Shell Programming and Scripting

Hi,

Please guide me in understanding the usage of exec command. I know 0,1 and 2 as file descriptors but what 3,4,5.... these descriptors mean.

exec 3>output
exec 4<input

what this signifies here?
I knw little basics of exec like it do not spawn any new shell to execute the commands but being in the same shell itexecutes the commands.
Please give me some idea so that it will clarify my doubt.

Thanks in advance.

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