Magento 2: How to solve an error when reindex by CLI?

There are no commands defined in the "indexer" namespace.
sudo php -f ./bin/magento indexer:reindex

Message is added to queue, wait to get your file soon magento 2

Export file problem in magento 2.3.2
php bin/magento cron:run

How To Migrate From Magento 1 To Magento 2 Without Breaking Anything.

You migration the latest magento 2 version . Mention the vertion for example data-migration-tool:2.3.3

composer config repositories.magento composer https://repo.magento.com
composer require magento/data-migration-tool:2.3.3


Edit config.xml file :-
Path: /vendor/magento/data-migration-tool/etc/opensource-to-opensource/1.9.2.
 <source>
        <database host="localhost" name="rene_copy_db" user="root" password="nopass" />
    </source>
    <destination>
        <database host="localhost" name="rene_m2_db" user="root" password="nopass" />
    </destination>
Also added this value crypt_key (from magento 1.9):
<crypt_key>sony</crypt_key>

From version mention  for example :/opensource-to-opensource/1.9.2.4/config.xml

Path: /vendor/magento/data-migration-tool/etc/opensource-to-opensource/1.9.2.4
map.xml

copy magento 1.9 media folder  to magento 2  media folder (pub/)

php bin/magento migrate:settings vendor/magento/data-migration-tool/etc/opensource-to-opensource/1.9.2.4/config.xml
php bin/magento migrate:data -a vendor/magento/data-migration-tool/etc/opensource-to-opensource/1.9.2.4/config.xml

Installing composer in Ubuntu / Linux

Step 1: sudo apt-get remove composer
Step 2: php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');"
Step 3: php -r "if (hash_file('sha384', 'composer-setup.php') === 'baf1608c33254d00611ac1705c1d9958c817a1a33bce370c0595974b342601bd80b92a3f46067da89e3b06bff421f182') { echo 'Installer verified'; } else { echo 'Installer corrupt'; unlink('composer-setup.php'); } echo PHP_EOL;"
sudo php composer-setup.php --install-dir=/usr/local/bin --filename=composer

Step 4: php -r "unlink('composer-setup.php');"

Step 5: sudo service apache2 restart
Step 6: System restart