How to restart CentOS or RHEL server safely

Restarting CentOS or RHEL server safely over ssh

RHEL/CentOS Linux commands that we can use to restart the server carefully:

1. shutdown command : All in one command to halt, power-off or reboot the machine.
2. systemctl command : Systemd’s systemctl command can reboot or shutdown your server too.
3. reboot command : Symbolic link and aliased to /sbin/systemctl to restart the CentOS/RHEL.
4. halt command : Again, symbolic link and alias set to /sbin/systemctl to halt the CentOS/RHEL. Shut down and halt the system. You still need to press the power-off button manually.
5. poweroff command : Symlink or soft link to /sbin/systemctl to power off your CentOS/RHEL box. Shut down and poweroff the system complety. Please note that there is no need to press the power off button.

The last three command acts as a shortcut to a longer command and saves some typing. Instead of typing “sudo shutdown -r now“, we can type “sudo reboot“.

Best way to gracefully restart CentOS or RHEL

The procedure is:

1. Synchronize cached writes to persistent storage as root user by flushing everything to avoid problem with PostgreSQL/MySQL/MariaDB, run:
   # sync;sync
2. Restart the CentOS/RHEL server, run:
   # shutdown -r now
3. An alternative and recommended way is to type as shutdown/reboot is soft link to /sbin/systemctl:
   # systemctl reboot
   OR
   # systemctl poweroff # <-- complete power off
4. Personally, if I were you, I would shutdown the database server before issuing the reboot command. Hence:
   # sync;sync
# systemctl stop postgresql
# systemctl stop mysql # MySQL/MariDB
# systemctl reboot

Understanding reboot or ‘systemctl reboot’ or ‘shutdown -r now’ commands

Open the terminal and run the following command:
ls -l /sbin/{halt,reboot,poweroff,shutdown}

Modern CentOS/RHEL symlinked to systemctl

• Flush all data to disk.
• Stop all running processes/services.
• All file systems are unmounted.
• The system will reboot.

Please do not pass the --force option to the systemct as it will reboot the box immediately without terminating any processes or unmounting any file systems. This will result in data loss for sure. Therefore avoid the following:
# systemctl --force --force reboot
# systemctl --force --force shutdown
So why --force option provided? It can be used in an emergency when the CentOS/RHEL system manager has crashed, and you need to shutdown the server. Hence, keeping verified backups are important for your systems.

When should I use the old shutdown command?

The shutdown command has additional options, including backward compatibility. For instance, display a message:

sudo shutdown -r now "RHEL 8 database server is going down for kernel update."
## OR ##
sudo shutdown -r 0 "RHEL 8 database server is going down for kernel update."
sudo shutdown -h now "CentOS 7 file-server is going down for disk replacement."

The “now” means immediately. We can pass the time string in the format “hh:mm” for an hour/minutes specifying the time to execute the shutdown specified in 24h clock format. Alternatively, it may be in the syntax “+m” referring to the set number of minutes m from now. Please note that “now” is an alias for “+0”, i.e., triggering an immediate shutdown. If no time argument is specified, “+1” is implied:

sudo shutdown -h +15 "CentOS NFS server is going down for ssd replacement in 15 mintues time. Please save all files"
sudo shutdown -r +5 "Developers save your files as devbox is rebooting in 5 mintues time."

We can cancel a pending shutdown. This may be used cancel the effect of an invocation of shutdown with a time argument that is not “+0” or “now”:
sudo shutdown -c

Command discussed here are compatible with different Linux distros such as Arch, Ubuntu, Debian, Fedora Linux, etc. Always read man pages.

How do I find out CentOS/RHEL systems reboot data and time?

Try the following commands:
last -x reboot | head -1
last -x shutdown | head -1
who -b
uptime -s

How to automate and reboot multiple CentOS/RHEL server hosted in the cloud

We can reboot many Linux machines, wait for it to go down (say for kernel update), come back up, and respond to commands using Ansible. See how to use Ansible to reboot Linux machine or server with playbooks.

Summing up

You learned about the proper way to either shut down or restart your CentOS/RHEL box, and we recommend that you keep verified backups to avoid data loss. See how to install and use rsnspahot on your RHEL/CentOS to make backups. Getting help is easy:
$ man shutdown
$ man systemctl
$ shutdown --help
Sample outputs:

shutdown [OPTIONS...] [TIME] [WALL...]

Shut down the system.

     --help      Show this help
  -H --halt      Halt the machine
  -P --poweroff  Power-off the machine
  -r --reboot    Reboot the machine
  -h             Equivalent to --poweroff, overridden by --halt
  -k             Don't halt/power-off/reboot, just send warnings
     --no-wall   Don't send wall message before halt/power-off/reboot
  -c             Cancel a pending shutdown