oclif-flag-type

static flags = {
  name: flags.string({
    char: 'n',                    // shorter flag version
    description: 'name to print', // help description for flag
    hidden: false,                // hide from help
    multiple: false,              // allow setting this flag multiple times
    env: 'MY_NAME',               // default to value of environment variable
    options: ['a', 'b'],          // only allow the value to be from a discrete set
    parse: input => 'output',     // instead of the user input, return a different value
    default: 'world',             // default value if flag not passed (can be a function that returns a string or undefined)
    required: false,              // make flag required (this is not common and you should probably use an argument instead)
    dependsOn: ['extra-flag'],    // this flag requires another flag
    exclusive: ['extra-flag'],    // this flag cannot be specified alongside this other flag
  }),

  // flag with no value (-f, --force)
  force: flags.boolean({
    char: 'f',
    default: true,                // default value if flag not passed (can be a function that returns a boolean)
    // boolean flags may be reversed with `--no-` (in this case: `--no-force`).
    // The flag will be set to false if reversed. This functionality
    // is disabled by default, to enable it:
    // allowNo: true
  }),
}