npm install multiple package names - command-line

Question

What does it do when I run this command:
npm install --save-dev package1 package2
It is definitely not installing multiple packages, but it looks to be essential. (For example https://www.browsersync.io/docs/gulp)
For me it throws following ERRs:
C:\1HLAVNI\Lukas\Webdesign\lukasradek>npm install --save-dev gulp-babel gulp-add-src
> bufferutil#1.1.0 install C:\1HLAVNI\Lukas\Webdesign\lukasradek\node_modules\bufferutil
> node-gyp rebuild
C:\1HLAVNI\Lukas\Webdesign\lukasradek\node_modules\bufferutil>if not defined npm_config_node_gyp (node "C:\Program Files\nodejs\node_modules\npm\bin\node-gyp-bin\\..\..\node_modules\node-gyp\bin\node-gyp.js" rebuild ) else (node "" rebuild )
gyp ERR! configure error
gyp ERR! stack Error: Can't find Python executable "python", you can set the PYTHON env variable.
gyp ERR! stack at failNoPython (C:\Program Files\nodejs\node_modules\npm\node_modules\node-gyp\lib\configure.js:401:14)
gyp ERR! stack at C:\Program Files\nodejs\node_modules\npm\node_modules\node-gyp\lib\configure.js:356:11
gyp ERR! stack at FSReqWrap.oncomplete (fs.js:123:15)
gyp ERR! System Windows_NT 10.0.10586
gyp ERR! command "C:\\Program Files\\nodejs\\node.exe" "C:\\Program Files\\nodejs\\node_modules\\npm\\node_modules\\node-gyp\\bin\\node-gyp.js" "rebuild"
gyp ERR! cwd C:\1HLAVNI\Lukas\Webdesign\lukasradek\node_modules\bufferutil
gyp ERR! node -v v6.3.1
gyp ERR! node-gyp -v v3.3.1
gyp ERR! not ok
npm WARN install:bufferutil#1.1.0 bufferutil#1.1.0 install: `node-gyp rebuild`
npm WARN install:bufferutil#1.1.0 Exit status 1
> utf-8-validate#1.1.0 install C:\1HLAVNI\Lukas\Webdesign\lukasradek\node_modules\utf-8-validate
> node-gyp rebuild
C:\1HLAVNI\Lukas\Webdesign\lukasradek\node_modules\utf-8-validate>if not defined npm_config_node_gyp (node "C:\Program Files\nodejs\node_modules\npm\bin\node-gyp-bin\\..\..\node_modules\node-gyp\bin\node-gyp.js" rebuild ) else (node "" rebuild )
gyp ERR! configure error
gyp ERR! stack Error: Can't find Python executable "python", you can set the PYTHON env variable.
gyp ERR! stack at failNoPython (C:\Program Files\nodejs\node_modules\npm\node_modules\node-gyp\lib\configure.js:401:14)
gyp ERR! stack at C:\Program Files\nodejs\node_modules\npm\node_modules\node-gyp\lib\configure.js:356:11
gyp ERR! stack at FSReqWrap.oncomplete (fs.js:123:15)
gyp ERR! System Windows_NT 10.0.10586
gyp ERR! command "C:\\Program Files\\nodejs\\node.exe" "C:\\Program Files\\nodejs\\node_modules\\npm\\node_modules\\node-gyp\\bin\\node-gyp.js" "rebuild"
gyp ERR! cwd C:\1HLAVNI\Lukas\Webdesign\lukasradek\node_modules\utf-8-validate
gyp ERR! node -v v6.3.1
gyp ERR! node-gyp -v v3.3.1
gyp ERR! not ok
npm WARN install:utf-8-validate#1.1.0 utf-8-validate#1.1.0 install: `node-gyp rebuild`
npm WARN install:utf-8-validate#1.1.0 Exit status 1
lukasradek#1.0.0 C:\1HLAVNI\Lukas\Webdesign\lukasradek
+-- gulp-add-src#0.2.0
| +-- event-stream#3.1.7
| | `-- split#0.2.10
| +-- streamqueue#0.1.3
| | `-- readable-stream#1.0.34
| `-- through2#0.4.2
| +-- readable-stream#1.0.34
| `-- xtend#2.1.2
| `-- object-keys#0.4.0
`-- gulp-babel#6.1.2
+-- babel-core#6.13.2
| +-- babel-code-frame#6.11.0
| | `-- js-tokens#2.0.0
| +-- babel-generator#6.11.4
| | +-- detect-indent#3.0.1
| | | `-- repeating#1.1.3
| | `-- lodash#4.14.1
| +-- babel-helpers#6.8.0
| +-- babel-messages#6.8.0
| +-- babel-register#6.11.6
| | +-- core-js#2.4.1
| | +-- home-or-tmp#1.0.0
| | +-- lodash#4.14.1
| | +-- path-exists#1.0.0
| | `-- source-map-support#0.2.10
| | `-- source-map#0.1.32
| +-- babel-runtime#6.11.6
| | `-- regenerator-runtime#0.9.5
| +-- babel-template#6.9.0
| | `-- lodash#4.14.1
| +-- babel-traverse#6.13.0
| | +-- globals#8.18.0
| | +-- invariant#2.2.1
| | | `-- loose-envify#1.2.0
| | | `-- js-tokens#1.0.3
| | `-- lodash#4.14.1
| +-- babel-types#6.13.0
| | +-- lodash#4.14.1
| | `-- to-fast-properties#1.0.2
| +-- babylon#6.8.4
| +-- json5#0.4.0
| +-- lodash#4.14.1
| +-- minimatch#3.0.2
| +-- path-exists#1.0.0
| +-- private#0.1.6
| +-- shebang-regex#1.0.0
| `-- slash#1.0.0
`-- object-assign#4.1.0
npm WARN optional Skipping failed optional dependency /chokidar/fsevents:
npm WARN notsup Not compatible with your operating system or architecture: fsevents#1.0.14
npm WARN lukasradek#1.0.0 No repository field.

Answer 1

Save as dependencies:
npm i package1 package2
Save as dev-dependencies:
npm i -D package1 package2
npm i --save-dev package1 package2

Answer 2

It is definitely not installing multiple packages
Why? You're installing package1 and package2 and marking them as devDependencies with --save-dev.
As stated in the documentation, you may combine multiple arguments, and even multiple types of arguments. In your case, you're combining 2 package names published on the registry.

Answer 3

npm install -save module1 module2
Example:
npm install -save morgan chalk errorhandler lusca dotenv path mongoose
The above command will install the latest versions of morgan, chalk, errorhandler, lusca, dotenv, path and mongoose.

Answer 4

The other answers here do not appear to answer the question. Specifically, doing an npm install --save-dev p1 p2 p3 does not cause p2 and p3 to be stored in my package.json file.
The following line installs all packages + saves it in dev dependencies for me:
for i in 'package1' 'package2'; do npm install --save-dev "$i"; done
I'm on Windows and use Cygwin, but this works on *nix systems, too.
Somewhat unrelated but very important if using Cygwin: if you do use Cygwin, you need to add noacl to your mount options, or it will mess up your module permissions and cause Windows to freak out.