As a root user can I execute commands in BASH as another user without requiring a password?

Question

I have a root user account on an RHeL server. On that server I have a simple script called user.sh that just returns the current user:

#!/bin/bash
echo $USER

when run from my root account the output is

bash user.sh
>>>root

From another script I would like to be able to temporarily switch between users without entering a password, storing the password in the script or a file, or modifying /etc/sudoers and execute user.sh and then return back to my initial root account. Is this at all possible?

Here's what I've tried so far:

1. Using delimeters to execute a block of code

   #!/bin/bash
   
   bash /user.sh
   
   su other_user <<EOF
   echo Current user: $USER
   EOF
   

   output:

   root
   Current user: root
   

2. switching to a user in bash, executing a command and then logging back out

   #!/bin/bash
   
   bash /user.sh
   
   su other_user
   bash /user.sh
   exit
   

   output: The script pauses execution and returns me to the terminal logged in as other_user however I will still be in my root account's directory that contains user.sh

   root
   [other_user@my_server]$
   

   if I then type exit I'm returned to my root account and the script completes execution

3. using the su - <username> -c /path/to/the/shellscript.sh to execute a script as a different account and then return

   #!/bin/bash
   
   bash /user.sh
   
   su - other_user -c /path/user.sh
   

   output:

   root
   -bash: /path/user.sh: Permission denied
   

4. using sudo -i -u other_user to log in as the user and execute the script which yields the same problem experienced with attempt #2 but I am redirected to other_user's home directory.

It may be worth noting that if I use method 2, while I'm logged in as other_user I am able to run bash user.sh and yield the desired output: other_user

Answer 1

In the second example, the $USER variable is expanded before su is executed. This can be prevented by quoting EOF.

su other_user <<'EOF'
echo Current user: $USER
EOF

Or you can execute the script to do it in the root shell, also using a here-doc:

su other_user <<END
bash user.sh
END

or you can use the -c option to su:

su other_user -c 'bash user.sh'