why is the following not working?

[bayeslearner]

It seems
USER_NAME=abc task ctx:greet_user works on linux
but
task ctx:greet_user USER_NAME=abc does not work on linux.

version: '3'

tasks:
  greet_user:
    desc: "Greet the user with a name."
    vars:
      USER_NAME:
        sh: 'echo "${USER_NAME:-DefaultUser}"'
    cmds:
      - echo "Hello, {{.USER_NAME}}!"

[mihakrumpestar]

I use it like this and it works:

task deploy PROJECT=caddy

version: '3'

vars:
  ENV: prod
  PROJECT: # Task parameter/arguments are not yet available in root interpolation/templating, Taskfile also does not support top level require
    sh: '[ -z "${PROJECT+x}" ] && { echo "Error: PROJECT is not defined" >&2; exit 1; } || echo "$PROJECT"'

It is possible that it is not available as environment variable in tasks, but only at top level.
I am doing the above, because command defined VARS/arguments (this is what you define after task command, before it, you define an ENV in shell and pass it to task) are not immediately available in top level VARS.

[vmaerten]

Hello @bayeslearner
If you want to have a default value for a variable, the recommanded way is to use the go templating :

  greet_user:
    desc: "Greet the user with a name."
    vars:
      USER_NAME: '{{.USER_NAME| default "DefaultUser"}}'
    cmds:
      - echo "Hello, {{.USER_NAME}}!"

Both USER_NAME=abc task ctx:greet_user and task ctx:greet_user USER_NAME=abc works