Here lies a list of useful commands I've come across. I'm sure I'll need at least some of them again in the future. This is where they'll stay for now.

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sudo hdparm -Tt /dev/sda
Check hard disk performance.

df -h
Display disk space usage for all mounted drives.

uname -a
Print out Linux kernel version and architecture as well as other useful system information.

cat /proc/sys/vm/swappiness
Print systems swappiness value.

xset dpms force off
Turn off the monitor. Any keypress or mouse action turns it on again.

Find out the code of a button on the keyboard. It also prints out all X events.

netstat -nr
Print the network gateway.

cat /proc/cpuinfo
Print processor information.

unzip -t <>
Test and display zip contents without extracting any files.

tar czf <backup.tar.gz> <path_to_archive>
Create tar.gz backup archive.

tar -xzf <backup.tar.gz>
Extract a tar.gz backup archive.

sudo -i
Log in as the root user.

du -hs <path>
Print the size of a directory. Leave out <path> to use the current directory.

crontab -e
Edit tasks run by cron (task scheduling).

sshd -v
See the version number of the ssh server.

tar -cj <path> | openssl des3 -salt > <encrypted_file>
Encrypt a file or directory with bzip2 compression.

cat <encrypted_file> | openssl des3 -d -salt | tar -xvj
Decrypt a encrypted file. It will be saved in the current directory.

which <executable>
Print the path to the installed executable file.

/sbin/mdadm --detail /dev/md0
Print the current RAID status.

tar -ztvf <backup.tar.gz>
List the contents of tar.gz archive without extracting.

watch -t 0.5 <command>
Run and print <command> every 0.5 seconds.

sudo usermod -s /bin/false <user>
Deny <user> shell access by setting their shell to false.

exec bash
Reload the current bash process to load any configuration changes.

cd -
Go back to previous directory.

sudo dmidecode -t 2
Display motherboard info.

sudo dpkg -i *.deb
Install all .deb in the current directory.

scp -r <source_path> <user>@<ip>:<remote_path>
Push file/folder to a remote server.

sudo dpkg-reconfigure tzdata
Change the timezone on a deb based system.

ssh-keygen -H -F <hostname>
Search for <hostname> in the known_hosts file.

tar -czf <backup.tar.gz> --exclude=<PATTERN1> --exclude=<PATTERN2> <path>
Create a tar.gz backup, excluding <PATTERN1> and <PATTERN2>.

find . -name <pattern> -print | xargs sed -i 's/<foo>/<bar>/g'
Find <foo> and replace it with <bar> across multiple files that match the <pattern>. e.g. *.txt

lsof -n -p <pid>
See the files that process <pid> has open.

mysql -u root -p -h localhost database_name < backup.sql
Import backup.sql into database as the mysql root user.

for f in *.wav; do ffmpeg -i "$f" -ab 320k "${f%.wav}.mp3"; done
Encode all .wav files in the current directory to 320Kbps mp3s.

sudo alsactl store
Save the current driver state for the selected soundcard to the configuration file.

lsof +c 0 /dev/snd/pcm* /dev/dsp*
See which processes have taken hold of your sound devices.

Print the current time and date.

dd if=/dev/device_name of=image.img bs=4M
Make an exact copy of a drive, such as a hard drive or a SD card from a Raspberry Pi.

sudo !!
Run the previous command with sudo.

mogrify -format png *.bmp
Convert images from one format to another preserving file names. Requires imagemagick.

for file in *.wav; do lame -m m --preset standard "$file" "${file%.wav}.mp3"; done
Convert mono wav files, in the current directory, to mono mp3's at decent quality. and with a small file size.

mongoimport -d <database> -c <collection> --type csv --file <data.csv> --headerline
Import a csv file into mongodb.

ssh-copy-id <user>@<ip>
Copy a ssh public key to a server.

htpasswd <password_file> <username>
Append user to <password_file> for apache2.

mogrify -resize <width>x<height> *.jpg
Resize all jpg images in folder. They will be overwritten.

for f in *.flac; do ffmpeg -i "$f" -q:a 2 "${f%.flac}.mp3"; done
Convert all flac files in the current directory to mp3, with a variable bit rate of 170-210 kbps.

ln -s <source_path> <link_path>
Create a symbolic link to a file or folder.

mongodump --db <database> --collection <collection>
Export a mongodb database in a importable mongodb format.

shntool split -f album.cue album.wav
Split an album, that is contained in one wav file, into separate tracks that are described in a cue file. Requires shntool.

for f in *.png; do convert "$f" -resize <width>x<height> "${f%.png}.jpg"; done
Convert and resize all png images in the current directory to jpg. Requires imagemagick.

find . -mtime 0
Recursively find files that have been modified more recently than 24 hours ago.

dig +short
Print the external/web/global IP address of the current machine.

rubberband -T 50:100 in.wav out.wav
Speed up a wav file from 50bpm to 100bpm.

for f in *.wav; do rubberband -T 85:90 "$f" "${f%.wav}_stretched.wav"; done
Stretch all wav files in the current directory from 85bpm to 90bpm.

for file in *.wav; do oggenc -q 6 "$file"; done
Convert all wav files in the current directory to Ogg Vorbis.

mongorestore -h --collection <collection> --db <database> <collection.bson>
Restore/import a mongodb collection from a mongodump.

sudo nmap -sP <broadcast_address>/<shorthand_subnet>
Show all nodes on the network.

sudo nginx -c /etc/nginx/nginx.conf -t
Check and test current nginx configuration.

ssh -ND 9999 <user>@<host>
Set up a SSH tunnel to browse the web using <host> as a proxy server.

sudo blkid
Display the UUID and filesystem for all block devices (drives) attached to the machine.

find <path> -iname "<pattern>"
Find all the files in the path (and sub directories, recursively), whose file names match the pattern.

find <path> -iname "<pattern>" | rename 's/<find>/<replace>/'
Rename multiple files recursively, starting at <path> . If a files name contains <find>, replace it with <replace>.

sudo kpartx -a <disk.img>
Mount the partitions of a disk image.

sudo kill -USR1 $(pgrep ^dd)
See progress of a running dd command.

sudo gpasswd -a user group
Add a user to a group.