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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. Commented 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. Commented 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. Commented Jan 15, 2020 at 21:32
  • @schily I can't find any information about set -o hashcmds at gnu.org/s/bash/manual/html_node/The-Set-Builtin.html or Google. Do you have any link where I can learn more about it? Commented Mar 15, 2024 at 15:23

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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. Commented 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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