I had to run a shell script to run automatically in detached screen during Linux system boot up via /etc/rc.local. This is needed because the server uses the tiny shell script to fetch data from remote host database and fill information into local MySQL server.
My idea was to su from root to www-data (Apache) user – the script has requirements to run with Apache user, then it has to run detached using GNU screen (multi terminal emulator. The tiny one line script I imagined would do the trick is like so:
# tty=$(tty); su www-data -c 'cd /home/user/www/enetpulse; screen -d /home/user/www/enetpulse/while_true.sh'; chmod 0720 $tty
I run this as root user to test whether it will work or not before I put it in /etc/rc.local but for my surprise got an error:
Must run suid root for multiuser support.
After a quick investigation on what is causing it I came across the solution which is to include screen arguments (-m -S shared). The working variant that gets around the error – i.e. successfully changes user privileges to Debian Apache user (www-data) and then detach with screen is:
# tty=$(tty); chmod a+rw $tty; su www-data -c 'cd /home/user/www/enetpulse; screen -d -m -S shared /home/user/www/enetpulse/while_true.sh'; chmod 0720 $tty;
That's all now script works out as planned on next server reboot