Files that contain the .index file extension are most commonly associated with the index files that are used for the Linux, Unix and Mac operating systems. The index files that are used by these operating systems store files that keep track of all the different types of files that your computer uses, how the files are used and which programs may access the files.
When an operating system is installed on a user's computer, the INDEX file is first created. That index file is then updated as files are added and removed to and from the user's computer hard drive.
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