Faq articles

Last change: 2025-01-06

Linux systems using systemd can use the systemctl command to list all available and active service units. Learn how to use the command with these tips.

Last change: 2025-01-06

Linux has multiple tools to process text files, including to search through files and filter out all unique words. Here is how to combine a few commands.

Last change: 2025-01-06

The version of systemd defines the available features and commands that is has to offer. Learn how to query the systemd version number.

Last change: 2025-01-06

Looking for all DNS requests made on a Linux system? Tools like dnstop, firejail, and tcpdump can help to find what is being queried.

Last change: 2025-01-06

Services that are controlled with systemd can be reloaded or restarted depending on their configuration. Use systemctl to perform the related restart tasks.

Last change: 2025-01-06

Systemd units can be enabled or disabled based on multiple factors, such as meeting specific dependencies. Learn how to see if a systemd service is enabled.

Last change: 2025-01-06

Systemd units can be configured to contain environment variables and passed along to the underlying application. Learn how to configure this unit setting.

Last change: 2025-01-06

SSH logs authentication attempts, but where? Find your SSH log file with these tips, such as in /var/log/auth.log or in the systemd journal.

Last change: 2025-01-06

The apt package manager can be used to download a package file without actually installing it. See how the download subcommand is used to achieve this.

Last change: 2025-01-06

A healthy service should not have configuration issues. Perform a configuration test of the SSH daemon (sshd) by first running it using the '-t' option.

Last change: 2025-01-06

Learn how to see the dependencies of a package that is to be installed or already installed on the system using the apt package manager.

Last change: 2025-01-06

Learn how to remove previously installed packages on a Linux system using the apt package manager, such as Debian and Ubuntu.

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Learn why and how packages may become unnecessary on Linux and how to remove them with the apt package manager on systems like Debian and Ubuntu.

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Learn how to troubleshoot issues with systemd units by verifying the unit files for any errors. One of the tools to help is systemd-analyze.

Last change: 2025-01-06

Troubleshoot issues like units being marked as 'not-found' in the output of systemctl list-units. This articles help with the steps to take.

Last change: 2025-01-06

Systemd timers are scheduled tasks for Linux systems. Show timer information with the systemctl command such as status, last execution, and its schedule.

Last change: 2025-01-06

Learn the difference between Linux kernel random sources /dev/random and /dev/urandom, and when to use which one. Spoiler: probably /dev/urandom.

Last change: 2025-01-06

Learn how to use the journalctl command to query the disk usage of the journal logs and how to clean or trim them by number, size, or age.

Last change: 2025-01-06

Got a busy Linux systems and wondering what the culprit is? Learn how to monitor them and focus on disk activity with tools like iotop, sar, vmstat, or iostat.

Last change: 2025-01-06

The file command is a powerful tool for Linux users. Learn how to see the file type of normal and special files on a Linux system.

Last change: 2025-01-06

File systems on Linux store a lot more than just the data itself. Learn how to see the size of a file on Linux systems using the ls or stat command.

Last change: 2025-01-06

Learn how to sort the output of the ls command and list a directory contents by its modification time. Here is which options to use.

Last change: 2025-01-06

Linux systems using systemd, use timers to schedule a repeating task. Learn how to configure these systemd timer units and fine-tune them.

Last change: 2025-01-06

Want to know if systemd is used on your Linux distribution? Learn how to quickly confirm that systemd is being used as your system and service manager.

Last change: 2025-01-06

Learn how to load and use your SSH key together with a SSH agent. This frequently asked question will explain the usage of ssh-add and ssh-agent.

Last change: 2025-01-06

Linux systems using systemd have the systemctl command available that can be used to show all service units, including a filter for only those that are enabled.

Last change: 2025-01-06

When making changes to systemd unit files, you may need to use systemctl daemon-reload. This article explains why and what happens next.

Last change: 2025-01-06

When systemd units are changed, a 'systemctl daemon-reload' might be needed. Need to know why? We can find the relevant units with some scripting.

Last change: 2025-01-06

Systemd units can be filtered using the SystemCallFilter setting. Learn how to see what syscalls are part of a particular syscall filter set.

Last change: 2025-01-06

Want to disable a systemd service unit, but wondering the difference between systemctl disable and systemctl mask? This article explains the differences.

Last change: 2025-01-06

Systemd allows customizing services with overrides. Learn how to edit an existing systemd service unit with the systemctl edit command.

Last change: 2025-01-06

Linux systems using systems may use the systemctl command to query services. Use a filter to reduce its output and only show all running services.

Last change: 2025-01-06

The background color of run0, part of systemd, can be changed with the '--background' option. This way another color can be configured instead of its default.

Last change: 2025-01-06

Linux has a wide range of tools to do data processing and text manipulation. Learn how to remove trailing whitespace from a file using the sed command.

Last change: 2025-01-06

Learn how to insert a line of text at the beginning of a file using the sed command. Great to automate repeating tasks and do data processing.

Last change: 2025-01-06

Frequently asked questions about data and text processing are covered in this section with focus on Linux tools and systems.

Last change: 2025-01-06

The systemctl command can be used to show the memory usage of a service managed by systemd. Use the subcommand 'status' to find the details about a unit.

Last change: 2025-01-06

Linux systems using systemd can use the systemctl command to show the all applied unit settings. This can be used on units like a service.

Last change: 2025-01-06

Systemd units have their own configuration file. The systemctl 'edit' command can be used to override settings of a systemd unit, including services.

Last change: 2025-01-06

Control groups allocate resources to a set of processes. Learn how to find the control group (cgroup) of a process by using /proc, pidof, or ps.

Last change: 2025-01-06

Want to see the control group in the output of the ps command? Here is how to tune your command options to include that.

Last change: 2025-01-06

Linux systems running systemd can use the timedatectl command to show time synchronization details. Learn how to use it, including its subcommands.

Last change: 2025-01-06

Linux systems using systemd store an unique identifier called the machine ID. Find this value using the hostnamectl command that comes with systemd.

Last change: 2025-01-06

The systemctl command has the list-dependencies option to show dependencies between units. But there are more options to query a little bit more information.

Last change: 2025-01-06

The systemctl command can be used to show all available systemd unit types. Here is how to find the available types and to select them.

Last change: 2025-01-06

Linux systems using systemd have the systemctl command available to show all active systemd units of one particular type using the '--type' option.

Last change: 2025-01-06

Learn how to define the maximum size that the systemd journal daemon may use on Linux systems for storing journals and limit its disk usage.

Last change: 2025-01-06

Use the journalctl command to show the size of the systemd journal logs. In this article we look how journalctl vacuuming works.

Last change: 2025-01-06

Linux systems using systemd store kernel events in the journal logs. Show these entries with the '--dmesg' or '-k' option, optionally with a date.

Last change: 2025-01-06

Systemd units define resources, such as a service, path, socket, or timer. They are usually managed with the systemctl command.

Last change: 2025-01-06

Linux systems with systemd use journal to store log entries. Learn how to filter these journal entries by specifying a date or time interval.

Last change: 2025-01-06

Learn how to continuously show new log entries on Linux systems using systemd with the journalctl command. The behavior will be like the 'tail -f' command.

Last change: 2025-01-06

Linux systems with systemd store log entries in a journal. Limit the number of log entries from the journal by filtering journalctl output by unit.

Last change: 2025-01-06

When changes are made to systemd unit files, such as service files, the systemd daemon needs to be reloaded. Use the daemon-reload subcommand to reload.

Last change: 2025-01-06

Systemd is a system and service manager on Linux distributions to start, stop, and monitor system services. Learn about systemd is and the main components.

Last change: 2025-01-06

Systemd units that are in a masked state are administratively disabled. While being in this state, they can not be started until they are unmasked.

Last change: 2025-01-06

The agent forwarding feature in SSH allows using your local SSH agent to be reached through an existing SSH connection. Learn when and how to use it.

Last change: 2025-01-06

The OpenSSH agent helps with authentication by making SSH keys available. Learn how to start the SSH agent when it is not running on your Linux system.

Last change: 2025-01-06

The SSH agent is a helper utility to temporarily store private keys when using public key authentication. Learn more about how this helps during daily tasks.

Last change: 2025-01-06

The SSH agent can be used to simplify authentication. As that is not always preferred behavior, we can disable the SSH agent when authenticating.

Last change: 2025-01-06

On Linux systems such as Arch Linux, pacman is the default package manager. Query the pacman package manager to show all installed packages.

Last change: 2025-01-06

Learn how to stop all processes of a single user using the killall command. To make this work, use the --user option and specify the username.

Last change: 2025-01-06

OpenSSH performs a host authenticity check when connecting to a system. Learn how to disable the check with the SSH option StrictHostKeyChecking.

Last change: 2025-01-06

Learn about SSH escape sequences and how they can help with terminating a SSH connection that does not respond to CTRL+C.

Last change: 2025-01-06

While protecting SSH keys is typically advised, it is not always feasible in automated processes. Learn how to remove the password or passphrase from a SSH key.

Last change: 2025-01-06

The SSH agent can load stored SSH keys into memory for authentication purposes. Use the ss-add command to show the available SSH keys that are loaded.

Last change: 2025-01-06

Frequently asked questions about SSH, such as SSH keys, configuration, and usage.

Last change: 2025-01-06

A zombie process, or defunct process, has completed execution, but has still an entry in the process table. Learn more about a zombie process on Linux.

Last change: 2025-01-06

How to kill a zombie process on Linux if it no longer responds to kill -9? Learn about zombie processes, including a few last steps that you can try.

Last change: 2025-01-06

When running a Linux system, you may need to find the process ID (PID) and process name. On Linux we can do this with the help of the pgrep command.

Last change: 2025-01-06

Each process on Linux has its own number. Discover this number is called the process ID (PID). Learn how to find the PID for a running process by name.

Last change: 2025-01-06

Linux administrators can interact in multiple ways with running processes. Learn how to find and stop a running process on Linux by searching for its name.

Last change: 2025-01-06

Frequently asked questions about running processes on a Linux systems, such as starting and stopping processes, query information, and monitoring them.

Last change: 2025-01-06

Show IP address information on Linux with the help of the ip command. Learn which subcommands to use to query the relevant details.

Last change: 2025-01-06

Show the IP address of your internet connection using the dig or the curl command. Learn which options can be used to find out this information.

Last change: 2025-01-06

Find the active DNS server being used by reviewing the network configuration, including common commands to query this information.

Last change: 2025-01-06

Use the find command to search for any files on Linux that are writable. To make this work, the -perm option of find can be used.

Last change: 2025-01-06

Disks will eventually fill up. Learn how to quickly see the size of a directory or folder on Linux systems using the du command.

Last change: 2025-01-06

Files starting with a dot are usually hidden on Linux. Learn how to see any hidden files on the command line or in the terminal using the ls command.

Last change: 2025-01-06

Learn how to see files smaller or bigger than a specific defined size on Linux, using the du command. Or use the find command to achieve a similar result.

Last change: 2025-01-06

Linux filesystems typically store file modification timestamps. Learn how to find the last modification time of a file or subdirectory in a specified directory.

Last change: 2025-01-06

Linux file systems use inodes, unique references to link to a file or directory. Learn how to see inode usage on a Linux file system or mount point.

Last change: 2025-01-06

Learn how to see used and remaining disk space on Linux systems by gathering the statistics of file systems and mount points using the df command.

Last change: 2025-01-06

Symbolic links may point to a directory or a file. Learn how to use the find command to discover symbolic links that refer to a directory.

Last change: 2025-01-06

Learn how to compare two directories on a Linux systems and see their differences between both or what files they are having in common.

Last change: 2025-01-06

Linux has in-depth details, including about network connections. Show the number of open connections using the ss command on Linux.

Last change: 2025-01-06

Linux has in-depth process information. Learn more about processes, such as when a process was started, using the ps tool.

Last change: 2025-01-06

There are multiple ways to see when a Linux system was started, such as using the uptime command, but also with commands like ps.

Last change: 2025-01-06

Frequently asked questions about system administration on Linux, the system state, how to perform common tasks, and troubleshooting issues.

Last change: 2025-01-06

Show the bandwidth usage and active connections by using the iftop tool on Linux. Learn how to use the tool to quickly find out this information.

Last change: 2025-01-06

Linux with systemd might doing DNS resolving using its resolver daemon. Learn how to inspect and clear the DNS cache when using the systemd resolver daemon.

Last change: 2025-01-06

Show counters related to the TCP connections by using the nstat command. This small utility will quickly retrieve the related statistics and display them.

Last change: 2025-01-06

Show CPU details such as architecture, vendor, caches, virtualization options, and even known CPU vulnerabilities such as Meltdown and Spectre.

Last change: 2025-01-06

Learn how to find the Linux distribution and version of a system. Use the right tool or file to find the relevant details.

Last change: 2025-01-06

Linux systems have a wide range of supporting utilities available. Show memory information such as memory type, the number of banks in use, and speed.

Last change: 2025-01-06

Need to delete the contents of a sensitive file on Linux? Instead of just deleting it with rm, have a look at some other options for a more secure deletion.

Last change: 2025-01-06

Linux may store the initial creation of a file. Learn how to use the stat command to find this initial creation time of a file, also known as its birth time.

Last change: 2025-01-06

The Linux kernel is marked tainted when a specific event happened that could impact reliable troubleshooting of kernel issues. Learn about the relevant events.

Last change: 2025-01-06

The Linux kernel can mark itself as being 'tainted'. Learn what it means when the Linux kernel is tainted and in particular the underlying cause.

Last change: 2025-01-06

Frequently asked questions about the Linux kernel and kernel security.

Last change: 2025-01-06

Show the network link details using the ip command to find out if a network has errors or dropped packets on a Linux system.

Last change: 2025-01-06

Use the network routing table to discover the default gateway on a Linux system. Here is how to use the ip command to achieve this

Last change: 2025-01-06

TCP or UDP ports can already be in use when starting up a service. Find out which process opened an UDP or TCP port on Linux by using the ss command.

Last change: 2025-01-06

Show which UDP/TCP ports are opened on a Linux system, including the related process. Use the ss tool to see more details about these sockets.

Last change: 2025-01-06

Frequently asked questions about networking, such as DNS, IP configuration, TCP/UDP details, and more.

Last change: 2025-01-06

Learn how to query the Time To Live (TTL) for a DNS record by using the dig tool on a Linux systems, including any relevant options.

Last change: 2025-01-06

Query the package manager to show installed packages on Ubuntu systems including version details. This can be done with the dpkg command and --list option.

Last change: 2025-01-06

Frequently asked questions about software and in particular the package manager. Learn how to install packages, query package versions, and more.

Last change: 2025-01-06

Retrieve device information from USB hubs and devices on Linux using the lsusb command. Learn how to use the command and its output.

Last change: 2025-01-06

Show the available hard disks in a system by using the right Linux tool. There are multiple options to pick, so let's have a look.

Last change: 2025-01-06

Show more detailed information about the available hard disks in the system. Specifications like speed, serial number, firmware, and other details.

Last change: 2025-01-06

The BIOS details can be displayed from within Linux itself. Learn how to query these details and where to find more information.

Last change: 2025-01-06

Frequently asked questions about hardware information on Linux systems, such as BIOS details, hard disks, USB devices, memory, and other details.

Last change: 2025-01-06

Find the biggest directories and files on disk by using the du command. The output can be sorted using numeric values to find the biggest entries.

Last change: 2025-01-06

Want to find all masked unit files on a Linux system running systemd? In this article we show how to do this with systemctl and query those units.

Last change: 2025-01-06

Limit the output from journalctl by defining the number of lines you want to see by using the '-n' option, optionally with the service itself.

Last change: 2025-01-06

Want to disable a service or specific systemd unit? Use the systemctl command to configure units and disable it on boot or completely.

Last change: 2025-01-06

Systemd can start and enable a unit, such as a service at the same time. Learn how to use systemctl more efficiently to achieve this this action.

Last change: 2025-01-06

Want to check the system for failed systemd units? In this article we show how to do this with systemctl and query the units with a failure state.

Last change: 2025-01-06

Frequently asked questions about systemd, systemctl, and journalctl. Learn by practical examples how to use these tools.

Last change: 2025-01-06

Frequently asked questions about file systems, file permissions, directories and files.

Last change: 2025-01-06

System hardening is the process of improving security defenses of desktop and servers. It is usually time-consuming, so let's decide when enough is enough.