chmod 1777: The Sticky Bit and Why /tmp Needs It

Write permission on a directory normally lets you delete anything in it, regardless of who owns the file. On a shared directory that is intolerable: any user could remove another user's temporary files. The sticky bit changes the rule so that removal and renaming require you to own the entry, own the directory, or be root.

This is why /tmp is 1777 and not 777. Every account needs to create temporary files there, and no account should be able to delete anyone else's. The same mode appears on /var/tmp and on /dev/shm for the same reason, and it is the single correct answer whenever someone reaches for 777 on a directory that several users share.

The trailing t in a listing is where the sticky bit shows up, in the position that would otherwise hold the others execute bit. Lowercase t means sticky and execute are both set, which is the working combination. Capital T means the sticky bit is set while others cannot traverse the directory at all, which is usually a mistake.

It restricts deletion, not writing. A file inside a sticky directory whose own mode is world writable can still have its contents replaced by anyone, because that permission comes from the file, not from the directory. The sticky bit stops the file being unlinked and replaced, not overwritten in place.

For a directory that does not need to be world writable, the better answer is usually a shared group with 2775 rather than 1777. Reach for the sticky bit when the set of users genuinely cannot be enumerated in a group, which in practice means system wide temporary space and little else.