BASH comments: Difference between # (hash) -vs- : (colon) commenting

Question

In bash GNU bash, version 4.2.46(2)-release (x86_64-redhat-linux-gnu) or any stable one, You can use # or : to comment

What's the difference between these 2 and usefulness?

I noticed # comment makes the whole line a comment, while : scope/effect is only until it reaches the first \n or ; character (in a given line).

[gigauser@someserver ~]$ # this is a # comment; echo this will not print
[gigauser@someserver ~]$ : this is a : comment; echo this will print
this will print

In the following code, why the last 2 comments didn't work as expected to treat them as a comment then (albeit :: works if it comes after a : ?

[gigauser@someserver ~]$ #
[gigauser@someserver ~]$ ##
[gigauser@someserver ~]$ ####
[gigauser@someserver ~]$ ####
[gigauser@someserver ~]$ : : : : : : : : : :
[gigauser@someserver ~]$ #
[gigauser@someserver ~]$ # # #
[gigauser@someserver ~]$ ####
[gigauser@someserver ~]$
[gigauser@someserver ~]$ :
[gigauser@someserver ~]$ : :
[gigauser@someserver ~]$ : : : : : : : : : : : : ::::::::
[gigauser@someserver ~]$ : : :::::
[gigauser@someserver ~]$
[gigauser@someserver ~]$
[gigauser@someserver ~]$ ::
-bash: ::: command not found
[gigauser@someserver ~]$ :::::
-bash: :::::: command not found
[gigauser@someserver ~]$

You *can* use `:` but it's not meant for comments. `#` is made for that https://stackoverflow.com/questions/3224878/what-is-the-purpose-of-the-colon-gnu-bash-builtin — micke, May 12 '21 at 17:07
Does this answer your question? [What is the purpose of the : (colon) GNU Bash builtin?](https://stackoverflow.com/questions/3224878/what-is-the-purpose-of-the-colon-gnu-bash-builtin) — micke, May 12 '21 at 17:07
@micke That post is very good but only focuses on `:` (`bash null command`) not sure if it gives comparison. I also found this post: https://www.shell-tips.com/bash/null-command/ BASH `NULL COMMAND` — AKS, May 13 '21 at 00:59
Yes I meant to highlight that while it's certainly possible to comment with `:`. It's not it's purpose, a bad idea and against all and any style guides. See: https://www.shell-tips.com/bash/comments/#5-using-unconventional-block-comments — micke, May 13 '21 at 12:28

Gordon Davisson · Accepted Answer · 2021-05-13T17:26:18.280

: isn't a comment at all, it's a command that doesn't do anything. The most important difference is that since it's a command, any shell syntax after it will be interpreted and acted on. For example:

: Check whether x > 1

Here the > 1 part will be interpreted as an output redirection, so it'll create a file named "1" and redirect the (empty) output to it. If it'd been x < 1 it would've tried to open "1" for input and (probably) failed.

: Check permissions on the user's home directory

Here the ' will be interpreted as starting a single-quoted string (which'll be an argument to :), and will probably wreak havoc with how the rest of the script is parsed.

: Test for completion (again)

Produces -bash: syntax error near unexpected token `('

The : command is sometimes useful if you want parameter expansions to be performed (because of their side effects), but don't want to do anything with the result.

: ${VAR:=defaultvalue}

Here, if $VAR isn't already set to something nonblank, the := modifier will set it to "defaultvalue". This is basically a shorthand for

if [ -z "$VAR" ]; then
    VAR=defaultvalue
fi

Edit:

As AKS pointed out with a link in the comments, one effect of : is that you can make block comments with it and a here-document with a quoted delimiter. That makes a functional difference from #.

: <<'COMMENT'
This block doesn't do anything, effectively a multi line comment block.
It'll also not get identified by linters and syntax highlighters as a comment.
COMMENT

Regular comments would have to start each line with #.

(Note: contrary to what the link says, you can use either single- or double-quotes on the here-doc delimiter, i.e. either <'COMMENT' or <"COMMENT". Without some sort of quoting on the delimiter, the shell will try to parse and expand any $ or backtick expressions it finds in the document, which can again cause trouble.)

BASH comments: Difference between # (hash) -vs- : (colon) commenting

1 Answers1