Here are the must used Linux/Ubuntu commands :

File System Management

pwd: Print the name of the current working directory.
date: print or set the system date and time.
ls: List information about the FILEs (the current directory by default).
cd: Change the shell working directory.
touch: Update the access and modification times of each FILE to the current time.
cat: Concatenate FILE(s) to standard output.
cp: Copy SOURCE to DEST, or multiple SOURCE(s) to DIRECTORY.
mv: Move SOURCE to DEST, or multiple SOURCE(s) to DIRECTORY.
rm: Remove (unlink) the FILE(s).
mkdir: Create the DIRECTORY(ies), if they do not already exist.
rmdir: Remove the DIRECTORY(ies), if they are empty.
echo: Print each text after echo.
vi:Usefull and simple editor
chmod: Change the mode of each FILE to MODE.
chown: Change the owner and/or group of each FILE to OWNER and/or GROUP.
find: Utility for comprehensive file and directory.
grep: Search for matching patterns in a file.
dd: Copy a file, converting and formatting according to the operands.
tail: Output the last part of files
head: Output the first part of files
more: Filter for paging through text one screenful at a time.
less: Like more with many other feture.

Process Management

ps: Report a snapshot of the current processes.
kill: Send a signal to a job.
top: Display Linux processes
htop: Display Linux processes with many fetures.
free: Display amount of free and used memory in the system

Device Management

df: Show information about the file system on which each FILE resides,
du: Summarize disk usage of the set of FILEs, recursively for directories.
lsmod: Show the status of modules in the Linux Kernel.
lsblk: List block devices like hard disk and partions.
fdisk: manipulate disk partition table
mount: Mount a filesystem.
umount: Unmount filesystems.

Package Management

apt: Provides a high-level commandline interface for the package management system.
apt-get update: Update package lists.
apt-get install: Install new packages.
apt-get remove: Remove packages.

Archive Management

tar: Saves many files together into a single tape or disk archive.
xz: Compress or decompress .xz and .lzma files
zip: Saves many files together into a single tape or disk archive in zip format.
unzip: Un zip archive file .

Networking

ping: Send ICMP ECHO_REQUEST to network hosts
netstat: Symbolically displays the contents of various network-related data structures.
ifconfig: Used to configure / show the kernel-resident network interfaces.
ip: Show and manipulate routing, network devices, interfaces and tunnels.
iptables: Administration tool for IPv4/IPv6 packet filtering and NAT.
wget: A non-interactive network retriever.
curl: curl is a tool for transferring data from or to a server.
ssh: Program for logging into a remote machine and for executing commands on a remote machine.
scp: Copies files between hosts on a network.
ssh-keygen: OpenSSH authentication key utility.
ssh-copy-id: Script that uses ssh to log into a remote machine.
rsync: Fast and extraordinarily versatile file copying tool.

System Information

man: An interface to the on-line reference manuals
uname: Print certain system information. With no OPTION, same as -s.
whoami: Print the user name associated with the current effective user ID.
sudo: execute a command as another user
hostnamectl: Query and change the system hostname and related settings.
history: Show history of command input by user

System Management

systemctl: Control the systemd system and service manager
journalctl: Query the systemd journal
crontab: Maintain crontab files for individual users
usermod: Modify a user account.
adduser: Add user to the system.
addgroup: Add group to the system.
passwd: Change user password.
reboot: halt, poweroff, reboot may be used to halt, power-off, or reboot the machine.

Other Management

awk: Pattern scanning and processing language
ln: Make links between files
tee: Read from standard input and write to standard output and files
cut: Remove sections from each line of files
tr: Translate, squeeze, and/or delete characters from standard input, writing to standard output.
sort: Write sorted concatenation of all FILE(s) to standard output.
uniq: Filter adjacent matching lines from INPUT (or standard input), writing to OUTPUT
git: Tool for manage versioning.

These commands are for interacting with a Linux system and by using them you can do many daily tasks.
Note that some of these commands require package installation (such as the zip command).

Good luck