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.
Flight mode is a setting on a mobile phone or wireless gadget that disables the device’s signal-transmitting ability but allows for the use of its other functions. As the term implies, the flight mode setting is typically engaged for safe use on an airplane where activities that require signal transmission are forbidden. A passenger using a device in flight mode is unable to place or receive calls and text messages but can still make use of the gadget’s other functions such as games or an MP3 player.Flight mode may also be referred to as airplane, radios off, stand-alone, or offline mode.
One key reason why signal transmission is generally prohibited while on board an aircraft is because the signal could interfere with the plane’s communication and navigation systems. Such transmissions may also interfere with cell networks on the ground. Most airline companies allow the use of cell phones, PDAs and other electronic devices as soon as the seat belt sign is turned off and until preparation for landing commences. However, the devices must be switched to flight mode before takeoff. In general, features like FM receiver, Bluetooth, Wi-Fi and GPS should still be operable even when the device is in airplane mode. In some gadgets, such as Apple’s iPad, iPhone and iPod Touch, even Wi-Fi and GPS are disabled. This is because these functions are permitted by some airlines and prohibited by others. In fact, there are even some airline companies that completely prohibit the use of all hand-held devices on their aircraft.
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