TOPIC: NETWORK BOOTING
A Practical Linux Administration Toolkit: Kernels, Storage, Filesystems, Transfers and Shell Completion
4th February 2026
Linux command-line administration has a way of beginning with a deceptively simple question that opens into several possible answers. Whether the task is checking which kernels are installed before an upgrade, mounting an NFS share for backup access, diagnosing low disk space, throttling a long-running sync job or wiring up tab completion, the right answer depends on context: the distribution, the file system type, the transport protocol and whether the need is a one-off action or a persistent configuration. This guide draws those everyday administrative themes into a single continuous reference.
Identifying Your System and Installed Kernels
Reading Distribution Information
A sensible place to begin any administration session is knowing exactly what you are working with. One quick approach is to read the release files directly:
cat /etc/*-releaseOn systems where bat is available (sometimes installed as batcat), the same files can be read with syntax highlighting using batcat /etc/*-release. Typical output on Ubuntu includes /etc/lsb-release and /etc/os-release, with values such as DISTRIB_ID=Ubuntu, VERSION_ID="20.04" and PRETTY_NAME="Ubuntu 20.04.6 LTS". Three additional commands, cat /etc/os-release, lsb_release -a and hostnamectl, each present the same underlying facts in slightly different formats, while uname -r reports the currently running kernel release in isolation. Adding more flags with uname -mrs extends the output to include the kernel name and machine hardware class, which on an older RHEL system might return something like Linux 2.6.18-8.1.14.el5 x86_64.
Querying Installed Kernels by Package Manager
On Red Hat Enterprise Linux, CentOS, Rocky Linux, AlmaLinux, Oracle Linux and Fedora, installed kernels are managed by the RPM package database and are queried with:
rpm -qa kernelThis may return entries such as kernel-5.14.0-70.30.1.el9_0.x86_64. The same information is also accessible through yum list installed kernel or dnf list installed kernel. On Debian, Ubuntu, Linux Mint and Pop!_OS the package manager differs, so the command changes accordingly:
dpkg --list | grep linux-imageOutput may include versioned packages, such as linux-image-2.6.20-15-generic, alongside the metapackage linux-image-generic. Arch Linux users can query with pacman -Q | grep linux, while SUSE Enterprise Linux and openSUSE users can turn to rpm -qa | grep -i kernel or use zypper search -i kernel, which presents results in a structured table. Alpine Linux takes yet another approach with apk info -vvv | grep -E 'Linux' | grep -iE 'lts|virt', which may return entries such as linux-virt-5.15.98-r0 - Linux lts kernel.
Finding Kernels Outside the Package Manager
Package databases do not always tell the whole story, particularly where custom-compiled kernels are involved. A kernel built and installed manually will not appear in any package manager query at all. In that case, /lib/modules/ is a useful place to look, since each installed kernel generally has a corresponding module directory. Running ls -l /lib/modules/ may show entries such as 4.15.0-55-generic, 4.18.0-25-generic and 5.0.0-23-generic. A further check is:
sudo find /boot/ -iname "vmlinuz*"This may return files such as /boot/vmlinuz-5.4.0-65-generic and /boot/vmlinuz-5.4.0-66-generic, confirming precisely which versions exist on disk.
A Brief History of vmlinuz
That naming convention is worth understanding because it appears on virtually every Linux system. vmlinuz is the compressed, bootable Linux kernel image stored in /boot/. The name traces back through computing history: early Unix kernels were simply called /unix, but when the University of California, Berkeley ported Unix to the VAX architecture in 1979 and added paged virtual memory, the resulting system, 3BSD, was known as VMUNIX (Virtual Memory Unix) and its kernel images were named /vmunix. Linux inherited vmlinuz as a mutation of vmunix, with the trailing z denoting gzip compression (though other algorithms such as xz and lzma are also supported). The counterpart vmlinux refers to the uncompressed, non-bootable kernel file, which is used for debugging and symbol table generation but is not loaded directly at boot. Running ls -l /boot/ will show the full set of boot files present on any given system.
Examining and Investigating Disk Usage
Why ls Is Not the Right Tool for Directory Sizes
Storage management is an area where a familiar command can mislead. Running ls -l on a directory typically shows it occupying 4,096 bytes, which reflects the directory entry metadata rather than the combined size of its contents. For real space consumption, du is the appropriate tool.
sudo du -sh /varThe above command produces a summarised, human-readable total such as 85G /var. The -s flag limits output to a single grand total and -h formats values in K, M or G units. For an individual file, du -sh /var/log/syslog might report 12M /var/log/syslog, while ls -lh /var/log/syslog adds ownership and timestamps to the same figure.
Drilling Down to Find Where Space Has Gone
When a file system is full and the need is to locate exactly where the space has accumulated, du can be made progressively more revealing. The command sudo du -h --max-depth=1 /var lists first-level subdirectories with sizes, potentially showing 77G /var/lib, 5.0G /var/cache and 3.3G /var/log. To surface the biggest consumers quickly, piping to sort and head works well:
sudo du -h /var/ | sort -rh | head -10Adding the -a flag includes individual files alongside directories in the same output:
sudo du -ah /var/ | sort -rh | head -10Apparent Size Versus Allocated Disk Space
There is a subtle distinction that sometimes causes confusion. By default, du reports allocated disk usage, which is governed by the file system block size. A single-byte file on a file system with 4 KB blocks still consumes 4 KB of disk. To see the amount of data actually stored rather than allocated, sudo du -sh --apparent-size /var reports the apparent size instead. The df command answers a different question altogether: it shows free and used space per mounted file system, such as /dev/sda1 at 73 per cent usage or /dev/sdb1 mounted on /data with 70 GB free. In practice, du is for locating what consumes space and df is for checking how much remains on each volume.
gdu: A Faster Interactive Alternative
Some administrators prefer a more modern tool for storage investigations, and gdu is a notable option. It is a fast disk usage analyser written in Go with an interactive console interface, designed primarily for SSDs where it can exploit parallel processing to full effect, though it functions on hard drives too with less dramatic speed gains. The binary release can be installed by extracting its .tgz archive:
curl -L https://github.com/dundee/gdu/releases/latest/download/gdu_linux_amd64.tgz | tar xz
chmod +x gdu_linux_amd64
mv gdu_linux_amd64 /usr/bin/gduIt can also be run directly via Docker without installation:
docker run --rm --init --interactive --tty --privileged
--volume /:/mnt/root ghcr.io/dundee/gdu /mnt/rootIn use, gdu scans a directory interactively when run without flags, summarises a target with gdu -ps /some/dir, shows top results with gdu -t 10 / and runs without interaction using gdu -n /. It supports apparent size display, hidden file inclusion, item counts, modification times, exclusions, age filtering and database-backed analysis through SQLite or BadgerDB. The project documentation notes that hard links are counted only once and that analysis data can be exported as JSON for later review.
Unpacking TGZ Archives
A brief note on the tar command is useful here, since it appears throughout Linux administration, including in the gdu installation step above. A .tgz file is simply a GZIP-compressed tar archive, and the standard way to extract one is:
tar zxvf archive.tgzModern GNU tar can detect the compression type automatically, so the -z flag is often optional:
tar xvf archive.tgzTo extract into a specific directory rather than the current working directory, the -C option takes a destination path:
tar zxvf archive.tgz -C /path/to/destination/To inspect the contents of a .tgz file without extracting it, the t (list) flag replaces x (extract):
tar ztvf archive.tgzThe tar command was first introduced in the seventh edition of Unix in January 1979 and its name comes from its original purpose as a Tape ARchiver. Despite that origin, modern tar reads from and writes to files, pipes and remote devices with equal facility.
Mounting NFS Shares and Optical Media
Installing NFS Client Tools
NFS remains common on Linux and Unix-like systems, allowing remote directories to be mounted locally and treated as though they were native file systems. Before a client can mount an NFS export, the client packages must be installed. On Ubuntu and Debian, that means:
sudo apt update
sudo apt install nfs-commonOn Fedora and RHEL-based distributions, the equivalent is:
sudo dnf install nfs-utilsOnce installed, showmount -e 10.10.0.10 can list available exports from a server, returning output such as /backups 10.10.0.0/24 and /data *.
Mounting an NFS Share Manually
Mounting an NFS share follows the same broad pattern as mounting any other file system. First, create a local mount point:
sudo mkdir -p /var/backupsThen mount the remote export, specifying the file system type explicitly:
sudo mount -t nfs 10.10.0.10:/backups /var/backupsA successful command produces no output. Verification is done with mount | grep nfs or df -h, after which the local directory acts as the root of the remote file system for all practical purposes.
Persisting NFS Mounts Across Reboots
Since a manual mount does not survive a reboot, persistent setups use /etc/fstab. An appropriate entry looks like:
10.10.0.10:/backups /var/backups nfs defaults,nofail,_netdev 0 0The nofail option prevents a boot failure if the NFS server is unavailable when the machine starts. The _netdev flag marks the mount as network-dependent, ensuring the system defers the operation until the network stack is available. Running sudo mount -a tests the entry without rebooting.
Troubleshooting Common NFS Errors
NFS problems are often predictable. A "Permission denied" error usually means the server export in /etc/exports does not include the client, and reloading exports with sudo exportfs -ar is frequently the remedy. "RPC: Program not registered" indicates the NFS service is not running on the server, in which case sudo systemctl restart nfs-server applies. A "Stale file handle" error generally follows a server reboot or a deleted file and is cleared by unmounting and remounting. Timeouts and "Server not responding" messages call for checking network connectivity, confirming that firewall rules permit access to port 111 (rpcbind, required for NFSv3) and port 2049 (NFS itself), and verifying NFS version compatibility using the vers=3 or vers=4 mount option. NFSv4 requires only port 2049, while NFSv2 and NFSv3 also require port 111. To detach a share, sudo umount /var/backups is the standard route, with fuser -m /var/backups helping identify processes that are blocking the unmounting process.
Mounting Optical Media
CDs and DVDs are less central than they once were, but some systems still need to read them. After inserting a disc, blkid can identify the block device path, which is typically /dev/sr0, and will report the file system type as iso9660. With a mount point created using sudo mkdir /mnt/cdrom, the disc is mounted with:
sudo mount /dev/sr0 /mnt/cdromThe warning device write-protected, mounted read-only is expected for optical media and can be disregarded. CDs and DVDs use the ISO 9660 file system, a data-exchange standard designed to be readable across operating systems. Once mounted, the disc contents are accessible under /mnt/cdrom, and sudo umount /mnt/cdrom detaches it cleanly when work is complete.
Transferring Files Securely and Efficiently
Copying Files with scp
scp (Secure Copy) transfers files and directories between hosts over SSH, encrypting both data and authentication credentials in transit. Its basic syntax is:
scp [OPTIONS] [[user@]host:]source [[user@]host:]destinationThe colon is how scp distinguishes between local and remote paths: a path without a colon is local. A typical upload from a local machine to a remote host looks like:
scp file.txt remote_username@10.10.0.2:/remote/directoryA download from a remote host to the local machine reverses the argument order:
scp remote_username@10.10.0.2:/remote/file.txt /local/directoryCommonly used options include -r for recursive directory copies, -p to preserve metadata such as modification times and permissions, -C for compression, -i for a specific private key, -l to cap bandwidth in Kbit/s and the uppercase -P to specify a non-standard SSH port. It is also possible to copy between two remote hosts directly, routing the transfer through the local machine with the -3 flag.
The Protocol Change in OpenSSH 9.0
There is an important change in modern OpenSSH that administrators should be aware of. From OpenSSH 9.0 onward, the scp command uses the SFTP protocol internally by default rather than the older SCP/RCP protocol, which is now considered outdated. The command behaves identically from the user's perspective, but if an older server requires the legacy protocol, the -O flag forces it. For advanced requirements such as resumable transfers or incremental directory synchronisation, rsync is generally the better fit, particularly for large directory trees.
Throttling rsync to Protect Bandwidth
Even with rsync, raw speed is not always desirable. A backup script consuming all available bandwidth can disrupt other services on the same network link, so --bwlimit is often essential. The basic syntax is:
rsync --bwlimit=KBPS source destinationThe value is in units of 1,024 bytes unless an explicit suffix is added. A fractional value is also valid: --bwlimit=1.5m sets a cap of 1.5 MB/s. A local transfer capped at 1,000 KB/s looks like:
rsync --bwlimit=1000 /path/to/source /path/to/dest/And a remote backup:
rsync --bwlimit=1000 /var/www/html/ backups@server1.example.com:~/mysite.backups/The man page for rsync explains that --bwlimit works by limiting the size of the blocks rsync writes and then sleeping between writes to achieve the target average. Some volume undulation is therefore normal in practice.
Managing I/O Priority with ionice
Bandwidth is only one dimension of the load a transfer places on a system. Disk I/O scheduling may also need attention, particularly on busy servers running other workloads. The ionice utility adjusts the I/O scheduling class and priority of a process without altering its CPU priority. For instance:
/usr/bin/ionice -c2 -n7 rsync --bwlimit=1000 /path/to/source /path/to/dest/This runs the rsync process in best-effort I/O class (-c2) at the lowest priority level (-n7), combining transfer rate limiting with reduced I/O priority. The scheduling classes are: 0 (none), 1 (real-time), 2 (best-effort) and 3 (idle), with priority levels 0 to 7 available for the real-time and best-effort classes.
Together, --bwlimitand ionice provide complementary controls over exactly how much resource a routine transfer is permitted to consume at any given time.
Setting Up Bash Tab Completion
On Ubuntu and related distributions, Bash programmable completion is provided by the bash-completion package. If tab completion does not function as expected in a new installation or container environment, the following commands will install the necessary support:
sudo apt update
sudo apt upgrade
sudo apt install bash-completionThe package places a shell script at /etc/profile.d/bash_completion.sh. To ensure it is loaded in shell startup, the following appends the source line to .bashrc:
echo "source /etc/profile.d/bash_completion.sh" >> ~/.bashrcA conditional form avoids duplicating the line on repeated runs:
grep -wq '^source /etc/profile.d/bash_completion.sh' ~/.bashrc
|| echo 'source /etc/profile.d/bash_completion.sh' >> ~/.bashrcThe script is typically loaded automatically in a fresh login shell, but source /etc/profile.d/bash_completion.sh activates it immediately in the current session. Once active, pressing Tab after partial input such as sudo apt i or cat /etc/re completes commands and paths against what is actually installed. Bash also supports simple custom completions: complete -W 'google.com cyberciti.biz nixcraft.com' host teaches the shell to offer those three domains after typing host and pressing Tab, which illustrates how the feature can be extended to match the patterns of repeated daily work.
Installing Snap on Debian
Snap is a packaging format developed by Canonical that bundles an application together with all of its dependencies into a single self-contained package. Snaps update automatically, roll back gracefully on failure and are distributed through the Snap Store, which carries software from both Canonical and independent publishers. The background service that manages them, snapd, is pre-installed on Ubuntu but requires a manual setup step on Debian.
On Debian 9 (Stretch) and newer, snap can be installed directly from the command line:
sudo apt update
sudo apt install snapdAfter installation, logging out and back in again, or restarting the system, is necessary to ensure that snap's paths are updated correctly in the environment. Once that is done, install the snapd snap itself to obtain the latest version of the daemon:
sudo snap install snapdTo verify that the setup is working, the hello-world snap provides a straightforward test:
sudo snap install hello-world
hello-worldA successful run prints Hello World! to the terminal. Note that snap is not available on Debian versions before 9. If a snap installation produces an error such as snap "lxd" assumes unsupported features, the resolution is to ensure the core snap is present and current:
sudo snap install core
sudo snap refresh coreOn desktop systems, the Snap Store graphical application can then be installed with sudo snap install snap-store, providing a point-and-click interface for browsing and managing snaps alongside the command-line tools.
Increasing the Root Partition Size on Fedora with LVM
Fedora's default installer has used LVM (Logical Volume Manager) for many years, dividing the available disk into a volume group containing separate logical volumes for root (/), home (/home) and swap. This arrangement makes it straightforward to redistribute space between volumes without repartitioning the physical disk, which is a significant advantage over a fixed partition layout. Note that Fedora 33 and later default to Btrfs without LVM for new installations, so the steps below apply to systems that were installed with LVM, including pre-Fedora 33 installs and any system where LVM was selected manually.
Because the root file system is in active use while the system is running, resizing it safely requires booting from a Fedora Live USB stick rather than the installed system. Once booted from the live environment, open a terminal and begin by checking the volume group:
sudo vgsOutput such as the following shows the volume group name, total size and, crucially, how much free space (VFree) is unallocated:
VG #PV #LV #SN Attr VSize VFree
fedora 1 3 0 wz--n- <237.28g 0Before proceeding, confirm the exact device mapper paths for the root and home logical volumes by running fdisk -l, since the volume group name varies between installations. Common names include /dev/mapper/fedora-root and /dev/mapper/fedora-home, though some systems use fedora00 or another prefix.
When Free Space Is Already Available
If VFree shows unallocated space in the volume group, the root logical volume can be extended directly and the file system resized in a single command:
lvresize -L +5G --resizefs /dev/mapper/fedora-rootThe --resizefs flag instructs lvresize to resize the file system at the same time as the logical volume, removing the need to run resize2fs separately.
When There Is No Free Space
If VFree is zero, space must first be reclaimed from another logical volume before it can be given to root. The most common approach is to shrink the home logical volume, which typically holds the most available headroom. Shrinking a file system involves data moving on disk, so the operation requires the volume to be unmounted, which is why the live environment is essential. To take 10 GB from home:
lvresize -L -10G --resizefs /dev/mapper/fedora-homeOnce that completes, the freed space appears as VFree in vgs and can be added to the root volume:
lvresize -L +10G --resizefs /dev/mapper/fedora-rootBoth steps use --resizefs so that the file system boundaries are updated alongside the logical volume boundaries. After rebooting back into the installed system, df -h will confirm the new sizes are in effect.
Keeping a Linux System Well Maintained
The commands and configurations covered above form a coherent body of everyday Linux administration practice. Knowing where installed kernels are recorded, how to measure real disk usage rather than directory metadata, how to attach local and network file systems correctly, how to extract archives and move data securely without disrupting shared resources, how to make the shell itself more productive, how to extend a Debian system with snap packages and how to redistribute disk space between LVM volumes on Fedora converts a scattered collection of one-liners into a reliable working toolkit. Each topic interconnects naturally with the others: a kernel query clarifies what system you are managing, disk investigation reveals whether a file system has room for what you plan to transfer, NFS mounting determines where that transfer will land and bandwidth control determines what impact it will have while it runs.