How do I find out my Dell server service tag using command line under RHEL / Fedora / Red Hat / CentOS / Scientific Linux operating systems?

You need to use the dmidecode command to find out various serial numbers including Dell servers service tag. This command is used to analyse BIOS DMI data. If this command is not install type the following command to install it:
# yum -y install dmidecode

Command To Find Out Dell Service Tag

Once installed type the following command to find out Dell service tag:
# dmidecode | grep -i serial
OR
# dmidecode -t 1
Sample outputs:

# dmidecode 2.9
SMBIOS 2.6 present.

Handle 0x0100, DMI type 1, 27 bytes
System Information
	Manufacturer: Dell Inc.
	Product Name: PowerEdge T110                 
	Version: Not Specified
	Serial Number: XYZABC
	SKU Number: Not Specified
	Family:  

The following command will only display service tag:
# dmidecode -s system-serial-number

How Do I Find Out Dell Service Tag Remotely?

You need to ssh into remote Dell server and type the same command:
$ ssh [email protected]
OR
$ ssh [email protected]
And type the above command:
# dmidecode -s system-serial-number
OR just pass the dmidecode command line option to ssh itself:
$ ssh [email protected] dmidecode -s system-serial-number