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/// Set default alsa device

Find desired device

$ cat /proc/asound/cards

then create /etc/asound.conf with following:

defaults.pcm.card 1
defaults.ctl.card 1

Replace "1" with the number of your card.

/// Display file sizes

$ du -sh *

# Include hidden files
$ du -sh .[!.]* * | sort -h
$ du -sch .[!.]* * | sort -h

/// Print octal file permissions

$ stat -c "%a %n" *
$ stat -c "%a %n" .*

/// Get files used by certain process

$ pgrep httpd | xargs -I {} -n 1 lsof -p {}

/// Set a SASL password for virsh

$ saslpasswd2 -a libvirt "username-here"

/// Create dummy files

# Assuming you want to create a file having 100M size.
$ dd if=/dev/zero of=/tmp/100M_dummy_file bs=100M count=1 status=progress

/// ESXi increase VM disk size, no reboot

# Part1:
# In the esxi GUI increase the VM's disk size.

# Part2:
# Open a shell to your VM and tell the kernel to rescan your device. In this case sda.
$ echo 1 > /sys/class/block/sda/device/rescan

# Install growpart
$ yum install cloud-utils-growpart
$ apt install cloud-guest-utils

# Expand your partition using growpart. (Yes there is a space between /dev/sda and the partition number. Odd syntax right?)
$ growpart /dev/sda 2

# And finally resize your file system to the newly acclaimed space
$ resize2fs /dev/sda2

/// Resize ext4 partition

!Make backups before messing with any data!

# Example given:
+--------------------------------+--------+
|       /dev/sda1 (ext4)         |  Free  |
+--------------------------------+--------+

# Unmount partition
$ umount /dev/sda1

# Use fdisk to remove the partition. Then add the same partition but larger at the same location.
$ fdisk /dev/sda

# Resize the fs on your partition
$ resize2fs /dev/sda1

# Check partition state
$ e2fsck /dev/sda1

# Remount your partition
$ mount /dev/sda1 /whatever

/// arp-scan to discover devices

$ arp-scan -I enp0s25 -l

/// SSH port forwarding

# Example hosts used:
# A: 192.168.1.10 (local machine)
# B: 10.10.10.10 (target server)
# C: 172.16.1.10 (jump host)
# D: 172.16.1.250 (second jump host)

# Local port forwarding: Connect local port 2222 to port 22 on machine B via jump host C
[user@A]$ ssh -L localhost:2222:10.10.10.10:22 user@172.16.1.10

# Use the forwarded connection with X11 forwarding
[user@A]$ ssh -p 2222 -X localhost

# Now X11 programs on B will display on A
[user@B]$ firefox

# Port forwarding through multiple jump hosts (C and D) to reach B
[user@A]$ ssh -L localhost:1236:10.10.10.10:8443 -J user@172.16.1.10,user@172.16.1.250 user@10.10.10.10

/// Debian: interface configuration

$ vim /etc/network/interfaces
auto "interface_name"
iface "interface_name" inet static
address XXX.XXX.XXX.XXX
netmask XXX.XXX.XXX.XXX
gateway XXX.XXX.XXX.XXX
dns-nameservers XXX.XXX.XXX.XXX XXX.XXX.XXX.XXX

$ systemctl restart networking

/// Debian: disable ipv6

# check if it is enabled
$ sysctl net.ipv6.conf.all.disable_ipv6

# disable
echo "net.ipv6.conf.all.disable_ipv6 = 1" >> /etc/sysctl.conf
echo "net.ipv6.conf.default.disable_ipv6 = 1" >> /etc/sysctl.conf
echo "net.ipv6.conf.lo.disable_ipv6 = 1" >> /etc/sysctl.conf

# apply changes
sysctl -p

/// Viewing CSV

$ column -t -s, file.csv | less -S

/// Date calculations with the date command

$ date -d "${date} - 31 days"
$ date -d "+1 days"
$ date -d "Sun Sep 11 07:59:16 IST 2012+10 days"
$ prog_end_date=$(date '+%C%y%m%d' -d "$end_date+10 days")

/// Bash sessions

found on this blog: https://www.thanassis.space/bashheimer.html

# Find current users bash handler
$ readlink /proc/$$/fd/1
/dev/pts/13

# Test with:
$ echo "Test" >> /dev/pts/13

# Get the pid of the current shell:
$ echo $$

# Start a gdb session on this pid:
$ sudo gdb --pid 1234

# Call the write history. This maybe has to be casted as an int:
$ call write_history("/tmp/foo")
$ call (int) write_history("/tmp/foo")

# Find your result:
$ less /tmp/foo