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.
In mobile computing, a jammer is a mobile communications device that transmits on the same frequency range as a cellphone to create strong cell tower interference and block cellphone signals and call transmission. Jammers are usually undetectable, and users may experience minimal effects such as poor signal reception. Jamming devices may be used in any location but are typically deployed where cellphone use may be disruptive, such as in libraries and restaurants.
A working cellphone must be in constant communication with its network provider via a cell tower or base station. By sending out radio waves in cellphone frequencies, jammers launch denial-of-service attacks (DoS), causing mobile devices to lose base station communication. Jammers come in a variety of shapes and sizes, including:
All jamming device types have three main parts, as follows:
Handheld jammers are capable of disrupting signals within nine to 30 meters, while more powerful jammers create a huge bubble stretching as far as a mile or 1.6 kilometers. In many countries, jammers are illegal, except in the military, law enforcement and other government agencies, where jammers are largely used to prevent bomb detonation or to isolate suspects in hostage situations.
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