Why use parens instead of square brackets in ''if !(ssh ...) then''? - shell

Question

I'm trying to understand the use of brackets in this line:
if !(ssh -q $user#$server "[ -d /some/directory ]")then
Usually the condition comes between [], what exactly is the use of () here?

Answer 1

There is a small "debatable" (See comments here) typo in your line. A space in front of your first bracket. In a shell script, the space is not needed. However, depending on some options from your shell, it might be needed to add a space when typing this line directly on the command-line. A rewrite with space would be :
if ! (ssh -q $user#$server "[ -d /some/directory ]"); then ... ; fi
There are several parts in this line
[ -d /some/directory ] : this is a call to the so called test command which can also be written as [ ... ]. test -d /some/directory and [ -d /some/directory ] are identical commands. It will check if the directory /some/directory exists or is accessible. (see man test for more information)
ssh -q $user#$server cmd: this will execute the command cmd as $user on $server. However it will do this quietly (-q) by not throwing any error output. So essentially, with cmd replaced by the above test, we check if that directory exists or is accessible for $user on $server.
man ssh :: -q : Quiet mode. Causes most warning and diagnostic messages to be suppressed
The full command is now enclosed between brackets. The brackets is a grouping which allows to execute several commands in unity. I.e. ( cmd1; cmd2 ) will be executed as one unit. Nonetheless, these brackets will do this in a separate instance. It will fork a sub-shell to do the execution. In this case only a single command is being executed in the sub-shell. It's unnecessary overhead. As you see, even though the syntax looks cleaner, it has an effect.
man bash :: (list) list is executed in a subshell environment. Variable assignments and builtin commands that affect the shell's environment do not remain in effect after the command completes. The return status is the exit status of list.
! cmd : this negates the exit status of cmd. If the command successfully executed (exit status=0) it will assume to be false. Or vice versa.
The if statement is written as if list; then thenlist; fi. The if statement is executed if the exit-status of list is zero. In this case list is ! (ssh -q $user#$server "[ -d /some/directory ]"), while a more common example of list is [ -e foo ] which is the test command.
Similar ways of writing the if-condition are now :
if ! ssh -q $user#$server "[ -d /some/directory ]"; then ...; fi
or you can just move the ! in the test, leading to :
if ssh -q $user#$server "[ ! -d /some/directory ]"; then ...; fi
Finally, if you want to keep the grouping for looks without the sub-shell, you can use { ... }, i.e.
if ! { ssh -q $user#$server "[ -d /some/directory ]"; }; then ...; fi
man bash :: { list; }
list is simply executed in the current shell environment. list must be terminated with a newline or semicolon.
This is known as a group command. The return status is the exit
status of list.

Answer 2

You need a space between ! and (...). One correct way to write this using bash shell:
if ! (ssh -q $user#$server "[ -d \"/some/directory\" ]"); then
echo "do something";
else
echo "do something else";
fi
This checks whether a directory /some/directory exists on a remote server using ssh.
man test to know about options, such as -d.
-d FILE
FILE exists and is a directory
! represents a logical (boolean) NOT or negation to the output of ().
-q option in ssh is for quiet mode.
For information on the single parentheses () in bash see https://www.gnu.org/software/bash/manual/html_node/Command-Grouping.html
Note, single parentheses are used to group command lines in a subshell. As OP suggested, () is not strictly needed/recommended here and the same if condition can be written without it,
if ! ssh -q $user#$server "[ -d \"/some/directory\" ]"; then
.....