Short commands, long names

A kernel file that reports software RAID. /proc is a live view of the running system (not a normal disk folder). md = Multiple Device, Linux’s name for software RAID. stat = status.

CPU model, cores, and flags (including vmx/svm for virtualization).

Detailed RAM and swap numbers.

A software RAID array as a virtual disk. Numbers: md0, md1, md2, sometimes md127. On Hetzner, md2 is often the big array — root may still be LVM on top (vg0-root), not md2 itself.

First SATA/SAS disk. The sd prefix is historical (SCSI disk). Next disks: sdb, sdc. Partitions: sda1, sda2.

First NVMe SSD. Partitions add p: nvme0n1p1. A second NVMe is usually nvme1n1.

Friendly names for Device Mapper volumes: vg0-root (LVM), cryptroot (LUKS), mpatha (multipath). Same objects also appear as /dev/dm-0, dm-1. In Rescue, ls /dev/mapper/ then mount the top of the stack.

First disk inside a KVM/Proxmox/Incus guest. Partitions: vda1, vda2. On the dedicated host you still see nvme or sd devices.

List of filesystems Linux mounts at boot. Wrong UUIDs here can stop the server from booting.

Shows whether you are on Debian, Ubuntu, and which version.

The file’s ID on a filesystem. Stores type, permissions, owner, size, timestamps, hard-link count, and pointers to data blocks. The name lives in the directory. ls -li prints the number. Hard links share one inode.

Tree of disks, partitions, RAID, LVM, and mount points. First command to learn.

How full each mounted filesystem is. -h = human (GiB), -T = type, -i = inodes.

How much space a folder uses. df is the tank; du is which boxes are heavy.

Interactive du. NCurses is the text UI library. Arrow keys to browse folders.

Create, stop, and inspect Linux software RAID arrays.

Interactive partition editor. -l lists tables without changing them.

GNU partition tool. Handles GPT disks larger than 2 TiB.

Format a partition. mkfs.ext4, mkfs.xfs, mkfs.btrfs, mkfs.vfat. This erases data on that slice. The partition type (83 / Linux filesystem) is only a label — mkfs is the real format.

Format a volume as swap (overflow RAM on disk).

Attach a filesystem to a folder, often /mnt in Rescue.

Detach a filesystem. Spelled umount (no n) from old UNIX 6-character names.

What is mounted where, in a tree. In Rescue, findmnt / is Rescue’s RAM root until you mount the OS at /mnt.

Prints UUID and filesystem type. Use UUIDs in /etc/fstab. In Rescue, pair with lsblk -f to see RAID, LVM, and LUKS layers.

Unlock or inspect LUKS encrypted disks. cryptsetup luksDump /dev/sdX shows the header. luksOpen creates /dev/mapper/cryptroot. Not used by a default Hetzner installimage.

Erase RAID/filesystem labels so installimage can reuse the disk. Destructive.

Repair a filesystem. Run from Rescue, not on a mounted disk.

Grow (or shrink, carefully) an ext4 filesystem after LVM grow.

Grow an XFS filesystem while it is mounted. There is no supported XFS shrink.

Repair an XFS filesystem. Run from Rescue, not while it is mounted.

Disk health. SMART = Self-Monitoring, Analysis and Reporting Technology.

A layer on a disk or RAID so you can grow volumes later without repartitioning everything. Ubuntu installer names can look like ubuntu--vg-root (dashes doubled).

A disk, partition, or RAID array given to LVM. Command: pvcreate, list: pvs.

A pool of PVs. Often named vg0. Command: vgcreate, list: vgs.

A slice of a VG you format and mount, like vg0-root. Command: lvcreate, list: lvs.

Mark a device as LVM physical volume.

Create a pool, e.g. vgcreate vg0 /dev/sda3.

Cut a named volume from the pool.

Grow an LV. -r also grows the filesystem.

Short status lists for each LVM layer.

Show files in a folder. ls -l is the long listing. ls -li adds inode numbers. ls -lah includes hidden files and human sizes.

Print full metadata for one file: size, inode, mode, owner, and timestamps. ls -l is the summary; stat is the card.

Show the folder you are in before you run ls.

Print a file to the screen. cat /proc/mdstat means “show RAID status”.

Filter lines that match a pattern. egrep is grep -E (extended patterns).

CPU model, cores, threads, architecture.

Inventory of the machine. Needs root for full detail.

PCI cards: NICs, NVMe, RAID. PCI = Peripheral Component Interconnect.

How many CPU threads Linux sees.

Read firmware tables. DMI = Desktop Management Interface (RAM size, manufacturer).

Hostname, OS, kernel, virtualization.

Timezone and clock.

Start/stop services on modern Debian and Ubuntu.

Read systemd logs. --disk-usage / --vacuum-time clean old logs.

Kernel name. uname -a is a full snapshot.

How long the server has been running, plus load averages.

Change file permissions.

Change file owner/group.

Run commands as if a mounted disk were the live OS. Used in Rescue to reset passwd.

Set a user’s password. Inside chroot it changes the installed OS, not Rescue.

Run one command as root.

Switch to another user, often root.

RAM and swap. Trust available more than free. -h = human sizes.

Running programs. ps aux is “all users, extra detail”.

Live CPU/RAM table. Named for the top consumers, not an acronym.

Friendlier top (by Hisham Muhammad). Color, mouse, easier sorting.

RAM, swap, and I/O in one repeating snapshot.

How busy each disk is. %util near 100% means the disk is saturated.

Which process is doing the most disk I/O.

Encrypted remote login. ssh root@IP from your laptop.

Copy files over SSH.

Create keys, or ssh-keygen -R IP to forget an old host key after reinstall.

Modern network tool. ip a addresses, ip r routes, -br brief.

Old network tool. Prefer ip on Debian and Ubuntu.

Test if an IP answers.

Fetch a URL from the command line.

Download a file from HTTP/HTTPS.

See listening ports and which process owns them. Replaces netstat. ss -tulpn is the usual check.

Beginner firewall on Ubuntu/Debian. Writes iptables/nftables rules for you.

IPv4 packet filter and NAT rules. Tables: filter (allow/drop), nat (port forwarding).

Newer kernel firewall. nft list ruleset shows the live policy. iptables may be a compatibility wrapper.

Rewrite packet IPs/ports. Used to publish a VM through the host’s public IP.

Change where a packet is going (port forwarding to a guest IP).

Change where a packet appears to come from so replies find their way back.

Two separate port spaces. TCP 80 and UDP 80 can both be used at once.

INPUT = traffic to this host. FORWARD = traffic through this host to a VM.

Debian/Ubuntu installer. apt update then apt upgrade or apt install.

Low-level package tool under apt.

Simple text editor. Ctrl+O save, Ctrl+X exit.

Editor. Press i to type, Esc then :wq to save and quit.

Keep sessions alive if SSH drops.

Pack or unpack .tar.gz files. The name is from magnetic tape.

Compress a file. GNU = GNU’s Not Unix.

Hetzner Rescue installer for Ubuntu/Debian, RAID, and partitions. Not a UNIX acronym.

Several disks acting as one. Not a backup. Often one layer under LVM (md2 → vg0-root).

Disk encryption. Appears as crypto_LUKS in lsblk -f until you unlock it to /dev/mapper/cryptroot. Not the Hetzner installimage default.

Kernel layer that builds virtual block devices. LVM, LUKS, and multipath all show up as /dev/dm-N. Use the /dev/mapper/ name when you can.

Modern partition map. Needed for disks larger than 2 TiB. GUID = Globally Unique Identifier.

Old partition map. Max 2 TB and 4 primary partitions. Avoid on big dedicated disks.

Oldest common Linux format still in the tools. First Linux filesystem that could manage about 2 TB. No journal: after a crash, fsck must scan the whole disk. Do not use for a new Hetzner OS install.

ext2 plus a journal for fast crash recovery. Backward compatible with ext2. No snapshots and no easy undelete — pick ext4 on a new dedicated server.

The usual Ubuntu/Debian format on Hetzner (installimage default). Journal, extents, delayed allocation, journal checksums, multi-block allocation, online grow. Can mount old ext2/ext3 volumes; old ext2/ext3 drivers cannot mount a modern ext4 volume.

Mature 64-bit journaling filesystem from SGI. Default on RHEL 7+. Delayed allocation, dynamic inodes, online grow (no shrink), xfs_repair. Strong for large files and /var.

Copy-on-write filesystem with snapshots, data checksums, compression, and subvolumes. Useful later for an Incus pool. Keep the OS on ext4/XFS unless you specifically want those features.

Btrfs (and similar filesystems) never overwrite a block in place. That is how snapshots stay cheap and consistent.

A short diary of upcoming writes. After a crash, Linux replays it instead of scanning the whole disk. ext3/ext4 and XFS have one. ext2 does not. Btrfs uses copy-on-write instead.

One-byte labels: 83 = Linux, 8e = Linux LVM, fd = Linux RAID, 82 = swap. GPT uses names like “Linux filesystem” instead. Labels are not the filesystem — mkfs still creates ext4/XFS/Btrfs.

Modern firmware instead of BIOS. EFI is the older name.

Small FAT32 partition, usually /boot/efi, that UEFI boots from.

Older firmware. Some Hetzner boxes still boot this way (legacy).

Boot from the network. How Hetzner Rescue starts, from RAM, not from your disks.

Fast SSD on the PCI bus. Device names look like /dev/nvme0n1.

Common disk cable/protocol. ATA = Advanced Technology Attachment. Devices often /dev/sdX.

Flash vs spinning platter. lsblk ROTA 0 = SSD/NVMe, 1 = HDD.

Stable ID for a filesystem. Prefer this over /dev/sda3 in fstab.

Processor and working memory.

Debian, Ubuntu, Proxmox, and so on.

How you log in remotely.

Address of the server, and names that point to addresses.

Two separate port spaces. TCP 80 and UDP 80 can both be used at once.

Linux hypervisor for VMs (Proxmox uses this). Hetzner “KVM console” instead means Keyboard-Video-Mouse remote screen.

System containers. Proxmox and Incus can run these.

A full guest OS on the dedicated host.

Starts Linux from the disk. installimage installs GRUB2.

Typed commands vs menu screens (installimage uses a TUI).

On df, du, free: sizes in G/M. On many other tools: show help.

installimage -a runs without the menu. ls -a shows hidden files.

fdisk -l lists partitions. ls -l is the long listing.

Depends on the command. wipefs -f is force. lsblk -f shows filesystems.

apt install -y does not ask “are you sure?”

df -i shows inode use (number of files), not bytes. ls -li prints each file’s inode number.

df -hT adds the filesystem type column (ext4, xfs).

Print extra detail. mdadm --verbose while creating RAID.

wipefs -n only shows what it would erase.

lvextend -r grows the filesystem with the volume.

lsblk -o NAME,SIZE,TYPE chooses columns.

ip -br a is a short address list.

find -xdev stays on one disk so you do not walk into /mnt by accident.