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