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Why is that some commands require the echo statement but others can simply be written without:
#!/bin/bash
aptitude upgrade
echo "mysql-server-5.1 mysql-server/root_password password $DB_PASSWORD" | debconf-set-selections
The commands that feed on stdin for some input to process, are usually fed by echo command. Echo dumps the string provided to it on stdout, which in turn is duplicated on stdin using the pipe “|”.
So for the commands which does not require input from stdin or uses some other method of input to process can be written without echo command.