File: coreutils.info,  Node: Conditional Executability,  Next: Multiple Changes,  Prev: Changing Special Mode Bits,  Up: Symbolic Modes

27.2.4 Conditional Executability
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There is one more special type of symbolic permission: if you use ‘X’
instead of ‘x’, execute/search permission is affected only if the file
is a directory or already had execute permission.

   For example, this mode:

     a+X

gives all users permission to search directories, or to execute files if
anyone could execute them before.