coreutils: Conditional Executability

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1 27.2.4 Conditional Executability
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1 There is one more special type of symbolic permission: if you use ‘X’
1 instead of ‘x’, execute/search permission is affected only if the file
1 is a directory or already had execute permission.
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1    For example, this mode:
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1      a+X
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1 gives all users permission to search directories, or to execute files if
1 anyone could execute them before.
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