Collection not found when running newman from shell script


Newman logs the following error when called from a shell script.

error: collection could not be loaded
  " unable to read data from file "1_authentication.postman.collection.json
  ' ENOENT: no such file or directory, open '1_authentication.postman.collection.json

This is the script:

newman run $collection

It works only if I replace the $collection parameter with the name of the file

newman run 1_authentication.postman.collection.json

Newman version: 5.2.4
OS: Ubuntu 20.04

I think I found a similar issue here but the proposed solution did not work.

Is this a newman issue or am I doing something wrong?
Thank you!