> ( : < dataset.json ) && echo YES # is dataset.json readable?

The subshell execution parentheses and the colon are superfluous here, just:

   < dataset.json && echo YES

> ( : >> result.json ) && echo YES # is result.json writable?

As a go-to idiom for a writability test, it gives me pause. If the file didn't exist, we created a zero-length one. That might be okay if we are going to write to it anyway as the next action.

If we are testing because we intend to overwrite it, why not just "> result.json" (which is by itself an idiom for truncating a file to zero length).

When would we every do this? Maybe before some command which takes the file name as a destination file argument rather than using output redirection, and which performs a lengthy computation before trying to open the file for writing. We can catch the permission error early.

I don't think I've ever coded such a test; normally you just do the operation that writes to the file and let that fail.

In POSIX C, there is a function access() for doing these kinds of tests. But it has a special purpose: it is meant to be used by a setuid root process to perform a permission test as if it were the real user/group (the one which elevated privilege to root). I.e. it's not can we do this operation, but should we do this operation (would we still be allowed, if we dropped privileges back to the original user).

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

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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

---

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.