How to run a command as a specific user in an init script?

If you have start-stop-daemon

start-stop-daemon --start --quiet -u username -g usergroup --exec command ...

For systemd style init scripts it's really easy. You just add a User= in the [Service] section.

Here is an init script I use for qbittorrent-nox on CentOS 7:

[Unit]
Description=qbittorrent torrent server

[Service]
User=<username>
ExecStart=/usr/bin/qbittorrent-nox
Restart=on-abort

[Install]
WantedBy=multi-user.target

On RHEL systems, the /etc/rc.d/init.d/functions script is intended to provide similar to what you want. If you source that at the top of your init script, all of it's functions become available.

The specific function provided to help with this is daemon. If you are intending to use it to start a daemon-like program, a simple usage would be:

daemon --user=username command

If that is too heavy-handed for what you need, there is runuser (see man runuser for full info; some versions may need -u prior to the username):

/sbin/runuser username -s /bin/bash -c "command(s) to run as user username"

Instead of sudo, try

su - username command

In my experience, sudo is not always available on RHEL systems, but su is, because su is part of the coreutils package whereas sudo is in the sudo package.