For a regular file, it grants you executable permission, that is, permission to run it as a program.
For a directory, the Execute bit grants you so-called Search Permission, meaning that you have authority to use names in that directory in a pathname.
Traditionally on Unix, it indicates that the file can be run as a Unix executable, with the Unix system call exec(2). This is how the shell runs such a program, not to mention other programs such as Interrogator. The first word of any command on the command line is often the name of a program file.
Usually this is either a compiled binary program, or a script textfile. You also need read permission in order to execute the file. Notice that a user who has read, but not execute, permission on a file can make a copy for themselves, turn on the executable bit and run it. For this reason, the Execute bit is supplanted by the Read bit, and in this usage has degenerated more into a status bit that indicates an executable, rather than anything that grants privileges. If you try to execute a regular file that does not have this bit set, it will give you a 'permission denied' error message. It's trying to tell you that the file is probably not an executable program. You may know better. Interrogator will typically dialog you if you try to execute such a file and give you the option of actually setting the executable bit before running it.
On MacOS X, and with files that come with MS Windows, the story is more complicated. The executable permission bit has the same meaning for Unix (BSD) executable programs in the same way that Unix has always worked. Unfortunately there are other reasons why files could have their executable bits on.
MacOS 9 Classic and 68000 applications are not Unix executables. They are Mac applications. The executable permission bit is ignored as running the program is entirely different. (They are run with the open command-line command, or by double clicking in Interrogator or any other graphical shell.)
Mac OS X or Cocoa applications actually have a Unix executable in their core usually at Contents/MacOS/. Its Executable bit is unfailingly on, as are all the directories in the package.
The original EXT has been deprecated for years. The EXT2 format is a more modern system; it is often the default partition type for Linux, and is probably the best debugged. More recently has been EXT3, which adds journaling and ACLs. Most systems besides Linux cannot use the EXT formats. Linux has been more successful writing drivers for other filesystem types.
Do not confuse EXT partitions with extended partitions. EXT filesystems were invented by the Linux community for their own use.
Extended partitions were invented by Microsoft when it became clear that their primary partition scheme didn't allow enough partitions. Recent versions of Microsoft's fdisk program enforced a pattern where disk partitions could only be created along these lines:
Primary 1 is the C disk, where MS Windows boots up from.
Primary 2 is an Extended partition, containing all of the logical partitions.
Primary 3 and 4 are not used.
Logical 1 is the D disk.
Logical 2 is the E disk, and so on.
These are all of course contained in the Primary 2 Extended partition.
Using other partitioning programs, you can make other primary partitions the extended partition, although only one extended partition is allowed on a disk at one time. Primary partitions 3 and 4 are sometimes used for Linux, BSD or Solaris.
As you might expect, the extended partition has a size that is fixed when you create it, and the total of the logical partitions within it cannot exceed this total.
Do not confuse extended partitions with EXT-type filesystems. Extended partitions are a Microsoft invention to subdivide partitions; it does not actually form a filesystem of its own.
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