Yes, `[` is a command, and has a man page and everything, but in a script it doesn't look like a command. It looks like something "special" is going on.

Whereas using `test` emphasises the point that you're just running another program. That all `if` does is check the output status of whatever program is being run, whether that's `test`/`[`, or `grep`, or anything else at all.

(Personally, I don't think that emphasis is necessary. But I've been using shell scripts long enough that I understand these nuances fairly well without having to think about them much any more. So I think that GP's point is a reasonable perspective to have, and worth considering.)

    while [ 1 ]; do ...; done

   while true; do ...; done

What's the reason for always using [ ?

(Making life easier for others and your self)

    test -d /nix && echo "$_ exists" || echo "$_ doesn’t exist"

(Not POSIX-compliant. Documented in Bash and Zsh.)

$_ is nto documented in the man page but is in the Info manual. It is not only set to the last argument of a command but also to the script name on script startup.

  $ bash --version
  GNU bash, version 4.4.20(1)-release (i686-pc-linux-gnu)
  [ ... ]

  $ cat underscore.sh
  #!/bin/sh
  # ^^ mistake here, should be /bin/bash

  test -e foo && echo $_ exists

  echo 'value of $_' = $_

  $ touch foo
  $ ./underscore.sh
  ./underscore.sh exists
  value of $_ = ./underscore.sh

But at the interactive prompt:

  $ test -e foo && echo $_ exists
  foo exists

In the first place, I code in POSIX, except in the rare situation of making something that is strictly geared toward Bash.

Magic global variables subject to hidden side effects are garbage; I already feel dirty enough when I have to use $?.

This piece of crap also breaks under the DEBUG trap:

  $ debug() {
  > :
  > }
  $ trap debug DEBUG
  $ test -e foo && echo $_ exists
  debug exists
  $ test -e bar && echo $_ exists
  !1!

Sorry, I'm not going back to a 1978 way of writing shell tests, in order to use some broken magic variable.

Why not?

  [ -d /nix ] && echo ...

  D=/nix test -d $D && echo $D exists …

  $ V=nada echo "x${V}x"
  xx

    while true; do ...; done

I might want to avoid an external process call just to create an unconditional loop.

  $ echo $BASH_VERSION
  4.4.20(1)-release
  $ type true
  true is a shell builtin

  $ type test
  test is a shell builtin
  $ type [
  [ is a shell builtin