Faq
How to reload or restart a systemd service?
Services that are controlled with systemd can be reloaded or restarted depending on their configuration. Use systemctl to perform the related restart tasks.
How to check if a systemd service is enabled?
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.
How to set environment variables in a systemd unit?
Systemd units can be configured to contain environment variables and passed along to the underlying application. Learn how to configure this unit setting.
How to see the SSH log?
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.
How to download a package with apt without installing it?
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.
How to test the sshd configuration for configuration errors?
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.
How to see the dependencies of a package with apt?
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.
How to remove a package with apt?
Learn how to remove previously installed packages on a Linux system using the apt package manager, such as Debian and Ubuntu.
How to remove unused packages with apt?
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.
How to verify a systemd unit for errors?
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.
Why does systemctl list-units show units as 'not-found'?
Troubleshoot issues like units being marked as 'not-found' in the output of systemctl list-units. This articles help with the steps to take.
How to see active systemd timers
Systemd timers are scheduled tasks for Linux systems. Show timer information with the systemctl command such as status, last execution, and its schedule.
What is the difference between /dev/random and /dev/urandom?
Learn the difference between Linux kernel random sources /dev/random and /dev/urandom, and when to use which one. Spoiler: probably /dev/urandom.
How to clear systemd journal logs by time
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.
How to monitor disk activity (I/O) on Linux
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.
How to see the file type on Linux
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.
How to see the size of a file
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.
How to display directory contents sorted by modification time
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.
How to schedule a periodic task with systemd
Linux systems using systemd, use timers to schedule a repeating task. Learn how to configure these systemd timer units and fine-tune them.
How to check if systemd is being used or active
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.
How to add a SSH key to the SSH agent
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.
How to see all enabled services with systemctl
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.
What does systemctl daemon-reload do?
When making changes to systemd unit files, you may need to use systemctl daemon-reload. This article explains why and what happens next.
How to check if 'systemctl daemon-reload' is needed
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.
How to see which syscalls are part of a systemd syscall filter set
Systemd units can be filtered using the SystemCallFilter setting. Learn how to see what syscalls are part of a particular syscall filter set.
What is the difference between systemctl disable and systemctl mask?
Want to disable a systemd service unit, but wondering the difference between systemctl disable and systemctl mask? This article explains the differences.
How to use systemctl edit to change a service?
Systemd allows customizing services with overrides. Learn how to edit an existing systemd service unit with the systemctl edit command.
How to see only running services with systemctl
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.
How to disable the background color of run0
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.
How to remove trailing whitespace from a file
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.
How to insert a line at the beginning of a file
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.
Data processing: Frequently Asked Questions
Frequently asked questions about data and text processing.
How to see memory usage of a service with systemctl?
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.
How to see the active settings of a systemd unit
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.
How to override the settings of a systemd unit
Systemd units have their own configuration file. The systemctl 'edit' command can be used to override settings of a systemd unit, including services.
How to see the cgroup of a process
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.
How to see cgroup in ps output
Want to see the control group in the output of the ps command? Here is how to tune your command options to include that.
How to see the time synchronization details with timedatectl
Linux systems running systemd can use the timedatectl command to show time synchronization details. Learn how to use it, including its subcommands.
How to show the systemd machine ID
Linux systems using systemd store an unique identifier called the machine ID. Find this value using the hostnamectl command that comes with systemd.
How to see the dependencies of a systemd unit
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.
How to see the available systemd unit types
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.
How to see all active systemd units of one type
Linux systems using systemd have the systemctl command available to show all active systemd units of one particular type using the '--type' option.
How to limit the disk usage of the systemd journal
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.
How to see the size of the systemd journal
Use the journalctl command to show the size of the systemd journal logs. In this article we look how journalctl vacuuming works.
How to see kernel messages with journalctl
Linux systems using systemd store kernel events in the journal logs. Show these entries with the '--dmesg' or '-k' option, optionally with a date.
What is a systemd unit?
Systemd units define resources, such as a service, path, socket, or timer. They are usually managed with the systemctl command.
How to see only recent journal entries
Linux systems with systemd use journal to store log entries. Learn how to filter these journal entries by specifying a date or time interval.
How to see new log entries automatically with journalctl
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.
How to see logging for a specific unit or service
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.
How to reload the systemd configuration
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.
What is systemd?
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.
What is a masked systemd unit?
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.
What is SSH agent forwarding?
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.
How to start the SSH agent?
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.
What is the purpose of the SSH agent?
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.
How to disable the usage of the SSH agent
The SSH agent can be used to simplify authentication. As that is not always preferred behavior, we can disable the SSH agent when authenticating.
How to show all installed packages with pacman
On Linux systems such as Arch Linux, pacman is the default package manager. Query the pacman package manager to show all installed packages.
How to stop all processes of a single user
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.
How to disable the SSH host key check?
OpenSSH performs a host authenticity check when connecting to a system. Learn how to disable the check with the SSH option StrictHostKeyChecking.
How to terminate a SSH connection that does not respond to CTRL+C
Learn about SSH escape sequences and how they can help with terminating a SSH connection that does not respond to CTRL+C.
How to remove the passphrase from a SSH key
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.
How to see the available SSH keys in the OpenSSH authentication agent
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.
SSH: Frequently Asked Questions
Frequently asked questions about SSH, such as SSH keys, configuration, and usage.
What is a zombie process?
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.
How to kill a zombie process
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.
How to show a running process name and its process ID (PID)
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.
How to find all process IDs by its process name
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.
How to kill a running process by its name
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.
Processes: Frequently Asked Questions
Frequently asked questions about running processes on a Linux systems, such as starting and stopping processes, query information, and monitoring them.
How to see the the network IP address of your system
Show IP address information on Linux with the help of the ip command. Learn which subcommands to use to query the relevant details.
How to see the IP address of your internet connection
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.
How to see which DNS server is used
Find the active DNS server being used by reviewing the network configuration, including common commands to query this information.
How to find writable files
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.
How to see the size of a directory
Disks will eventually fill up. Learn how to quickly see the size of a directory or folder on Linux systems using the du command.
How to see hidden files
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.
How to see files greater than a specific size
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.
How to find when the last modification happened in a directory
Linux filesystems typically store file modification timestamps. Learn how to find the last modification time of a file or subdirectory in a specified directory.
How to see inode usage
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.
How to see used and free disk space
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.
How to find symbolic links that point to a directory
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.
How to compare two directories and find the differences
Learn how to compare two directories on a Linux systems and see their differences between both or what files they are having in common.
How to see the number of open connections on Linux
Linux has in-depth details, including about network connections. Show the number of open connections using the ss command on Linux.
How to see when a process was started
Linux has in-depth process information. Learn more about processes, such as when a process was started, using the ps tool.
How to see when the system was started (uptime)
There are multiple ways to see when a Linux system was started, such as using the uptime command, but also with commands like ps.
System administration: Frequently Asked Questions
Frequently asked questions about system administration, the system state, and how to perform common tasks.
How to see active connections and bandwidth usage on Linux
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.
Show to clear the DNS cache with systemd
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.
How to show network TCP statistics and counters
Show counters related to the TCP connections by using the nstat command. This small utility will quickly retrieve the related statistics and display them.
How to see CPU details
Show CPU details such as architecture, vendor, caches, virtualization options, and even known CPU vulnerabilities such as Meltdown and Spectre.
Methods to find the Linux distribution and version
Learn how to find the Linux distribution and version of a system. Use the right tool or file to find the relevant details.
How to see memory information such as type and speed
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.
How to securely delete a file and its contents
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.
How to see the creation date of a file
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.
What is a tainted kernel
The Linux kernel is marked tainted when a specific event happened that could impact reliable troubleshooting of kernel issues. Learn about the relevant events.
How to find the specific cause of a tainted kernel
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.
Kernel: Frequently Asked Questions
Frequently asked questions about the Linux kernel and kernel security.
How to see errors and dropped packets on a network interface on Linux
Show the network link details using the ip command to find out if a network has errors or dropped packets on a Linux system.
How to see the default gateway on Linux
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
How to see which process is using a port
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.
How to see open ports on Linux
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.
Networking: Frequently Asked Questions
Frequently asked questions about networking, such as DNS, IP configuration, TCP/UDP details, and more.
How to see the TTL value of a DNS record
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.
How to show all installed packages on Ubuntu
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.
Package manager: Frequently Asked Questions
Frequently asked questions about software, such as package manager, package versions, and how to configure them.
How to list all USB devices
Retrieve device information from USB hubs and devices on Linux using the lsusb command. Learn how to use the command and its output.
How to see the available hard disks
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.
How to see hard disk specifications and details
Show more detailed information about the available hard disks in the system. Specifications like speed, serial number, firmware, and other details.
How to see BIOS details
The BIOS details can be displayed from within Linux itself. Learn how to query these details and where to find more information.
Hardware: Frequently Asked Questions
Frequently asked questions about hardware information on Linux systems, such as BIOS details, hard disks, USB devices, memory, and other details.
How to find the biggest directories on disk
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.
How to see all masked units with systemctl
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.
How to see the last X lines with journalctl
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.
How to disable a systemd unit with systemctl
Want to disable a service or specific systemd unit? Use the systemctl command to configure units and disable it on boot or completely.
How to start and enable a unit with systemctl
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.
How to show failed units with systemctl
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.
Systemd: Frequently Asked Questions
Frequently asked questions about systemd, systemctl, and journalctl. Learn by practical examples how to use these tools.
File systems: Frequently Asked Questions
Frequently asked questions about file systems, file permissions, directories and files.