Users & groups

cat /etc/passwd – list system users
cat /etc/group – list system user groups
useradd user – create new user
passwd user – edit user password (Skip username to edit your own password)
usermod -g group user – changes user’s primary group
usermod -a -G group user – add existing user to secondary group

Archives

tar -zcf archive.tar.gz dir1/ dir2/subdir file.txt – create new archive with name archive.tar.gz, include dir1/, dir2/subdir, file.txt directories & files
tar -xf archive.tar.gz -C extract/to/dir – extract files from archive.tar.gz to extract/to/dir (Omit destination path & -C option to extract to same directory)

zip

zip -r archive.zip dir1/ dir/subdir file.txt – create new archive with name archive.zip, include dir1/, dir2/subdir, file.txt directories & files
unzip archive.zip -d extract/to/dir – extract files from archive.zip to extract/to/dir (Omit destination path & -d option to extract to same directory)

Software

dpkg --list | grep php – list installed packages, optionally filter by name with grep
apt-cache search kde – search for packages available to install
apt-get install firefox – install package
apt-get remove --purge skype – uninstall package and all of its configuration files (To retain configuration files, skip --purge option)
apt-get autoremove – remove all orphaned, not used packages (Packages, which were used as components for other packages, now all removed)

Hardware

lsblk – list all devices (Including CD-Roms, USB flashes etc.) and mount points (If mounted)
mount /dev/sdb /mnt – mounts /dev/sdb/ device to /mnt path (Path must exist)
umount /dev/sdb – unmounts device

Processes

ps aux | grep nginx – list all running processes, optionally filter by name (Process name, command will be filtered) with grep
kill 123 – kill process with PID 123 (PID can be retrieved with ps aux)
pkill nginx – kill process by name

Mysql

mysqldump --host=localhost --user=db_user --password=db_password db_name | gzip > db.sql.gz – creates dump of db_name database, logging in with provided credentials and gzips it to db.sql.gz file
zcat db.sql.gz | mysql --host=localhost --user=db_user --password=db_password db_name – imports gzipped database dump to db_name database