Filesystem, paths and essential commands

Mental model

Linux presents one directory tree rooted at /. /etc holds configuration, /var variable data and logs, /home user homes, /usr much installed software, /tmp temporary files and /proc a virtual view of kernel/process state. Absolute paths start with /; relative paths start from the current working directory. ~ expands to your home in a shell.

Guided command lab

mkdir -p ~/academy/{notes,backup}
cd ~/academy
pwd
printf 'Cloud class\nLinux lab\n' > notes/day1.txt
ls -lah notes
cat notes/day1.txt
cp -i notes/day1.txt backup/
mv notes/day1.txt notes/linux.txt
stat notes/linux.txt
file notes/linux.txt
find ~/academy -type f -name '*.txt'
diff -u backup/day1.txt notes/linux.txt

mkdir -p creates missing parents; brace expansion creates two directories. > replaces content, whereas >> appends. cp -i and mv -i request confirmation before overwrites. diff exits 0 when files match and 1 when they differ; 1 is not always an execution failure.

Read and edit

Use less filename to scroll, /term to search and q to quit. In nano save with Ctrl+O and exit Ctrl+X. In vi/vim, i enters insert mode, Esc returns to normal, :wq saves/exits and :q! discards unsaved changes. Check whether the editor is installed before assuming it is present.

Links and archives

ln notes/linux.txt notes/hard-link.txt
ln -s linux.txt notes/soft-link.txt
tar -czf backup/notes.tar.gz notes/
tar -tzf backup/notes.tar.gz
mkdir -p restore
tar -xzf backup/notes.tar.gz -C restore

A hard link points to the same inode and generally cannot cross filesystems; a symbolic link stores a path and can become dangling. List an unfamiliar archive before extraction and use a dedicated directory.

Delete deliberately

Use rm -i notes/soft-link.txt for the link. rm usually bypasses a desktop trash folder. Before recursive deletion, print and inspect the exact path. Do not use destructive wildcard commands as a first exercise.

Assignment and verification

Create a course directory containing three lessons; archive it, restore it and compare one file with diff. Explain why deleting a symbolic link does not delete its target, and why editing one hard-link name changes the content visible through the other.

Official reference

GNU Coreutils manual

Interview and revision check

Why does a symbolic link break after moving its target?

The link stores a path. If that path no longer resolves, the link is dangling even though the target data may exist elsewhere.