration file

Recommended method using .netrc

A configuration file is a text file that must contain:

  • local directory on the local machine where the files will be stored.
  • remote URL to the Nextcloud service.

For example:

# Config file for Nextcloud sync cron

local: /home/fbar/mydata
remote: https://nextcloud.example.com

And the ~/.netrc file contains the user credentials:

default
login foobar
password p@ssw0rd

Note: the .netrc file must reside in the home directory. It is not possible for a different file to be used.

Alternative not using .netrc

If the username and password is included in the configuration file, they will be used instead of the .netrc file.

This method is not recommended, because the password will be passed to nextcloudcmd via command line parameters which exposes it to others on the system.

# Config file for Nextcloud sync cron

local: /home/fbar/mydata
remote: https://nextcloud.example.com
username: foobar
password: p@ssw0rd

Unsynced folders

Optionally the configuration file can also contain:

  • unsyncedfolders file with names of folders on remote machine that shall not be synced.

The file is passed to nextcloudcmd as the --unsyncedfolders option.

# Config file for Nextcloud sync cron

local: /home/fbar/mydata
remote: https://nextcloud.example.com
unsyncedfolders: /home/fbar/nosync.lst

Logging and errors

The default log directory is called "._sync_nextcloud" inside the local directory, or it can be specified via command line arguments.

The log directory contains these files:

  • sync.log contains a line for each run of the Nextcloud command line client. The line contains a timestamp and whether the run was "ok" or "fail". If ok, the duration it took to run is included.

  • nextcloudcmd.txt contains the output from the previous run of the Nextcloud command line client (regardless of whether it was successful or not). This file is useful for debugging why the sync client failed.

  • sync.pid contains the process identifier (PID) number of the script currently running for that local directory. The PID file should not be present when there is none runnning.

  • failures.txt contains information about previous failures, if the most recent run was not successful. The information in it determines the delay before the client is allowed to run again. If the most recent run was successful this file is deleted, because it breaks the previous sequence of failures.

The log directory is created if it does not already exist.

Example

Install the Nextcloud command line client. For example, on CentOS 7, this can be done by running:

sudo yum install epel-release
sudo yum install nextcloud-client

Create a configuration file.

If the local directory does not exist, create it. The user running the cron job needs permissions to read/write/access the local directory, as well as reading the configuration file.

Optionally, test the configuration file. Manually run the script to check the configuration is correct. Note: this can take a long time to run, since it will perform the initial sync.

nextcloud-sync-cron.sh --verbose /home/username/myncs.conf

Configure cron by editing the crontab file:

crontab -e

For example, this job runs the script once every minute:

 * * * * * /home/username/nextcloud-sync-cron.sh /home/username/myncs.conf

Details

Retries after failure

Retries and configuration errors

If the client fails because of a problem with the values in the configuration file, subsequent runs are prevented until the config file is fixed. For example, the password was wrong. The script compares the modified timestamp on the config file with the failures.txt file.

If the problem is fixed without needing to update the configuration file, the script can be allowed to run either by: changing the modified timestamp of the config file; or simply deleting the failures.txt file.

Retries and other errors

Other failures results in subsequent retries being skipped until a delay is reached. This delay increases with subsequent failures.

The delay starts at 1 minute, and doubles with subsequent failures, with the maximum delay of 24 hours. That is the delays are: 1 minute on the first failure, then 2, 4, 8 minutes etc.

Exit status

0: ok

Success.

2: usage error

Command line arguments were incorrect.

3: unexpected error

A command in the script failed.

4: already running

Another instance of the script is already running for the local directory.

Solution: wait until the other script finishes running.

5: Skipping

A previous run of the script failed and the time delay before another sync can be attempted has not been reached. Try again later.

Solution: try again later, or delete the failures.txt file to clear the failures causing it to delay synching.

6: bad value in configuration file

The configuration file contains incorrect values. The script will not run.

Solution: edit the config file to provide the correct values.

7: values in configuration file incorrect

The configuration file contains incorrect values. The script used them in a previous sync, but they were rejected. For example, the username and password are incorrect.

The script will not attempt another sync until the configuration file has been modified.

Solution: edit the config file to provide the correct values. But if the values are correct (i.e. the problem was fixed elsewhere), update the timestamp of the configuration file or delete the failures.txt file.

Known limitations

Security issues

If the .netrc file is not used, the password is passed to the Nextcloud command line client via command line arguments. This is a limitation of the Nextcloud command line client.

Some errors will produce output

Certain types of errors will cause the script to print out an error message on stdout. Cron, by default, emails the user if the job produces output. Therefore, the user could receive many emails before the problem is fixed (especially if the cron job runs frequently).

Normally, error messages will be written to the log file and no output is produced (unless the verbose option is provided). Until the script is ready to write to the log file, error messages appear on stderr. Usually, these are all problems that will be raised the first time the script is run. For example:

  • the configuration file cannot be read;
  • the configuration file contain incorrect values; or
  • the local directory does not exist or has the wrong permissions.

Therefore, it is advised to wait until the cron job runs, at least once, before leaving it unattended. However, these errors can also arise if incorrect changes are made to the configuration file or local system.

Performance

Every time the script is run, it needs to scan the entire local directory for changes.

The nextcloudcmd program is not designed to be a sync client. There is a long-standing issue for a proper non-GUI sync client to be built.

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