Precautions

timeshift --help
sudo timeshift --list

df -h                            # to see space
dpkg --list | grep linux-image   # to see the kernel images
sudo apt purge IMG1 IMG2 IMG3    # to purge several of the oldest images

timeshift --help
sudo timeshift --list

sudo dmesg -xT
sudo dmesg --level=emerg,alert,crit,err,warn -T
sudo journalctl --pager-end
sudo journalctl --since=today
sudo journalctl --since=2019-12-31
less /var/log/Xorg.0.log

idle=nomwait acpi_backlight=vendor rcu_nocbs=0-7
# https://www.kernel.org/doc/html/latest/admin-guide/kernel-parameters.html

sudo hdparm -i /dev/sda
sudo hdparm -I /dev/sda

# show areas:
sudo hdparm -N /dev/sda
sudo hdparm --dco-identify /dev/sda

# VERY dangerous:
sudo hdparm -N pSSSSSSSSSSS /dev/sdx  # remove HPA, set visible area/max sectors to SSSSSSSSSSS
sudo hdparm --dco-restore /dev/sdx    # revert DCO back to factory defaults; may brick drive
# Also, GParted has a feature to remove HPA ?  But I don't see it.

sudo ps -ex | grep oom

sudo journalctl | grep 'Out of memory'
# To see more, take the timestamp when that occurred, and do:
sudo journalctl | grep 'TIMESTAMP'

systemctl status systemd-oomd

# to turn it off, persistently across reboots:
sudo systemctl mask systemd-oomd.service

# Which processes have adjustments so less likely to kill:
export pids=""
export sep=""
for pid in `grep '-' /proc/*/oom_score_adj | cut -d / -f 3`
do
pids="$pids$sep$pid"
sep=","
done
ps -p $pids
grep '-' /proc/*/oom_score_adj

# Set application to be harder to kill:
for pid in `pidof keepassxc`
do
sudo echo '-200' >"/proc/$pid/oom_score_adj"
done
# Some processes seem to have hardcoded values,
# will give "permission denied".

grep 'ACTIVE_CONSOLES=' /etc/default/console-setup	# see "/dev/tty[1-6]" or similar
sudo sed -i "s/#NAutoVTs=/NAutoVTs=/" /etc/systemd/logind.conf
# then reboot

df -khT
df -khT -x squashfs -x tmpfs		# don't show snap devices

sudo du -h --summarize --exclude=/home --exclude=/sys --exclude=/proc --exclude=/run /
sudo du -h --summarize -c --exclude=/home --exclude=/sys --exclude=/proc --exclude=/run /*

sudo du -h --summarize -c /home/*
du -sh $HOME/* | sort -h
du -sh $HOME/.[0-9a-z]* | sort -h

sudo df -i

grep logrotate /etc/crontab /etc/cron.*/* /etc/anacrontab /var/spool/cron/crontabs/*
cat /etc/logrotate.conf
man logrotate

# As a quick fix:
sudo rm /var/log/*.gz

# List installed packages sorted by size:
dpkg-query -Wf '${Installed-Size}\t${Package}\n' | sort -rn | less

sudo bash -c "! pgrep -x flatpak && rm -r /var/tmp/flatpak-cache-*"

# show current kernel:
uname -r

# show all kernels except the one currently running:
sudo dpkg --list 'linux-image*'|awk '{ if ($1=="ii") print $2}'|grep -v `uname -r`

# For each image you want to remove, do:
sudo apt purge linux-image-NNNNNNNNNNNNN-generic

# These shouldn't be necessary:
sudo apt autoremove
sudo update-grub

# show current kernel:
uname -r

# show all kernels:
rpm -q kernel

# For each kernel you want to remove, do:
sudo dnf remove kernel-NNNNNNNNNNNNN

tmpfs  /tmp      tmpfs  noatime,nodiratime,nodev,nosuid,mode=1777,size=1G,defaults  0  0
tmpfs  /var/crash  tmpfs  noatime,nodiratime,nodev,nosuid,mode=1777,size=1G,defaults  0  0

# Download and extract the test package.
# Then:

sudo ./install-sh
sudo apt install php-cli php-xml pkg-config

phoronix-test-suite --help
# have to accept some license terms etc

# can run via cli or GUI
# GUI will give a CLI-type interface; type 'quit' to get out

# https://www.phoronix-test-suite.com/documentation/phoronix-test-suite.html

phoronix-test-suite list-available-suites
phoronix-test-suite list-available-tests | less

# I want: simple, few dependencies, run in a few minutes, test CPU RAM disk.

# Best to run tests after a fresh boot, no other apps running.
# Run each test 3 times and average them.

phoronix-test-suite benchmark pts/sysbench
# My 2011 Dell Inspiron N5010 with Kubuntu 20.10:
# (Intel Core i3 370M @ 2.39GHz, 8 GB RAM, 320 GB WD3200BEVT-7 with ext4/LUKS)
# gave CPU 2100, RAM 9.8 M
# My 2021 KDE Slimbook 15 with KDE Neon 5.20.5:
# (AMD Ryzen 7 4800H @ 2.9GHz, 16 GB RAM, 256 GB NVMe SSD with ext4)
# gave CPU 17600, RAM 13.6 M

phoronix-test-suite benchmark pts/leveldb
# My 2011 Dell Inspiron N5010 with Kubuntu 20.10:
# (Intel Core i3 370M @ 2.39GHz, 8 GB RAM, 320 GB WD3200BEVT-7 with ext4/LUKS)
# gave seq fill 12 MB/S, hot read 1.76 usec/op, random read 1.71 usec/op.
# My 2021 KDE Slimbook 15 with KDE Neon 5.20.5:
# (AMD Ryzen 7 4800H @ 2.9GHz, 16 GB RAM, 256 GB NVMe SSD with ext4)
# gave seq fill 23 MB/S, hot read 7 usec/op, random read 7 usec/op.

phoronix-test-suite benchmark pts/fio
# Choose sub-test 3 (sequential read), option 4 (Linux AIO), Buffered, Not Direct,
# 4 KB block size.
# My 2011 Dell Inspiron N5010 with Kubuntu 20.10:
# (Intel Core i3 370M @ 2.39GHz, 8 GB RAM, 320 GB WD3200BEVT-7 with ext4/LUKS)
# gave 48 MB/S.
# My 2021 KDE Slimbook 15 with KDE Neon 5.20.5:
# (AMD Ryzen 7 4800H @ 2.9GHz, 16 GB RAM, 256 GB NVMe SSD with ext4)
# gave 1132 MB/S.

default-sample-format = float32ne  
default-sample-rate = 44100  
alternate-sample-rate = 48000  
default-sample-channels = 2    
default-fragments = 2  
default-fragment-size-msec = 125  
resample-method = soxr-vhq      
nice-level = -11    
realtime-priority = 9  
rlimit-rtprio = 9

xdg-open https://errors.ubuntu.com/user/$(sudo cat /var/lib/whoopsie/whoopsie-id)

apt install PACKAGENAME-dbgsym
# or:
apt install PACKAGENAME-dbg

gdb NAMEOFAPP 2>&1 | tee $HOME/gdb-NAMEOFAPP.txt
(gdb) run ARG1 ARG2 ARG3
# get app to crash/fail
(gdb) thread apply all bt
(gdb) q

xinput --list
xinput --list DEVICEID
# output shows 7 buttons, 2nd is middle button
xinput set-button-map DEVICEID 1 0
# test it, then add it as a startup application

Precautions

Problems and troubleshooting

Magic key-sequences

Clean up space on disk

Performance

Run Performance Tests

Tweaking/fixing video

Tweaking/fixing mouse/trackpad/scrolling/video latency/tearing:

Tweaking/fixing audio

Reporting Bugs

Devices

Webcam

Keyboard

Battery / Power Management

Display

Mouse

Touchpad

Miscellaneous

Bootloader: