pwd [print working directory]
date [sho current date and time]
cal [calendar]
cat /proc/cpuinfo [cpu info]
cat /proc/cpuinfo | grep processor | wc -l [No. of cpus]
cat /proc/cpuinfo | grep 'core id' [No. of cores]
cat /proc/meminfo [memory info]
free -h [memory usage info]
uptime [show up time]
uname -a [show system and kernel]
cat /etc/issue [or head -n1 /etc/issue] [show distro]
who [Show who is logged into the system]
whoami [show your username]
whereis ls (any cmd) [or which ls]
df -Th [show disk usage]
du [show directory space usage]
[du -Sh | sort -rh | head -n 15] or [du -s * | sort -n | cut -f2 | xargs -d "\n" du -sh]
last reboot [Show system reboot history]
hostname [Show system host name]
hostname -I [Display the IP addresses of the host]
lspci -tv [Display PCI devices]
lsusb -tv [Display USB devices]
lshw [Displays information about system’s hardware configuration]
lsblk [Displays block devices related information]
hdparm -i /dev/sda [Show info about disk sda, needs root access]
fdisk -l [Display disks partitions sizes and types, needs root access]
dmidecode [Displays hardware information from the BIOS]

Enviroment Variables:
env [show enviroment variables]
echo $HOME [home directory path]
echo $PATH [bin paths]
echo $SHELL [shell bin file path]
export NAME=value [Set $NAME to value]

Multiple Commands:
cmd1 | cmd2 [pipes]
cmd1 ; cmd2 [Run cmd1 then cmd2]
cmd1 && cmd2 [Run cmd2 if cmd1 is successful]
cmd1 || cmd2 [Run cmd2 if cmd1 is not successful]

File and Directory:
pwd [print working directory]
mkdir dir1 [create a directory called 'dir1']
rmdir dir [remove dir1, if it's empty]
ls [list files and folders]
cd dir1 [change to dir1]
cd ~ [change to user's home directory]
cd .. [change to parent directory]
rm -rf dir1 [remove dir1 recursivly and forcably]
touch file1 [create file1]
cat file1 [show file1 content]
less file1 [View and paginate file1]
file file1 [Get file1 type]
cp file1 file2 [Copy file1 to file2]
cp -r dir1 dir2 [Recursively Copies dir1 to dir2. dir2 is created if it does not exist]
mv file1 file2 [Move/Rename file1 to file2]
rm file1 [Delete file1]
head file1 [Show first 10 lines of file1, , -n: number of lines]
tail file1 [Show last 10 lines of file1, -n: number of lines]
stat file [file info]
stat -c "%a %n" * [list by permission numerical mode]
ln -s file1 link1 [create symbolic link link1 to file1]
sort file1 (sort the content of file1 alphabetically, r: reverse, n: numbers)
sort -o file1 file2 (sort the content of file1 alphabetically and write the result to a file2)
wc file1 [count number of words in file1, nbr of line, nbr of words, byte size, -l (lines), -w (words), -c (byte size), -m (number of characters)]

ls Parameters:
-a Show all (including hidden)
-R Recursive list
-r Reverse order
-t Sort by last modified
-S Sort by file size
-l Long listing format
-1 One file per line
-m Comma-​sep​arated output
-Q Quoted output
-h Human Readable
-s Size

grep Doc *txt [search inside text files for 'Doc'][-i: case insensitive, -r: recursive]
find /dir/ -name name* [Find files starting with name in dir]
find /dir -user name [Find files owned by name in dir]
find /dir -size +100M [Find files larger than 100MB in /dir]
locate file [Find file (quick search of system index)]

CTRL+0: save
CTRL+X: exit
CTRL+C: show line number
CTRL+SHIFT+-: goto line number #
CTRL+W: find
ALT+W: find next
CTRL+\: replace

4 read (r)
2 write (w)
1 execute (x)

chmod 775 file [Change mode of file to 775, -r: recursive]
chown user:group file
chmod u+rwx,g+rw,o+r file
chmod a+rwx file

[u]: user
[g]: group
[o]: other

id [Display the user and group ids of your current user]
last [Display the last users who have logged onto the system]
who [Show who is logged into the system]
w [Show who is logged in and what they are doing]
adduser user1 [create a user named 'user1']
passwd user1 [change user1 password]
deluser [delete user1]
userdel [delete user1]
addgroup group1 [create a new group named group1]
delgroup group1 [delete group1]
usermod -g group1 use1 [add user1 to group1]
usermod -g user1 user1_new_name [change account name]
usermod -aG group1 user1 [add groups to a user with-out loosing the ones he's already in]

ping host [ping host and output results]
whois domain [get whois information for domain]
dig domain [get DNS information for domain]
dig -x host [reverse lookup host]
wget file [download file]
wget -c file [continue a stopped download]
wget --user user --password pass [authorized]
hostname [Show system host name]
hostname -I [Display the IP addresses of the host]
hostname -i [Display the network address of the host name]
netstat -nutlp [Display listening tcp and udp ports and corresponding programs]
ip addr show [Displays IP addresses and all the network interfaces]
ip address add dev eth0 [Assigns IP address to interface eth0]

tar cf dir1.tar dir1 [create a tar named dir1.tar containing dir1]
tar xf dir1.tar [extract dir1 from dir1.tar]
tar czf dir1.tar.gz dir1 [create a tar with Gzip compression]
tar xzf dir1.tar.gz [extract a tar using Gzip]
tar cjf file.tar.bz2 [create a tar with Bzip2 compression]
tar xjf file.tar.bz2 [extract a tar using Bzip2]
gzip dir1 [compresses dir1 and renames it to dir1.gz]
gzip -d dir1.gz [decompresses dir1.gz back to dir1]
zip -r dir1

Processes & Monitoring:
ps [display your currently active processes]
top (or ps -ef) [display all running processes]
kill pid [kill process id pid]
kill -9 pid [violent kill]
killall proc1 [kill all processes named proc1]
program & [Start program ​ in the background]
bg [lists stopped or background jobs; resume a stopped job in the background]
fg [brings the most recent job to foreground]
fg n [brings job n to the foreground]
pstree -paul [Visualizing processes in tree model]
pgrep firefox [Find Firefox process ID]

Monitoring Tools:
ps [display your currently active processes]
top [display all running processes]
htop, glances
mpstat 1 [Display processor related statistics]
vmstat 1 [Display virtual memory statistics]
iostat 1 [Display I/O statistics]
tail -n 100 /var/log/syslog
free -h [memory usage info]
df -Th [show disk usage]
du [show directory space usage]
watch df -Th [Execute "df -Th", showing periodic updates, very useful]
tcpdump -i eth0 [Capture and display all packets on interface eth0]
tcpdump -i eth0 'port 80' [Monitor all traffic on port 80 ( HTTP )]
lsof [List all open files on the system]
lsof -u user1 [List files opened by user1]
pstree [Visualizing processes in tree model]
tload [graphic representation of system load average]
iftop [Displays Bandwidth Usage]
nload [Displays Network Usage]
IPTraf [An IP Network Monitor]

File Transfer:
scp file.txt user1@server1:/tmp [Secure copy file.txt to the /tmp folder on server1]
scp user1@server1:/var/www/*.html /tmp [Copy *.html files from server1 to the local /tmp folder]
scp -r server1:/var/www /tmp [Copy all files and directories recursively from server1 to the current system's /tmp folder]
rsync -a /home /backups/ [Synchronize /home to​ /backups/home]
rsync -avz /home user1@server1:/backups/ [Synchronize files/directories between the local and remote system with compression enabled]

screen [running commands in sessions (even when you logout from ssh)] ctrl+A followed by 'd' to exit: screen -r to restore session, screen -ls to show all sessions, screen -r id, to restore to a specific session.
history [display a history of your commands]
hault or poweroff [shutdown the computer]
reboot [reboot the computer]
clear [clear the terminal screen]
exit [exit terminal]