Troubleshooting

Don’t panic!

Most pipeline errors are easy to fix once you know what caused them. The pipeline provides detailed logs to help you identify and resolve issues quickly.

How to assess an error

When the pipeline fails, follow these steps to understand what went wrong:

1. Check the terminal output

   The terminal will show you which step failed. Look for lines starting with ERROR or FAILED. This gives you a quick indication of where the problem occurred.

2. Read the .nextflow.log file

   This file contains detailed information about the pipeline execution. It’s located in your working directory (where you ran the nextflow run command).

   # View the last 100 lines of the log
   tail -n 100 .nextflow.log
   
   # Search for errors in the log
   grep -i error .nextflow.log

   What will you see in the log file

   The log file will show you information about which process failed (a.k.a., which processing steps returned an error), what exactly is the error message, which files were used, and which command was executed. Importantly, it will point you to the specific working directory in which the process was run. In there, you will find additional, more specific log files.

3. Check the work directory

   Each process creates a subdirectory in the work/ folder. The terminal output shows you the work directory for the failed process (e.g., work/a1/b2c3d4...).

   Inside this directory, you’ll find:

   • .command.sh - The script that was executed
   • .command.out - Standard output from the process
   • .command.err - Error messages from the process
   • .command.log - Combined output and errors

   # Navigate to the failed process directory
   cd work/a1/b2c3d4...
   
   # Check the error output
   cat .command.err

4. Look for specific error messages

   Sometimes the error message will be self-explanatory, and you will be able to understand the root of the issue. If not, you can look in the common errors and solutions section for some help.

Common errors and solutions

Here are the most frequent issues users encounter and how to fix them:

Missing or incorrect input folder

Error message:

ERROR ~ Validation of pipeline parameters failed!

 -- Check '.nextflow.log' file for details
The following invalid input values have been detected:

* --input (input): the file or directory 'input' does not exist

Cause: The pipeline cannot find your input folder.

Solutions:

• Use absolute paths instead of relative paths: /full/path/to/data not ./data
• Verify your BIDS directory structure matches the inputs guide
• Check file naming follows BIDS conventions (you can use the online validator tool)

Missing age column or not formatted correctly

Error message:

ERROR ~ Missing 'age' column in participants.tsv

Cause: The participants.tsv file doesn’t contain the required age column.

Solution: Add an age column to your participants.tsv file. See the inputs guide for the correct format. Remember that columns must be tab-separated, not space-separated.

Docker not running

Error message:

ERROR ~ Cannot connect to the Docker daemon

Cause: Docker Desktop is not running or Docker is not installed.

Solutions:

• Start Docker Desktop
• Wait a few seconds for Docker to fully start
• If on an HPC cluster, use -profile apptainer instead of -profile docker

Incorrect shell selection

Error message:

ERROR: No b-values are below the dti_max_shell_value threshold of 1200 for subject...

Cause: The pipeline’s default shell selection (b ≤ 1200 for DTI, b ≥ 700 for fODF) doesn’t match your acquisition protocol.

Solution: Specify your shells manually using --dti_shells and --fodf_shells. See the shell selection section for details. Example:

--dti_shells "0 1500" \
--fodf_shells "0 1500 3000"

For heterogeneous datasets

If you have heterogeneous datasets with varying acquisition protocols, you can specify a different threshold for both DTI and fODF fitting. To do so, use the --dti_max_shell_value and/or the --fodf_min_shell_value.

FreeSurfer license missing

Error message:

No license file path provided. Please specify the path using --fs_license parameter.

Cause: You’re using the segmentation profile without providing a FreeSurfer license.

Solution: Get a free license from here and specify it with:

--fs_license /path/to/license.txt

Permission denied errors

Error message:

ERROR ~ Permission denied: '/path/to/output/sub-01/...'

Cause: The pipeline cannot write to the output directory.

Solutions:

• Check you have write permissions to the --outdir location
• Ensure the output directory is not owned by a different user
• On shared systems, verify the directory is not read-only

Still having issues?

If you’ve tried the solutions above and are still stuck:

1. Search existing issues

   Check if someone else has encountered the same problem: GitHub Issues

2. Open a new issue

   If you can’t find a solution, create a new issue on GitHub with:

   • Your command (the full nextflow run command)
   • Error message (from .nextflow.log or .command.err)
   • Pipeline version (the -r flag value, e.g., 0.2.2)
   • System information (OS, Docker/Apptainer version)
   • Input data description (age range, acquisition parameters)