Tech Tudor

Over the course of my Linux use, I've found these three commands to be very helpful and they are worth committing to memory.

DD, the low-level data dumping utility is a dangerous but also one of the most useful commands when used lucidly. For instance, burning an ISO image to USB stick is just one command away:

sudo dd if=ubuntu-18.04.1-desktop-amd64.iso of=/dev/sdb bs=1M Furthermore, if you want to "hard-wipe" your disk completely so that all data is permanently erased beyond recovery (for example, before handing your laptop in an exchange offer, or to a technician for repairs), all you need to remember is this one command:
sudo dd if=/dev/zero of=/dev/sda
This will wipe your disk entirely and there is no recovery possible, though some sysadmins are of the opinion that you should repeat the above at least three times to be absolutely sure!

DU, or the disk usage command is used for quickly estimating the drive space used in a folder or partition. For example, to display the space used by each file and folder in the current directory, you can simply run:

techtudor@ubuntu:/data/iso$ du -sh *
2.2G ubuntu-18.04.1-desktop-amd64.iso

Another common use case is that you want to sort all current folder contents in the order of their size:

techtudor@ubuntu:/data/java$ du -sh * | sort -h
512     apache-ant-1.9.4-bin.tar.bz2.sha512
44K     hamcrest-core-1.3.jar
112K    flyingsaucer-R8-users-guide.pdf
136K    sqlite-jdbc-3.7.2-javadoc.jar
772K    yuicompressor-2.4.8.jar
848K    libGDX
957K    log4j
15M     tomcat

DF is finally used for estimating disk space but for the entire disk rather than a directory or folder. df -l is all you need to show which partition is how much full.
techtudor@ubuntu:/data/java$ df -l
Filesystem     1K-blocks      Used Available Use% Mounted on
udev             1936212         0   1936212   0% /dev
tmpfs             393184      1380    391804   1% /run
/dev/sda2       51343840  12112488  36593528  25% /
tmpfs            1965900     25860   1940040   2% /dev/shm
tmpfs               5120         4      5116   1% /run/lock
tmpfs            1965900         0   1965900   0% /sys/fs/cgroup
/dev/sda3      383526908 133183184 250343724  35% /data
tmpfs             393180        20    393160   1% /run/user/1000

After mastering DD, DU & DF commands, and making a habit of using them in your daily errands, you'll rarely ever need to open your actual file-manager such as nautilus or thunar!