What's an upper 'T' at the end of unix permissions?

Question

What's that 'T' in the permissions mean, and how does it work? Is it related to this 'sticky bit' thing I've heard about but never quite understood?

Answer 1

I thought I'd combine the info from the previous answers and show another way of looking at it.

Nautilus can help you see how things are here. Consider this test directory I created:

Now, consider this second one:

As you can see, in both cases the sticky bit is set. In the capital-T version, the execute bit is unset. In the lowercase-t version, the x bit is set.

For directories, here's what Wikipedia has to say:

The most common use of the sticky bit today is on directories. When the sticky bit is set, only the item's owner, the directory's owner, or the superuser can rename or delete files. Without the sticky bit set, any user with write and execute permissions for the directory can rename or delete contained files, regardless of owner. Typically this is set on the /tmp directory to prevent ordinary users from deleting or moving other users' files. This feature was introduced in 4.3BSD in 1986 and today it is found in most modern Unix systems.

Answer 2

When the 'T' is present, it means that 'x' permission is has not been given on the directory/file. If you give 'x' permission then a 't' will be shown.

You can set the 'T'/'t' with a 4 character permssion, rather than a 3 character. 0777, for example, whould give rwx to owner, group, and world. 1754 would give rwxr-xr-T permission.

The first character controls the SUID, SGID, and Stickbit.

0 = All bits off 1 = Sticky bit on 2 = SGID bit on 4 = SUID bit on 5 = SUID and Sticky bit on 6 = SUID and SGID bit on 7 = All bits on.

To learn more go here: on this