bash line continuation and comments

Question

Any explanation why the comment is echo'ed in the third example?

$ echo a b \
> c # test
a b c

$ echo a b c \
> # test
a b c

$ echo a b c \
> \ # test
a b c  # test

$ echo a b c \
> \  # test
a b c 

Answer 1

In bash, # starts a comment only if it is at the beginning of a word. As per this rule, neither of the following two command lines have comments:

$ echo a b c# test
a b c# test
$ echo a b c \ # test
a b c  # test

In the first case above, # is part of the word c# and # is not at the beginning.

In the second case above (your case), # is part of the word # (where the first character is a blank and # is the second character). # is not at the beginning and therefore does not start a comment.

Normally, the shell treats a blank as a word separator. By escaping the blank, it loses that special interpretation and becomes just another character.

The use or not of a continuation line does not change this.

Documentation

From the section entitled Comments in man bash:

a word beginning with # causes that word and all remaining characters on that line to be ignored. [Emphasis added.]

Answer 2

Seems like # test is the next word/argument and since comments have to start with # the third command has no comment.