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There exist shell scripts where comments have a space between the hash sign and the actual comment

# a comment at the beginning of a line
echo foo # a comment trailing after a command

and other that do not have one

#another comment at the beginning of a line
echo bar #another comment trailing after a command

Does this decision have some sort of impact on the actual execution of the scripts or is it (just) a coding style issue?

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    After you enabled hashcommands via set -o hashcmds for recent Bourne Shell versions, A line that starts with #something is seens as a special command. But this only applies to the end of the file $HOME/.shrc where this feature is usually enabled for interactive shells. The default for all scripts is disabled hash commands. BTW: hash commands include a method to edit shell aliases witout a need to quote the text.
    – schily
    Jan 15, 2020 at 14:41
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    Another exception is the shebang line (characters hash sign and exclamation mark (#!)) at the beginning of scripts.
    – Paulo Tomé
    Jan 15, 2020 at 14:59
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    One big difference between the shell and other languages like awk or perl is that there should be a space or non-word token before the # in order to be recognized as a comment marker: echo 3#foo will print 3#foo not just 3.
    – mosvy
    Jan 15, 2020 at 21:32

2 Answers 2

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There’s no non-stylistic difference; POSIX specifies that, when recognising tokens in the shell,

If the current character is a '#', it and all subsequent characters up to, but excluding, the next <newline> shall be discarded as a comment. The <newline> that ends the line is not considered part of the comment.

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  • This is only true as long as you do not use POSIX extensions, see my comment on the original question.
    – schily
    Jan 15, 2020 at 14:43
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I'm not aware of any shell in which a whitespace would matter. For example, in man bash it says:

   In a non-interactive shell, or an interactive shell in which
   the interactive_comments option to the shopt builtin is enabled
   (see SHELL BUILTIN COMMANDS below), a word beginning with #
   causes that word and all remaining characters on that line to
   be ignored.

It specifically says that all characters after # are ignored.

Personally, I prefer to put a whitespace after #.

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