5.4 Special Directories and Files

5.4 Special Directories and Files

Not every directory behaves like a private home folder. Shared temporary areas need extra rules so users cannot erase each other’s work. Shortcuts across the tree use symbolic links. Topic 5.4 focuses on /tmp, /var/tmp, the sticky bit, ls -d, and ln -s.

Temporary directories: /tmp and /var/tmp

Both locations are usually world-writable so any user can create temp files. Without an extra control, directory write permission would let User A delete User B’s file. The sticky bit solves that classic problem.

The sticky bit on directories

When sticky is set, users may still create files if the directory is writable, but they generally may delete or rename only their own files (root still can manage everything). That is why sticky directories fit shared temp space.

In ls -l output, sticky directories often look like:

drwxrwxrwt ... /tmp

The final t in rwxrwxrwt replaces the usual others-execute x. A capital T can appear if sticky is set without others-execute.

sudo chmod +t /var/tmp
sudo chmod 1777 /shared/dropbox

Octal 1000 adds sticky; 1777 is a common shared-scratch mode (sticky + wide rwx). Essentials wants recognition of sticky-on-directory for /tmp, not a deep setuid lesson.

Worked example: Alice creates /tmp/alice.out; Bob creates /tmp/bob.out. With sticky set, Bob cannot delete alice.out even though he can write to /tmp. He can still delete bob.out.

Sticky-bit scenarios

ls -ld /tmp                 # look for ...rwxrwt
touch /tmp/$USER-test
# other user: rm /tmp/<yours>-test → fails when sticky set
rm /tmp/$USER-test          # owner cleanup OK

Applications pick /var/tmp when a temp file should survive reboot; both directories commonly show sticky in ls -ld. If a classmate can delete your /tmp file, the first troubleshooting glance is whether sticky is still present on the directory.

Listing directories with ls -d

By default, ls dirname shows contents. ls -d shows the directory entry itself.

ls -ld /tmp
ls -ld /home/*/project

Use ls -ld when you care about directory permissions, not a flood of temporary filenames.

Symbolic links with ln -s

A symbolic link stores a path pointing to another file or directory:

ln -s /usr/share/doc/demo/README.md ~/readme-link
ln -s /var/log ~/logs

Long listings show type l and often linkname -> target:

lrwxrwxrwx 1 alice alice 12 Jul 23 10:00 docs -> /srv/docs

Link mode bits are commonly rwxrwxrwx; the target’s permissions still govern access to the real data when you follow the link.

Worked example: ln -s /home/alice/projects/notes/todo.txt ~/Desktop/todo.txt puts a Desktop shortcut on the real notes file. Move the real file without updating the link and the Desktop entry becomes broken.

Relative targets (ln -s ../shared/assets assets) travel with a project tree; absolute targets are better when link and target may move separately. ln -s /tmp/missing ~/broken-link still creates type l—fix the target or rm the pointer only. Always confirm with ls -l that you see -> pointing where you intended before you rely on the shortcut.

Hard links vs symlinks (awareness only)

Essentials emphasizes ln -s. Plain ln creates a hard link (same inode, usually same filesystem, not for directories). Shortcuts to directories or cross-filesystem pointers → think symbolic.

Practical checklist for Topic 5.4

1. Expect /tmp and often /var/tmp to be sticky shared temp spaces.
2. Recognize t / T in directory modes as sticky indicators.
3. Use ls -ld to inspect a directory’s own mode.
4. Create shortcuts with ln -s and verify with ls -l.
5. Deleting a symlink removes the pointer, not necessarily the target.

Together with accounts (5.1–5.2) and chmod/chown (5.3), special directories and symlinks complete Topic 5. Quick self-check: ls -ld /tmp (expect sticky t), create a disposable ln -s, then remove only the link name when cleaning up.