Edit

Custom Base Class

Use inheritance to share functionality between common commands. Here is an example of a command base class that has some common shared flags.

For large CLIs with multiple plugins, it's useful to put this base class into its own npm package to be shared.

// src/baseCommand.ts
import {Command, Flags, Interfaces} from '@oclif/core'

enum LogLevel {
  debug = 'debug',
  info = 'info',
  warn = 'warn',
  error = 'error',
}

export type Flags<T extends typeof Command> = Interfaces.InferredFlags<typeof BaseCommand['globalFlags'] & T['flags']>

export abstract class BaseCommand<T extends typeof Command> extends Command {
  // add the --json flag
  static enableJsonFlag = true

  // define flags that can be inherited by any command that extends BaseCommand
  static globalFlags = {
    'log-level': Flags.enum<LogLevel>({
      summary: 'Specify level for logging.',
      options: Object.values(LogLevel),
      helpGroup: 'GLOBAL',
    }),
  }

  protected flags!: Flags<T>

  public async init(): Promise<void> {
    await super.init()
    const {flags} = await this.parse(this.constructor as Interfaces.Command.Class)
    this.flags = flags
  }

  protected async catch(err: Error & {exitCode?: number}): Promise<any> {
    // add any custom logic to handle errors from the command
    // or simply return the parent class error handling
    return super.catch(err)
  }

  protected async finally(_: Error | undefined): Promise<any> {
    // called after run and catch regardless of whether or not the command errored
    return super.finally(_)
  }
}

// src/commands/my-command.ts

export default class MyCommand extends BaseCommand<typeof MyCommand> {
  static summary = 'child class that extends BaseCommand'

  static examples = [
    '<%= config.bin %> <%= command.id %>',
    '<%= config.bin %> <%= command.id %> --json',
    '<%= config.bin %> <%= command.id %> --log-level debug',
  ]

  static flags = {
    name: Flags.string({
      char: 'n',
      summary: 'Name to print.',
      required: true,
    }),
  }

  public async run(): Promise<Flags<typeof MyCommand>> {
    for (const [flag, value] of Object.entries(this.flags)) {
      this.log(`${flag}: ${value}`)
    }

    return this.flags
  }
}

For a more complex example, here's how we do this for the Salesforce CLI.