A shell variable is a variable that is available only to the current shell. In contrast, an environment variable is available system wide and can be used by other applications on the system.A shell is the operating system's command interpreter. It processes the commands entered on the command line or read from a shell script file.
A jumper is a tiny metal connector that is used to close or open part of an electrical circuit. It may be used as an alternative to a dual in-line package (DIP) switch. A jumper has two or more connecting points, which regulate an electrical circuit board.
A jumper is made of material that conducts electricity, and is sheathed in a nonconductive plastic block to prevent accidental circuit shorts. A jumper positioned over two or more pins creates a connection that activates certain setting instructions. Jumpers are like on/off switches. They may be removed or added to enable component performance options. A group of jumper pins is a jumper block, which has at least one pair of contact points with a small metal pin at the end. A sleeve or shunt is draped over the pins to allow electric currents to pass over other circuit points. Older PCs used jumpers to set voltage and central processing unit (CPU) speed. Moreover, jumpers and jumper blocks were used to reset basic input/output system (BIOS) configuration and clear complimentary metal oxide semiconductor (CMOS) information. Older PCs contained at least one jumper and, in many cases, a bank of DIP switches. It was common to find 30 to 40 jumper pairs on a motherboard. Because of poor documentation, some systems were difficult to set correctly, and motherboards eventually had fewer labeled and numbered jumper blocks. Jumpers are found on modern hard drives but are rarely seen on motherboards. In most cases, settings are configured automatically or via software. Configuration settings are often stored in non-volatile random access memory (NVRAM). The jumper’s main advantage is its one-time configuration, which makes it less vulnerable to corruption or power failure than firmware. Jumper altering requires that settings be physically changed.
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