How do I find out BIOS version using a Linux command line option?

To find out BIOS version under Linux operating systems use the dmidecode command which is a tool to analyse BIOS data. You must login as root to run dmidecode command:

dmidecode | less

Sample outputs:

dmidecode 2.10
SMBIOS 2.5 present.
45 structures occupying 1832 bytes.
Table at 0xCFF6A000.

Handle 0x0000, DMI type 0, 24 bytes
BIOS Information
        Vendor: Phoenix Technologies LTD
        Version: 2.1
        Release Date: 06/23/2008
        Address: 0xE4F00
        Runtime Size: 110848 bytes
        ROM Size: 1024 kB
        Characteristics:
                PCI is supported
                PNP is supported
                BIOS is upgradeable
                BIOS shadowing is allowed
                ESCD support is available
                Boot from CD is supported
                Selectable boot is supported
                EDD is supported
                3.5"/2.88 MB floppy services are supported (int 13h)
                ACPI is supported
                USB legacy is supported
                LS-120 boot is supported
                ATAPI Zip drive boot is supported
                BIOS boot specification is supported
                Targeted content distribution is supported

Another output from my laptop:

sudo dmidecode | less

Sample outputs:

BIOS Information
        Vendor: Dell Inc.
        Version: A02
        Release Date: 02/03/2010
        Address: 0xF0000
        Runtime Size: 64 kB
        ROM Size: 2112 kB
        Characteristics:
                ISA is supported
                PCI is supported
                PC Card (PCMCIA) is supported
                PNP is supported
                BIOS is upgradeable
                BIOS shadowing is allowed
                Boot from CD is supported
                Selectable boot is supported
                3.5"/720 KB floppy services are supported (int 13h)
                Print screen service is supported (int 5h)
                8042 keyboard services are supported (int 9h)
                Serial services are supported (int 14h)
                Printer services are supported (int 17h)
                CGA/mono video services are supported (int 10h)
                ACPI is supported
                USB legacy is supported
                AGP is supported
                Smart battery is supported
                BIOS boot specification is supported
                Function key-initiated network boot is supported
                Targeted content distribution is supported
        BIOS Revision: 0.2
        Firmware Revision: 0.2

Handle 0x0100, DMI type 1, 27 bytes
System Information
        Manufacturer: Dell Inc.
        Product Name: Precision M6500                 
        Version: Not Specified
        Serial Number: 55Lxxxx
        UUID: 4a2yzzzeC-3500-xxx-yyy-Bzzzzyyy5y
        Wake-up Type: Power Switch
        SKU Number: Not Specified
        Family: