# is not always a comment symbol in Bash

When I started using Nix, mainly for temporary development environments, I noticed something unexpected: # in some shell commands was treated like a “normal” character. I always thought # would mark the start of a comment, but that’s not always the case.

Here’s an example of # working as a comment symbol:

$ echo Hello # World
Hello

And here’s how you use it as a non-comment symbol:

$ echo Hello#World
Hello#World

Mind-blowing, isn’t it?

So I went to check Bash’s man page:

[…] a word beginning with # introduces a comment. A word begins at the beginning of a line, after unquoted whitespace, or after an operator. The comment causes that word and all remaining characters on that line to be ignored. […]

That’s interesting! Only a word beginning with # starts a comment. That’s different from other programming languages. For example, in Python any # not inside a string starts a comment, no leading whitespace required.

Bash always amazes me with its quirks.