scripts-fabq/notes/linux.md

Linux

Table of Contents

• Linux
  • Table of Contents
  • System Information
    • Hardware Information
    • Software Information
  • User Management
    • User Information
    • Super User Management
    • Switch User
  • System Management
    • Change password of a tar/openssl archive
    • Verify two possibly identical folders recursively
  • NFS
  • Network Diagnostics
  • Diagnosis
    • Debian Upgrade Issues
    • Wayland Issues
  • Fonts

System Information

Hardware Information

To gather detailed information about your hardware, use the following commands:

• lscpu: Displays information about the CPU architecture, including details about cores, threads, and CPU family.
• lshw: Provides a comprehensive listing of hardware components. Use lshw -short for a more concise view.
• hwinfo: Offers detailed information about hardware components and can be more verbose than lshw.
• lsscsi: Lists SCSI devices, including disks and other SCSI-attached hardware.
• lsusb: Shows information about USB devices connected to your system.
• dmidecode: Retrieves hardware information from the BIOS. Use:
  • dmidecode -t processor for CPU details
  • dmidecode -t memory for RAM details
  • dmidecode -t bios for BIOS information

CPU information

lscpu
cat /proc/cpuinfo
grep -c 'model name' /proc/cpuinfo

• lscpu: Structured summary of CPU architecture, cores, threads, and NUMA topology.
• cat /proc/cpuinfo: Raw per-core details including model name, flags, and frequencies.
• grep -c 'model name': Quick count of logical CPU cores.

GPU information

lspci | grep -i vga

CPU frequency scaling driver

Check which driver is managing CPU frequency scaling (e.g., intel_pstate, acpi-cpufreq):

cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_driver

Software Information

Finding information on the Linux distribution

• lsb_release -a: Displays detailed information about the Linux distribution, including the distributor ID, description, release number, and codename.
• cat /etc/debian_version: Displays the version of the Debian distribution if you're running a Debian-based system (like Ubuntu).
• cat /etc/os-release: Displays information about the operating system, such as the name, version, and ID of the distribution.
• cat /etc/*release: Searches for any files in the /etc/ directory that contain the word release and displays their contents. This typically includes more detailed distribution information.
• cat /etc/*version: Similar to cat /etc/*release, but looks for files containing the word version. It can provide additional version-related details.
• hostnamectl: Displays system information related to the hostname and other metadata about the system. This may include the operating system, kernel version, and architecture.

Finding Path to Binary

To find the location of an executable binary, use:

type <binary-name>

This command will show the path to a binary executable, ex composer, if it's available in your PATH.

Number of Words in a File

To count the number of words in a file, use:

wc <filepath>

This command will show the number of words along with other details like lines and characters.

Number of Lines in a File

To count the number of lines in a file, use:

wc -l <filepath>

User Management

User Information

Add users

This variation of the adduser command uses the --gecos option to pre-fill the user's information (Full name, Room number, Work Phone, Home Phone, and Email) non-interactively, allowing you to automate user creation with predefined details.

adduser --gecos "<full-name>,,,<email>" <username>

This variation creates a system user with a Bash shell, no password login (--disabled-password), a specified home directory (/home/<username>), and adds the user to a new group, while using the --gecos option to set the full name as <service-description>.

adduser --system --shell /bin/bash --gecos '<service-description>' --group --disabled-password --home /home/<username> <username>

List Users

To list all users from the /etc/passwd file in alphabetical order, use:

awk -F':' '{ print $1}' /etc/passwd | sort

Super User Management

Disable Root Login

To disable root login via SSH, perform the following steps:

1. Edit the SSH Configuration File:

   nano /etc/ssh/sshd_config
   

   Comment out the line containing PermitRootLogin.

2. Change Shell for Root User:

   nano /etc/passwd
   

   Find the line starting with root and change /bin/bash to /sbin/nologin.

   systemctl restart ssh
   

Add User to Sudo Group

adduser <username> sudo

Update Sudoers File to Remove Password Requirement

Edit the sudoers file with the default editor:

visudo

Edit the sudoers file with nano:

EDITOR=nano visudo

Add the following line to allow the user to execute commands without a password:

<username>     ALL=(ALL) NOPASSWD:ALL

Switch User

Switch to Another User as Sudoer

sudo -i -u <username>

Switch to Another User as Root

su - <username>

Run command as specific user

sudo -u <username> <command>

Change shell of a user

chsh -s /bin/bash <username>
chsh -s /usr/sbin/nologin <username>

Change user with specific shell

sudo -u <username> bash

System Management

Ensure hostname or add alias

nano /etc/hosts
# Add the hostname alias:
# 127.0.1.1 <hostname-intranet>

nano /etc/hostname
# Set the main hostname:
# 127.0.1.1 <hostname-intranet> <hostname-short>

hostnamectl set-hostname <hostname-intranet>

Tar backup for a large number of small files

Create a tar archive and transfer it to a remote server:

tar -c /path/to/dir | ssh <username>@<hostname-intranet> 'tar -xvf - -C /absolute/path/to/remotedir'

Compress and transfer a folder, then store it as a .tar.gz file:

tar zcvf - /folder | ssh <username>@<hostname-intranet> "cat > /backup/folder.tar.gz"

Transfer a compressed .tar.gz file and extract it on the remote server:

cat folder.tar.gz | ssh <username>@<hostname-intranet> "tar zxvf -"

Alternative: change directory on the remote server before extracting:

cat folder.tar.gz | ssh <username>@<hostname-intranet> "cd /path/to/dest/; tar zxvf -"

List time zones

Use timedatectl to list available time zones or check the current settings.

timedatectl
timedatectl list-timezones

Configure time zone

Configure the time zone using timedatectl or by manually setting a symbolic link to /etc/localtime.

timedatectl set-timezone "America/Toronto"

Alternatively, manually set the time zone by linking the correct file:

mv /etc/localtime /etc/localtime-old
ln -s /usr/share/zoneinfo/<timezone> /etc/localtime

Find a specific service

systemctl list-units --type=service | grep <service-name>

Bind mount a directory

Make a directory available at another path, useful during chroot recovery or container setup:

mount --bind /dev /mnt/<newroot>/dev

Chroot into another system

Enter a mounted system's root as if it were the running OS. Useful for recovery, initramfs rebuilds, or bootloader fixes:

chroot /mnt/<newroot>

Typically preceded by binding the required pseudo-filesystems:

mount --bind /dev  /mnt/<newroot>/dev
mount --bind /proc /mnt/<newroot>/proc
mount --bind /sys  /mnt/<newroot>/sys
chroot /mnt/<newroot>

Rebuild initramfs

After kernel or driver changes, rebuild the initramfs and refresh the GRUB configuration:

update-initramfs -u
update-initramfs -u -k all

• update-initramfs -u: Rebuilds the initramfs for the currently running kernel.
• -k all: Rebuilds for all installed kernels.

Rebuild initramfs for a specific kernel version:

update-initramfs -c -k $(uname -r)

• -c: Create a new initramfs (instead of updating).
• -k $(uname -r): Targets the currently running kernel version.

Update GRUB:

update-grub

• Scans for kernels and regenerates /boot/grub/grub.cfg.

Change password of a tar/openssl archive

Decrypt the archive

To decrypt an openssl-encrypted archive using a password stored in a file:

1. Store your password in a temporary file:

   nano $HOME/<filename>
   

2. Decrypt the archive:

   openssl aes-256-cbc -d -pbkdf2 -in <archive>.tar.gz -out <archive>.tar -pass file:$HOME/<filename>
   

3. Re-encrypt the archive with a new password:

   nano $HOME/<filename>
   openssl aes-256-cbc -e -pbkdf2 -in <archive>.tar -out <archive>-new.tar.gz -pass file:$HOME/<filename>
   rm $HOME/<filename>
   

Decode / Extract

Decrypt and extract the contents directly into a directory:

nano $HOME/<filename>
openssl aes-256-cbc -d -pbkdf2 -in <archive>.tar.gz -pass file:<filename> | tar xz -C .
rm $HOME/<filename>

Verify two possibly identical folders recursively

With diff

diff -r <dir1>/ <dir2>/

With rsync

Dry run — shows differences without copying any data:

rsync -avn <dir1>/ <dir2>/

• -n: dry run, no changes made.

With cmp

#!/bin/bash

dir1="<dir1>/"
dir2="<dir2>/"

# Check if both directories exist before proceeding.
if [ ! -d "$dir1" ] || [ ! -d "$dir2" ]; then
    echo "One or both directories do not exist."
    exit 1
fi

# Iterate through all files in dir1 and compare with corresponding files in dir2.
for file1 in $(find "$dir1" -type f); do
    file2="${file1/$dir1/$dir2}"
    if [ ! -f "$file2" ]; then
        echo "File $file2 not found."
    else
        cmp --silent "$file1" "$file2" || echo "Files $file1 and $file2 differ."
    fi
done

NFS

Show NFS exports from a server:

showmount -e <hostname>
showmount -e localhost

List active exports and their options on the server:

exportfs -v

Network Diagnostics

Measure HTTP response timing:

Breaks down the full request lifecycle — useful for diagnosing DNS, TLS, or TTFB issues:

curl -o /dev/null -s -w \
  'Lookup: %{time_namelookup}s\nConnect: %{time_connect}s\nAppConnect: %{time_appconnect}s\nTTFB: %{time_starttransfer}s\nTotal: %{time_total}s\n' \
  https://<hostname>

• time_namelookup: DNS resolution time.
• time_connect: TCP connection time.
• time_appconnect: TLS handshake time.
• time_starttransfer: Time to first byte (TTFB).
• -o /dev/null: Discards the response body.

High-frequency ping:

Flood-style ping to stress-test latency or detect intermittent packet loss:

ping -i 0.002 <host>

• -i 0.002: Send a packet every 2ms. Requires root.

Jumbo frame ping:

Test whether the network path supports large MTU frames (useful for diagnosing MTU mismatches):

ping -s 1472 -i 0.01 <host>

• -s 1472: Payload size of 1472 bytes (1472 + 28-byte IP/ICMP header = 1500-byte MTU).
• Increase -s to test jumbo frames (e.g., -s 8972 for 9000-byte MTU).

Diagnosis

Debian Upgrade Issues

Apt Logs

less /var/log/apt/history.log

Wayland Issues

System Logs

Examine system logs and hardware information for troubleshooting Wayland issues:

lspci -k | grep -A 3 -E "(VGA|3D)"
lsmod | grep -i "drm\|gpu\|nouveau\|amdgpu\|i915"
glxinfo | grep "OpenGL version"

GPU Information

Search the system logs for any errors or warnings related to GPU and Wayland:

journalctl -b | grep -i "drm\|gpu\|display\|wayland\|monitor"
journalctl -b | grep -i "gnome-shell"

Journal Filtering by Date and Keyword

Search logs within a specific time window:

journalctl --since "<date>" --until "<date>" | grep -i <keyword>

Example:

journalctl --since "2026-01-01" --until "2026-01-02" | grep -i btrfs

Kernel microcode events:

journalctl -k | grep -i "microcode"

• -k: Show only kernel messages (equivalent to dmesg output via the journal).

Fonts

Download and Install Fonts

1. Download the Font Archive:

   wget https://<font-archive-url>
   

2. Extract the Font Files:

   tar -xzvf <font-archive>.tar.gz
   

3. Copy the Font Files:

   Local font directory

   cp -v *.ttf ~/.local/share/fonts/
   

   Global font directory - Package manager managed

   cp -v *.ttf /usr/share/fonts
   

   Global font directory - User managed

   cp -v *.ttf /usr/local/share/fonts
   

Update the Font Cache

sudo su -
fc-cache -fv
fc-cache -frv

• -f: Force re-generation of cache files, overriding timestamp checking.
• -r: Erase all existing cache files and rescan.
• -v: Display status information while busy.