I'm successfully passing them using the env property in launch.json:

{
  "version": "0.2.0",
  "configurations": [
    {
      "type": "pwa-node",
      "request": "launch",
      "name": "Launch Program",
      "skipFiles": [
        "<node_internals>/**"
      ],
      "program": "${workspaceFolder}/index.js",
      "env": {
        "TEST_VAR": "foo"
      }
    }
  ]
}

Here is an example, you set the value in the environment block.

    {
// Use IntelliSense to learn about possible attributes.
// Hover to view descriptions of existing attributes.
// For more information, visit: https://go.microsoft.com/fwlink/?linkid=830387
"version": "0.2.0",
"configurations": [
    {
        "name": "g++ - (GDB 9.2) Build and debug active file with RepoCodeInspection",
        "type": "cppdbg",
        "request": "launch",
        "program": "${fileDirname}/bin/${fileBasenameNoExtension}",
        "args": [],
        "stopAtEntry": false,
        "cwd": "${workspaceFolder}",
        "environment": [
            {
                "description": "same as commands below by using 'setenv ...",
                "info": "cant debug b/c of libBase/libRecipe now requiring dependency to boost for stacktrace dumps",
                "name": "LD_LIBRARY_PATH",
                "value": "/libs/:./"
            }
        ],
        "externalConsole": false,
        "MIMode": "gdb",
        "setupCommands": [
            {
                "description": "Enable pretty-printing for gdb",
                "text": "-enable-pretty-printing",
                "ignoreFailures": true
            }
        ],
        "logging": {
            "trace": false,
            "traceResponse": false
        },
        "preLaunchTask": "RepoCodeInspection",
        
    },
    {
        "name": "g++ - (GDB 9.2) Attach to a running process",
        "type": "cppdbg",
        "request": "attach",
        "processId":"${command:pickProcess}",
        "program": "${fileDirname}/bin/${fileBasenameNoExtension}",
        "MIMode": "gdb",
        "setupCommands": [
            {
                "description": "Enable pretty-printing for gdb",
                "text": "-enable-pretty-printing",
                "ignoreFailures": true
            }
        ],
        "logging": {
            "trace": false,
            "traceResponse": false
        },
        "preLaunchTask": "RepoCodeInspection",
       
    },
]

}

• Doesn't work, why?

According to this post from user weinand...

The ".env" file is read and processed after VS Code has substituted variables in the launch config. So your debugged program will indeed see the environment variable "FOO" with the correct value but VS Code's variable substitution in the launch.json will not see it.

The reason for this is that ".env" files are a node.js concept and not a generic platform mechanism. So VS Code does not know anything about .env files, but the node.js debugger knows about .env files.

... this functionality in launch.json is specific for applications running on Node.js, although that's not what M$ explains in their documentations for VSCode.

• Possible solution

For Python applications (possibly for other platforms as well) environment variables defined in a .env file (or whatever name you like) will be available for your application as long as the following configuration is present in launch.json...

{
    "version": "0.2.0",
    "configurations": [
        {
            [...]
            "envFile": "${workspaceFolder}/.env", // Path to the ".env" file.
            [...]
        }
    ]
}

Note that just exporting a variable...

export SOMEVAR_A=1234

... will not make the environment variable SOMEVAR_A available for the application being executed by the VSCode debugger nor for the settings - especially inside "env" and "args" ("configurations") - in launch.json as, for example, in this case...

{
    "version": "0.2.0",
    "configurations": [
        {
            [...]
            "env": {
                "SOMEVAR_A": "${env:SOMEVAR_A}"
            },
            "args": [
                "${env:SOMEVAR_A}"
            ]
            [...]
        }
    ]
}

NOTE: In our tests the ${env:SOMEVAR_A} syntax did not work in any scenario. That is, didn't work for the application ("env") and didn't work for the settings ("args") in launch.json.

PLUS I: Dirt Hack

For values present in "args" ("configurations") you can use the hack below...

{
    "version": "0.2.0",
    "configurations": [
        {
            [...]
            "envFile": "${workspaceFolder}/.env",
            "args": [
                "`source \"${workspaceFolder}/.env\";echo ${SOMEVAR_A}`"
            ]
            [...]
        }
    ]
}

... as the configuration in "envFile" doesn't work.

Notice, although, that the following construction...

[...]
"args": [
    "`echo ${SOMEVAR_A}`"
]
[...]

... would also work for "args" as long as the environment variable "SOMEVAR_A" has been previously exported in the conventional way.

The same reasoning would work for a tasks (tasks.json), but in both cases we can't guarantee that.

TIP: An .env File Example

SOMEVAR_A="abcd"
SOMEVAR_B="efgh"
SOMEVAR_C=123456

PLUS II: Export Variables

There are cases where you will need to export variables (eg. export SOMEVAR_A="abcd") so that they can be consumed by certain resources. In these cases there may be problems, because the fact that we export variables prevents (we don't know why) that they are seen in the context of the "envFile" configuration "envFile": "${workspaceFolder}/.env".

A workaround to get around these limitations is to add set -a before the variables set and set +a after it. With this we were able to meet the two scenarios as this example...

#!/usr/bin/env bash

set -a    
SOMEVAR_A="abcd"
SOMEVAR_B="efgh"
SOMEVAR_C=123456
set +a

... or in a more compatible and safe way use set -a/set +a as in this example...

[...]
"args": [
    "`set -a;source \"${workspaceFolder}/.env\";set +a;echo ${SOMEVAR_A}`"
[...]

VSCode's support for environment variables is a mess!

• Conclusion

We don't know if the limitations we are dealing with here are from VSCode's own design or are bugs. Anyway, it doesn't seem to make much sense.

These procedures were tested on Manjaro Linux (Arch based).

[Ref(s).: https://unix.stackexchange.com/a/79077/61742 , https://stackoverflow.com/a/30969768/3223785 ]

As described in the documentation, you need to use the args attribute. E.g.

{
    // Use IntelliSense to learn about possible attributes.
    // Hover to view descriptions of existing attributes.
    // For more information, visit: https://go.microsoft.com/fwlink/?linkid=830387
    "version": "0.2.0",
    "configurations": [
        {
            "type": "node",
            "request": "launch",
            "name": "Debug App",
            "program": "${workspaceFolder}/main.js",
            "args": ["arg1", "arg2", "arg3"]
        }
    ]
}