$1.$ What is the purpose of the PATH variable?

a$.$ Set the PATH variable so that it causes the shell to search the following directories in order:

$/$usr$/$local$/$bin

$/$usr$/$bin$/$X$11$

$/$usr$/$bin

$/$bin

$/$Developer$/$Tools

$$The bin directory in your home directory

$$The working directory

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b$.$ If there is a file named doit in $/$usr$/$bin and another file with the same name in your ~$/$bin$,$ which one will be executed? $($Assume that you have execute permission for both files.$)$

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c$.$ If your PATH variable is not set to search the working directory, how can you execute a program located there?

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d$.$ Which command can you use to add the directory $/$usr$/$sbin to the end of the list of directories in PATH?