git submodule update --remote does not work

I cloned a git repository, mainrepo, which has a submodule submodule1. When I try to get the latest update from submodule1:

$ cd mainrepo

$ git submodule add git@bitbucket.org:myaccount/submodule1.git

$ git submodule update --remote submodule1
Usage: git submodule [--quiet] add [-b branch] [--reference <repository>] [--] <repository> [<path>]
   or: git submodule [--quiet] status [--cached] [--recursive] [--] [<path>...]
   or: git submodule [--quiet] init [--] [<path>...]
   or: git submodule [--quiet] update [--init] [-N|--no-fetch] [--rebase] [--reference <repository>] [--merge] [--recursive] [--] [<path>...]
   or: git submodule [--quiet] summary [--cached|--files] [--summary-limit <n>] [commit] [--] [<path>...]
   or: git submodule [--quiet] foreach [--recursive] <command>
   or: git submodule [--quiet] sync [--] [<path>...]

$ git submodule update --remote
Usage: git submodule [--quiet] add [-b branch] [--reference <repository>] [--] <repository> [<path>]
   or: git submodule [--quiet] status [--cached] [--recursive] [--] [<path>...]
   or: git submodule [--quiet] init [--] [<path>...]
   or: git submodule [--quiet] update [--init] [-N|--no-fetch] [--rebase] [--reference <repository>] [--merge] [--recursive] [--] [<path>...]
   or: git submodule [--quiet] summary [--cached|--files] [--summary-limit <n>] [commit] [--] [<path>...]
   or: git submodule [--quiet] foreach [--recursive] <command>
   or: git submodule [--quiet] sync [--] [<path>...]

I am sure I once tried updating a submodules this way, the docs says to do so too, yet I do not understand why it does not work.

But if I do a fetch and merge directly in the submodule, it gets updated to the latest repository commit as I want it:

$ cd submodule1

$ git fetch && git merge master

Why doesn't the submodule update --remote submodule1 command work?

Maybe that might help:

$ vim mainrepo/.git/config
[core]
        repositoryformatversion = 0
        filemode = true
        bare = false
        logallrefupdates = true
[remote "origin"]
        fetch = +refs/heads/*:refs/remotes/origin/*
        url = git@bitbucket.org:myaccount/mainrepo.git
[branch "master"]
        remote = origin
        merge = refs/heads/master
[submodule "local/src/utils"]
        url = git@bitbucket.org:myaccount/submodule1.git

$ vim .gitmodules
[submodule "submodule1"]
        path = submodule1
        url = git@bitbucket.org:myaccount/submodule1.git

Solution

I would recommend reading this crash course on Submodules from git: https://git-scm.com/book/en/v2/Git-Tools-Submodules

My guess is that, since the repository you cloned already has a submodule (check for .gitmodules in the repo's root!) you shouldn't have to git submodule add it again.

Cloning the repository and then running git submodule update --recursive should suffice!

Alternatively, git clone --recurse-submodules does this automatically for you.