linux - bash script writing shell commands -


why commands require echo statement others can written without:

#!/bin/bash

aptitude upgrade

echo "mysql-server-5.1 mysql-server/root_password password $db_password" | debconf-set-selections

the commands feed on stdin input process, fed echo command. echo dumps string provided on stdout, in turn duplicated on stdin using pipe "|". commands not require input stdin or uses other method of input process can written without echo command.


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