Others' files stay put

bob deleted alice's plan without the right to write to the file itself. The sticky bit closes that gap in a shared directory.

Deleting erases a name from the directory's list, so the directory's rights decide it. The devs group has w on /srv/team, and that was enough for bob.

The sticky bit adds a condition: only the file's owner, the directory's owner, or root may delete or rename a file. The bit sits in the others' x position as the letter t. That is how /tmp, where everyone writes, is set up:

bash

ls -ld /tmp

drwxrwxrwt: anyone can create a file in /tmp, but no one can delete someone else's. Such a directory protects others' files with a refusal: rm: cannot remove '…': Operation not permitted.

chmod +t directory sets the bit. As a number it is a 1 in the same place as setgid's 2, and both at once are chmod 3775.

Warning

On Ubuntu, sticky also gets in the way of editing together: in a sticky directory the group writes to, another person's file will not open for appending — tee -a and >> get Permission denied even though the group has w. This is the kernel's protection against file spoofing: cat /proc/sys/fs/protected_regular prints 2. So sticky goes on a directory where everyone drops their own files.

Check yourself 1 question

The devs team needs two directories: docs for documents everyone edits together, and inbox for reports where each person drops their own. Which modes do you give them?

With sticky on docs, Ubuntu will not let anyone append to another person's document: echo x >> file as a colleague gets Permission denied.
1777 opens docs for writing to everyone, not just devs, and does not pass the group to new files. inbox without sticky does not protect reports from deletion.
Without sticky on inbox, bob can delete alice's report: the group can write to the directory, so rm goes through.

Editing together needs a shared group for new files — setgid, 2775. For a directory where each person drops their own files, sticky adds protection from deletion — 3775; nobody needs to append to others' files there.

Key points

The directory's rights decide deleting and renaming; sticky (t) leaves them to the file's owner, the directory's owner, and root.
Setgid and sticky together are chmod 3775, shown as drwxrwsr-t.
On Ubuntu, sticky in a group directory blocks appending to others' files: put it where everyone writes their own.

A shared directory: setgid, sticky, and umask

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