If you search you will find different answers to this. You can do this in multiple ways, here I will talk about 2 ways, single command and multiple commands.
First the idea is to switch from say root user to a named user you created or was created for you on your Linux server to run commands as not the root user. The reason you want to do this is so that everything isn’t owned by the root user. Or you are installing something like PHP Composer which barfs on you if you run it as root user.
You will see some saying to use su others saying to use sudo (some bs options etc.) You will also see really wrong answers on Stack. I have no idea why you would use sudo over su, you can google that. But I do know that su switches users. Here is an article goes into more detail of su vs sudo and when you use both.
Single command syntax
So the first way is to run a single command directly inline. If you are the root user you simply use su The syntax to do so is as follows:
su - username "commandToExecute [command options and arguments]"
It has been my experience that the Double ” Quotes are required or else the shell gets confused. You may be able to use single quotes if you don’t use any variables within the quotes.
Multiple commands syntax
To more easily issue multiple commands or long commands you need to use Linux heredoc syntax.
Heredoc uses <
su - $username <<SHT cd $serverDir php $composerFile install SHT
Like I said you can use any Delimiter you want. It is tradition to use all caps for the word, it makes it easier to spot. The ending word (EOF here) has to have no spaces or words before it. You can list any number of commands within that syntax and all will be executed by the user.
NOTE: After the ending EOF the shell returns the user to whatever user you were/are logged in as before the lines of code. If you are logged in as root, you are returned to root. Also when you issue the su command you are moved out of the directory you are in. That is why I used cd to move back to the directory I needed to be in.