Create a new console command

The "Make" extension can be used to create a new console command:

vendor/bin/typo3 make:command
typo3/sysext/core/bin/typo3 make:command

You will be prompted with a list of installed extensions. If your newly created extension is missing, check if you installed it properly.

Enter the command name to execute on CLI [myextension:dosomething]:
This name will be used to call the command later on. It should be prefixed with your extensions name without special signs. It is considered best practise to use the same name as for the controller, in lowercase.
Should the command be schedulable? (yes/no) [no]:
If you want the command to be available in the backend in module System > Scheduler choose yes. If it should be only callable from the console, for example if it prompts for input, choose no.

Have a look at the created files

The following files will be created or changed:

Page tree of directory src/extensions
$ tree src/extensions
└── my-test
    ├── Classes
    |   └── Command (*)
    |   |   └── DoSomethingCommand.php (*)
    ├── Configuration
    |   └── Services.yaml (*)
    ├── composer.json
    └── ext_emconf.php

Call the new command

After a new console command was created you have to delete the cache for it to be available, then you can call it from the command line:

vendor/bin/typo3 cache:flush
vendor/bin/typo3 myextension:dosomething
typo3/sysext/core/bin/typo3 cache:flush
typo3/sysext/core/bin/typo3 myextension:dosomething

Next steps

You can now follow the console command tutorial and learn how to use arguments, user interaction, etc.