Shutdown the machine immediately

# shutdown -h now

Reboot the machine immediately

 # shutdown -r now

Shutdown the machine with user defined message

# shutdown -h now 'System is going down for upgrade'

Scheduling the shutdown with 24 hour format

# shutdown -h 20:00

Cancel a running shutdown

You can cancel the running shutdown by using -c option as,

# shutdown -c

 -r Requests that the system be rebooted after it has been brought down
 -h Requests that the system be either halted or powered off after it has been broughtdown, with the choice as to which left up to the system
 -H Requests that the system be halted after it has been brought down
 -P Requests that the system be powered off after it has been brought down
 -c Cancels a running shutdown. TIME is not specified with this option, the firstargument is MESSAGE
 -k Only send out the warning messages and disable logins, do not actually bring thesystem down

See log of system shutdown

# last reboot
 reboot system boot 2.6.18-238.12.1. Sun Jun 5 07:56 (17+00:07)
 reboot system boot 2.6.18-238.9.1.e Sat Apr 30 05:08 (36+02:44)
 reboot system boot 2.6.18-238.9.1.e Fri Apr 22 02:38 (8+02:25)
 reboot system boot 2.6.18-238.5.1.e Sat Mar 5 19:13 (47+06:21)
 reboot system boot 2.6.18-238.1.1.e Sat Jan 22 06:44 (42+12:25)

Solaris

You need to use the shutdown command as follows (you must login as root)

# shutdown -y -g0 -i S