keybinding build_with with cmd in sublime - sublimetext3

Question

I want to call a specific build command stored in an external plugin of Sublime. The .sublime.build looks like this.
{
"selector": "text.html.markdown.knitr",
"working_dir": "${project_path:${folder}}",
"env": { "LANG": "en_US.UTF-8" },
"cmd": [ "Rscript -e \"library(knitr); knit('$file', output='$file_path/$file$
"shell": true,
"variants":
[
{
"name": "Run",
"working_dir": "$file_path",
"shell_cmd": "Rscript -e \"rmarkdown::render(input = '$file')\""
}
]
}
It uses cmd to simply call an external command. I would like to create a keybinding that automatically selects the "Run" variant of the .sublime.build. I tried with the following code:
{ "keys": ["ctrl+shift+b"], "command": "build", "args": {"build_system": "/Packages/knitr/knitr-Markdown.sublime-build", "variant": "Run" }},
Unfortunately, the shell returns
shell_cmd or cmd is required
[cmd: None]
[dir: /Users/serg/Desktop]
[path: /Library/Frameworks/Python.framework/Versions/2.7/bin:/Users/serg/anaconda3/bin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin:/Library/TeX/texbin:/usr/local/MacGPG2/bin:/opt/X11/bin]
[Finished]
Any help is appreciated

Answer 1

First of all, your current build file is invalid, the JSON itself is not valid. I'm not sure if this is a copy & paste error. If not, use a JSON Validator to fix the syntax.
Next, you need to provide the command in the correct syntax. cmd is expecting the command as an array. Since your cmd is incomplete, I'll provide a different example.
Wrong syntax
"cmd": ["compiler --arg source.c"]
Correct syntax
"cmd": ["compiler, "--arg", "source.c"]
For reference, here's one of the build files from the R-IDE package:
{
"selector": "text.html.markdown.rmarkdown",
"working_dir": "$file_path",
"cmd": [
"Rscript", "-e",
"rmarkdown::render('$file_name', encoding = 'UTF-8')"
],
"osx":{
"path": "/Library/TeX/texbin:/usr/local/bin:$PATH"
}
}