[blueflow]

Find the problem:

  echo "$(tr -dc A-Za-z0-9 </dev/urandom | dd count=1 bs=16 2>/dev/null)"
       ^-- SC2005 (style): Useless echo? Instead of 'echo $(cmd)', just use 'cmd'.

I have experienced so many situations where shellcheck is giving harmful advice or warns me about the thing that is exactly my intention.

[xorcist]

The style issue shellcheck reports here is probably what we both suspect it is.

The call to echo has a likely purpose is to trim some spaces from a string, but it's not at all obvious why this is done or what the benefit is. In the best case, it leaves the reader pondering this weird semi-no-op.

If you think shellcheck, and probably most readers, is wrong in this assertion, just speak your mind and let's learn from one another. But I certainly can't guess your intention here.

[rascul]

I would probably do it this way:

tr -dc A-Za-z0-9 </dev/urandom | dd count=1 bs=16 2>/dev/null; echo

A few less characters to type, you get your newline, and shellcheck doesn't complain.

[ndsipa_pomu]

Using "echo" at all is a problem. It's recommended to switch to "printf" instead as echo has unfixable problems, especially with strings that start with a dash.

  printf "%s\n" "$(tr -dc A-Za-z0-9 </dev/urandom | dd count=1 bs=16 2>/dev/null)"

If you don't require the line feed, then you can just use:

  tr -dc A-Za-z0-9 </dev/urandom | dd count=1 bs=16 2>/dev/null

[blueflow]

The thing with echo is repeated again and again but not a problem in the code i've just posted (due to A-Za-z0-9).

And i do require the line feed. Following shellcheck's advice broke the code.

[ndsipa_pomu]

Not a problem until the random string starts with a dash and you get unexpected results from "echo" interpreting it as an option instead of an argument. Using "printf" bypasses that issue as it's clear what the argument is.

Just tested it and ShellCheck is happy with the printf version:

  printf "%s\n" "$(tr -dc A-Za-z0-9 </dev/urandom | dd count=1 bs=16 2>/dev/null)"

It's also got a stylistic improvement (in my opinion, anyhow) in that the line feed is explicit in the printf command rather than being almost a side effect of echo.

Edit: I see your point about the "tr" not enabling a dash, so my example falls a bit flat, but my point stands in other scenarios. It's good practise to avoid echo where feasible. I think the double quotes will also protect in this instance. The problem with relying on the tr string to not foul up echo is that you may later adapt the code and use a different translate string which could then introduce a tricky bug to track down.

[ndsipa_pomu]

I'd also add that in the cases where you disagree with shellcheck (it doesn't get everything correct all the time), you can include a disabling comment just before the line to tell shellcheck that you know best:

# shellcheck disable=SC2005

[saagarjha]

…what’s the problem?