Item — HN Reader

They are NOT superflous, and all you need to prove it is `zsh` (but there are others that follow suit in similar fashions):

  zsh% echo "hello world" > data
  
  zsh% < data && echo "READABLE"   # <- will print the contents of data
  hello world
  READABLE
  
  zsh% : < data && echo "READABLE" # <- this will not
  READABLE

So, if you want something that "everyone can use" without going into details about the difference between commonly used shells.. you'd use the null-command.

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and given that we use the null-command, it _WILL_ behave different with or without subshell.. and all you need is `bash --posix` to prove it:

    % cat subshell.sh
    #!/bin/bash --posix
    ( : < missing.json ); echo AFTER    # <- will echo AFTER
    
    % ./subshell.sh
    ./subshell.sh: line 2: missing.json: No such file or directory
    AFTER
    
    % cat no-subshell.sh
    #!/bin/bash --posix
    : < missing.json; echo AFTER        # <- this will not
    
    % ./no-subshell.sh
    ./no-subshell.sh: line 2: missing.json: No such file or directory

    % : ^- apparently.. there is a difference

The output above is not truncated, `no-subshell.sh` will stop executing due to the broken read.

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One should never trust things just because they are written, but that also applies to comments on HN. Originally when I read your message I actually thought I made a mistake, I was very close to writing an apology comment and adding a note to the blog post, but not close enough - I had to test it again.

I'm thankful for the watchful eyes and scrutiny when reading things online, that's good - keep it up, but your message is factually wrong - on so many levels.

3 comments

> : < missing.json; echo AFTER # <- this will not

This is required by POSIX in the section "Consequences of Shell Errors". If a redirection error occurs in a special built-in command, a non-interactive shell is required to exit.

The bare redirection without : does not have this problem:

  < missing.json; echo AFTER

Therefore, in a POSIX-conforming shell, we do not need to wrap it in a subshell.

> but your message is factually wrong - on so many levels.

How many? Can you count the levels? I didn't know that in zsh "< file" dumps to standard output, which is suppressed by :. If I used Zsh, I would know that sort of thing.

It’s very common that the most upvoted comment on a post is a mistaken correction. People seem to love comments seemingly proving that the post is wrong without actually checking anything. And once it has enough upvotes a big discussion may follow up with personal attacks on the author or other questionable comments, while the people trying to point out that the post actually is accurate get either buried under the critic storm or downvoted to oblivion because people just don’t like the truth. This happens in every controversial topic.

One of the reasons I stopped writing.

I have never been in this situation before but I do agree with you, and honestly it feels very bad. I saw the comment when it was posted and, perhaps naively, thought "oh well, no one will care - its just another fluff comment".

Fast-forward to seeing that comment climb up the ranks, posted by a person who has 270x my karma, who seemingly gets upvotes by just.. writing things? no proof? no rationale? nothing?

Yeah, feels bad. I'm super happy so many are enjoying the article, and this situation can't take too much away from that, but man.. discouraging for sure.

Don’t worry, everyone who reads the comment will read yours as well, which is itself interesting.

> Yeah, feels bad. I'm super happy so many are enjoying the article, and this situation can't take too much away from that, but man.. discouraging for sure.

Again, don’t worry too much about that. The story got upvoted to the front page and lots of people will read it, that’s what matters. Not that a misguided post got a bunch of upvotes.

As a programmer you will be in the situation many times when you write something that has to be a certain way for reason (like portability) and someone won't see it (possibly a more senior person). That should not feel bad at all, or discouraging; there is no egg on your face!

Or, you might not even write that something yourself; you just understand why it is that way and leave it alone in the refactoring that you are doing; then you get a review comment like "you can get rid of this from here", and you have to respond "no you can't, because ...".

  zsh% < data > /dev/null && echo READABLE   # <- fixes it, sorta

Maybe zsh still reads the entire file and copies it to /dev/null, which would be a severe performance problem for large files.

The colon by itself, without the subprocess, also prevents the dumping to stdout:

  zsh% : < data && echo READABLE

However, with the subprocess parentheses, we can redirect the nonexistence diagnostic to /dev/null, which I don't see mentioned or exemplified in the article:

  zsh% : < nonexistent 2> /dev/null && echo READABLE
  zsh: no such file or directory: nonexistent
  zsh% (: < nonexistent) 2> /dev/null && echo READABLE

The normal way of testing whether something exists and is readable that you would actually use in production script looks more like this:

  $ test -r file && echo YES
  $ [ -r file ] && echo YES

so we are talking about obfuscated coding.

I would say that "if you want something everyone can use", the -r test would be the first candidate.

BTW, why not bring up the strawman of tcsh?

  tcsh% < nonexistent && echo READABLE
  Invalid null command.
  tcsh% : < nonexistent && echo READABLE
  nonexistent: No such file or directory.

Yes, if we want an obfuscated readability test which works literally in any shell that is currently still in deployment in systems that permit new scripts to be installed and run, it looks like do need that colon.

However, fish doesn't like &&:

  fish> < nonexistent && echo yes
  fish: Expected a command, but instead found a redirection
  fish> : < nonexistent && echo yes
  fish: Unsupported use of '&&'. In fish, please use 'COMMAND; and COMMAND'.
  : < nonexistent && echo yes
                  ^

Does it support test -r?

  fish> test -r nonexistent
  fish> test -r nonexistent; and echo yes
  fish> test -r /etc/hosts; and echo yes
  yes

Not parentheses though:

  fish> ( test -r /etc/hosts ); and echo yes
  fish: Illegal command name “( test -r /etc/hosts )”

We clearly have to restrict the idea of what "everyone can use" to POSIX-like shells; there is no getting around shell differences absolutely, other than for perhaps trivial command invocations.

I think you should take a deep breath and read this updated FAQ:

https://refp.se/articles/your-shell-and-the-magic-colon#why-...

I am not asking anyone to put this in production, it is a tongue-in-cheek/cute way of explaining/showing what the null-command _can_ do, not what it _must_ do.

No, any distro that stopped shipping entities mentioned to use only null-command variants.. well, I'd save whatever your end goal is to a meeting with them.

Just enjoy the article for what it is — a piece of trivia.

Best Regards, Filip Roséen - refp

I understand; it's just "here are these cool, obscure things you can do with the colon". I had no idea it was supposed to be "here are these cool, obscure things yuo can do with the colon, carefully coded to work on numerous POSIX-like shells that are in widespread use.

To me, it's "cool" that you can just do "< test-file && echo exists", on a shell whose developers haven't decided to imbue a whole bunch of new requirements into redirection. It's completely legit to note that the existence test is not coming from the : command, but from redirections, and redirections can be set up without a command. The : is only there to work around shell quirks.

For decades, I used "> file" to truncate files to zero length, never with a colon command; the example makes it look as if the colon is required in order for the command line to be valid and for the effect to take place.

BTW I noticed that zsh not only does the cat-like dumping to stdout in non-interactive mode, but it also does the redirection to the pager!!! So if you drop that test into a non-interactive script, and it finds an existing file, if that script is run in a terminal session, it will pause for input.