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| Regular file | | touch file; write with editors or echo/cat |
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| Directory | | mkdir dir; remove with rmdir or rm -r |
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| Symbolic link | | ln -s target linkname |
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| Block device | | Usually in /dev (e.g., /dev/sda, /dev/loop0) |
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| Character device | | /dev/null, /dev/tty |
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| FIFO (named pipe) | | mkfifo pipe; read/write with cat/echo |
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| Unix socket | | Created by programs (e.g., services); list with ls -l or ss -x |
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Explore inode numbers:
Hard vs. symbolic links: mkdir ~/lab-nodes && cd ~/lab-nodes
printf 'hello\n' > a.txt
ln a.txt a.hard # hard link (same inode)
ln -s a.txt a.sym # symlink (different inode)
ls -li
rm a.txt && cat a.hard # still works; inode persists via hard link
cat a.sym # will fail: dangling symlink
---
Discovering Filesystems & Devices
Two essential tools:
df — usage of mounted filesystems
- Shows how full mounted filesystems are and where they are mounted.
- Helpful flags:
-h (human), -T (type), target path to narrow output.
df -hT
# Focus on a path
sudo mkdir -p /mnt && df -hT /mnt
lsblk — block devices and partitions
- Shows block devices (disks, partitions, loop devices) regardless of mount state.
- Helpful flags:
-f (FSType/Label/UUID), -o to customize columns.
lsblk
lsblk -f
lsblk -o NAME,MAJ:MIN,RM,SIZE,RO,TYPE,FSTYPE,LABEL,UUID,MOUNTPOINTS
Relationship between df and lsblk
lsblk lists devices (e.g., /dev/sda2, /dev/loop0) and where they’re mounted (MOUNTPOINTS column).df reports space usage per mounted filesystem.
Mapping tip: Find the row in lsblk -f whose MOUNTPOINTS equals the df mountpoint; the corresponding NAME (e.g., sda2) is the device backing that filesystem.
Exercise: Identify the device backing your home directory and its filesystem type. df -hT ~
lsblk -f | grep $(df --output=source ~ | tail -1)
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Working with Nodes
Core commands (read man pages for details):
- List & inspect:
ls -la, tree (optional), file, stat.
- Create:
touch, mkdir, mkfifo, ln -s, (rarely) mknod for manual device nodes.
- Copy/Move/Delete:
cp (-r for dirs), mv, rm (-r recursive, -i interactive).
- Permissions & ownership:
chmod, chown, chgrp, umask.
- Search:
find, grep.
- View:
cat, less, head, tail, wc.
- Space usage:
du -sh DIR, df -hT.
Mini-lab: mkdir -p ~/lab-fs/dir1/dir2
cd ~/lab-fs
touch notes.txt
echo "sample" > dir1/data
mkfifo dir1/pipe
ln -s dir1/data link-to-data
ls -lR
file notes.txt dir1/pipe link-to-data
find . -type f -size +0
chmod 640 notes.txt && stat notes.txt
- Note: Sockets are typically created by running daemons. You can spot them with
find -type s (e.g., in /run/ or /var/run/).
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Creating & Using a Loopback Filesystem (45–60 min)
Goal: Create a file-backed block device with losetup, format it as ext4, mount it, and verify via df and lsblk.
Create a sparse backing file
mkdir -p ~/lab-loop && cd ~/lab-loop
truncate -s 100M loop.img # fast; or: dd if=/dev/zero of=loop.img bs=1M count=100
ls -lh loop.img
Attach the file to a free loop device
sudo losetup -fP loop.img # chooses next free /dev/loopX
losetup -a # show all loop mappings
lsblk:
Make an ext4 filesystem on the loop device
sudo mkfs.ext4 -L LAB_FS /dev/loopX
What happens: ext4 creates a superblock and inode tables, then initializes data structures.
Mount and validate
sudo mkdir -p /mnt/labfs
sudo mount "$LOOPDEV" /mnt/labfs
df -hT /mnt/labfs
lsblk -f | grep $(basename "$LOOPDEV")
Create some content and examine space usage:
sudo sh -c 'echo "hello ext4" > /mnt/labfs/hello.txt'
sudo ls -la /mnt/labfs
sudo du -sh /mnt/labfs
Unmount and detach (clean up)
sudo umount /mnt/labfs
sudo losetup -d /dev/loopX
rm -f loop.img
- Safety tip: Always unmount before detaching the loop device. Use
lsof | grep /mnt/labfs if the mount is busy.
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Mount Tables & Where Linux Stores Them
mount without args lists mounts.- Modern systems reflect mounts in
/proc/self/mounts and /proc/mounts. Many distros keep /etc/mtab as a compatibility symlink.
- Persistent mounts are configured in
/etc/fstab (be careful!).
Explore:
cat /proc/mounts | head
mount | head
cat /etc/fstab
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Assessment: Lab Submission Tasks
Complete the following tasks and submit a report with your commands and output.
- Device to Filesystem Mapping
- Run
df -hT and lsblk -f.
- Identify the line in each result that corresponds to your
/ (root) filesystem.
- Links and Inodes
- Create a file
original.txt, a hard link hard.link, and a symbolic link symbolic.link to it.
- Provide the output of
ls -li and explain why the inode numbers are the same or different.
- Loopback Filesystem Creation
- Create a 150 MB
loop.img, format it with ext4 (label MY_LOOP), and mount it on /mnt/mydata.
- Create a file
proof.txt inside it containing your name.
- Provide the final output of
df -hT /mnt/mydata and lsblk -f showing your mounted device.
find Command Practice
- Provide the
find command that lists all files in your home directory (~) larger than 1 MB and modified in the last 7 days.
- Provide the
find command to list all Unix domain sockets under /run.
- Real-World Scenario: The Automated Organizer
- Imagine your
Downloads directory is cluttered. Your task is to write a sequence of commands to automatically clean it up.
- Setup: Create a
~/cleanup_target directory with at least 10 files, including:
- Three
.log files (e.g., program1.log).
- Three
.tmp files (e.g., data.tmp).
- One file larger than 10 MB (e.g.,
truncate -s 15M large_dataset.dat).
- Some other miscellaneous files.
- Automation Task:
- Create subdirectories:
logs, temp, and large_files.
- Move all
.log files into logs.
- Move all
.tmp files into temp.
- Move all files larger than 10 MB into
large_files.
- Create a
cleanup_report.txt listing the contents of the new subdirectories.
- Submission: Provide the exact sequence of commands you used.
- Bonus: For bonus points, provide a solution that does not use shell expansion (like
*.log). Instead, use a more robust tool like find with -exec or xargs.
Submit a short report with command history snippets and brief explanations (≈1–2 pages).
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Troubleshooting
mkfs.ext4: device is busy — Ensure it’s not mounted: mount | grep loop; unmount, then retry.umount: target is busy — Find open files: sudo lsof +f -- /mnt/labfs or fuser -vm /mnt/labfs.- No free loop device — Create one:
sudo modprobe loop; or manually: sudo losetup /dev/loop10 loop.img.
- Wrong device formatted — Always confirm with
lsblk -f before mkfs.*. In a VM, take a snapshot first.
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Optional Extensions
- Create partitions inside the loop device (
sudo sfdisk "$LOOPDEV"), then partprobe and format /dev/loopXp1.
- Compare
mkfs.ext2 vs. mkfs.ext4 space usage and features (journaling, extent maps).
- Mount with options:
sudo mount -o noatime,nodiratime "$LOOPDEV" /mnt/labfs and measure stat times.
---
== Quick Command Cheat Sheet == # Discover
pwd; ls -la; stat FILE; file FILE; du -sh DIR; df -hT; lsblk -f
# Make nodes
mkdir DIR; touch FILE; ln -s TARGET LINK; mkfifo PIPE
# Links vs inodes
ln SRC DEST # hard link (same inode)
ln -s SRC DEST # symlink (diff inode)
# Permissions
chmod MODE PATH; chown USER:GROUP PATH
# Loopback + ext4
truncate -s 100M loop.img
sudo losetup -fP loop.img; losetup -a
sudo mkfs.ext4 -L LAB_FS /dev/loopX
sudo mkdir -p /mnt/labfs && sudo mount /dev/loopX /mnt/labfs
df -hT /mnt/labfs; lsblk -f | grep loopX
sudo umount /mnt/labfs; sudo losetup -d /dev/loopX; rm -f loop.img
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