What is the most portable syntax for `if` statements?

Question

(NOTE: There is a very similar question, asked yesterday oddly enough, but it doesn't cover the main syntaxes I'm concerned with. If I were to update the other question to include them it might render the answers no longer valid, so I'll focus here on what was missed in the other question. Anyone who wants the whole picture should look at both questions and their accepted answers.)

I typically write my if statements as follows:

if [[ -z "$SOMETHING" ]]; then
  # blah blah blah
fi

which works well for me under Bash and Z Shell, but I recently came across a syntax which drops the square brackets entirely:

if command -v given-command > /dev/null 2>&1; then
  echo given-command is available
else
  echo given-command is not available
fi

Is the version without any brackets portable? How does it compare to the options presented here?

Is my standard version mainly only supported on Bash and Z Shell (like the [[ -w /home/durrantm ]] && echo "writable" version mentioned on that other question)?

Answer 1

Yes, your standard version is not POSIX-standard, as the question you linked to notes. However, the following is:

if some-command args; then ...

I'd say this is the only portable form of if, since, this is what if is all about. if does something if some-command (which can be a function, a shell builtin, or an executable¹) returns 0 for the arguments and input it is run with.

When you run if [ ] or if [[ ]], [ ] and [[ ]] are programs or shell builtins which return 0 if the condition tests true. If you convert [ to test, you'll see that you're always running the form without the brackets:

if [ ... ]
# same as 
if /bin/test ...

---

¹To be more precise, let's look at how the if construct is defined:

The if command shall execute a compound-list and use its exit status to determine whether to execute another compound-list.

The format for the if construct is as follows:

if compound-list 
then
    compound-list
[elif compound-list 
then
    compound-list] ...
[else
    compound-list]
fi

Note that a compound list is not the same as a compound command.