Comments (Bash Reference Manual)

3.1.3 Comments

In a non-interactive shell, or an interactive shell in which the interactive_comments option to the shopt builtin is enabled (see The Shopt Builtin), a word beginning with ‘#’ causes that word and all remaining characters on that line to be ignored. An interactive shell without the interactive_comments option enabled does not allow comments. The interactive_comments option is on by default in interactive shells. See Interactive Shells, for a description of what makes a shell interactive.