bash - Pass commands as input to another command (su, ssh, sh, etc)

Question

I have a script where I need to start a command, then pass some additional commands as commands to that command. I tried

su
echo I should be root now:
who am I
exit
echo done.

... but it doesn't work: The su succeeds, but then the command prompt is just staring at me. If I type exit at the prompt, the echo and who am i etc start executing! And the echo done. doesn't get executed at all.

Similarly, I need for this to work over ssh:

ssh remotehost
# this should run under my account on remotehost
su
## this should run as root on remotehost
whoami
exit
## back
exit
# back

How do I solve this?

I am looking for answers which solve this in a general fashion, and which are not specific to su or ssh in particular. The intent is for this question to become a canonical for this particular pattern.

Answer 1

Adding to tripleee's answer:

It is important to remember that the section of the script formatted as a here-document for another shell is executed in a different shell with its own environment (and maybe even on a different machine).

If that block of your script contains parameter expansion, command substitution, and/or arithmetic expansion, then you must use the here-document facility of the shell slightly differently, depending on where you want those expansions to be performed.

1. All expansions must be performed within the scope of the parent shell.

Then the delimiter of the here document must be unquoted.

command <<DELIMITER
...
DELIMITER

Example:

#!/bin/bash

a=0
mylogin=$(whoami)
sudo sh <<END
    a=1
    mylogin=$(whoami)
    echo a=$a
    echo mylogin=$mylogin
END
echo a=$a
echo mylogin=$mylogin

Output:

a=0
mylogin=leon
a=0
mylogin=leon

2. All expansions must be performed within the scope of the child shell.

Then the delimiter of the here document must be quoted.

command <<'DELIMITER'
...
DELIMITER

Example:

#!/bin/bash

a=0
mylogin=$(whoami)
sudo sh <<'END'
    a=1
    mylogin=$(whoami)
    echo a=$a
    echo mylogin=$mylogin
END
echo a=$a
echo mylogin=$mylogin

Output:

a=1
mylogin=root
a=0
mylogin=leon

3. Some expansions must be performed in the child shell, some - in the parent.

Then the delimiter of the here document must be unquoted and you must escape those expansion expressions that must be performed in the child shell.

Example:

#!/bin/bash

a=0
mylogin=$(whoami)
sudo sh <<END
    a=1
    mylogin=$(whoami)
    echo a=$a
    echo mylogin=$mylogin
END
echo a=$a
echo mylogin=$mylogin

Output:

a=0
mylogin=root
a=0
mylogin=leon