Inline comments for Bash?

My preferred is:

Commenting in a Bash script

This will have some overhead, but technically it does answer your question

echo abc `#put your comment here` \
     def `#another chance for a comment` \
     xyz etc

And for pipelines specifically, there is a cleaner solution with no overhead

echo abc |        # normal comment OK here
     tr a-z A-Z | # another normal comment OK here
     sort |       # the pipelines are automatically continued
     uniq         # final comment

How to put a line comment for a multi-line command