Apparently when tox prints the executed commands it does not keep them in their original form and drops the double quotes, making them really confusing and invalid.

Content in tox.ini:

commands =
        bash -c "git ls-tree --full-tree --name-only -r HEAD plugins | grep -E '\.ya?ml$' | xargs -I FILE -n1 -P 4 ansible-lint --force-color -q -p -x $SKIPPED FILE"

Sample output:

ansible-lint runtests: commands[1] | bash -c git ls-tree --full-tree --name-only -r HEAD plugins | grep -E '\.ya?ml$' | xargs -I FILE -n1 -P 4 ansible-lint --force-color -q -p -x $SKIPPED FILE

As you can see the quotes that mark the bash command to be executed are lost, so if anyone tries to copy/paste the command, it will fail to run because what is printed on screen is invalid.