cd - Change directory.
$ cd /path/to/directory

ls - List directory contents.
$ ls -l /path/to/directory

pwd - Print working directory.
$ pwd

echo - Display a line of text.
$ echo "Hello World"

touch - Create a new empty file.
$ touch newfile.txt

mkdir - Create a new directory.
$ mkdir new_directory

rmdir - Remove an empty directory.
$ rmdir empty_directory

rm - Remove files or directories.
$ rm file.txt
$ rm -r directory

cp - Copy files and directories.
$ cp source.txt destination.txt
$ cp -r source_directory new_directory

mv - Move or rename files and directories.
$ mv oldname.txt newname.txt
$ mv file.txt /path/to/directory

cat - Concatenate files and print on the standard output.
$ cat file.txt

less - View file contents with pagination.
$ less file.txt

grep - Search text using patterns.
$ grep "search_pattern" file.txt

find - Search for files in a directory hierarchy.
$ find /path/to/search -name "filename"

chmod - Change file mode bits.
$ chmod +x script.sh

chown - Change file owner and group.
$ chown user:group file.txt

tar - Archive files.
$ tar cvf archive_name.tar /path/to/directory

curl - Transfer data from or to a server.
$ curl http://example.com

wget - Non-interactive network downloader.
$ wget http://example.com/file

history - Show command history.
$ history

alias - Create an alias for a command.
$ alias ll='ls -l'

exit - Exit the shell session.
$ exit

man - Display the manual page for a command.
$ man ls

env - Display environment variables.
$ env

export - Set environment variable.
$ export PATH="$PATH:/new/path"

unset - Remove environment variable or function.
$ unset VARIABLE_NAME

ps - Report a snapshot of current processes.
$ ps aux

kill - Send a signal to a process.
$ kill -9 PID

top or htop - Display and manage the top processes.
$ top

ssh - OpenSSH SSH client (remote login program).
$ ssh user@host

scp - Secure copy (remote file copy program).
$ scp file.txt user@host:/path/to/destination