Essentials

General

Command	Description
echo "TEXT"	Output any text "TEXT" provided
whoami	Find out what user we're currently logged in as
man man	Shows info about man command and lists all the sections of the manual
strace objFile	Lists all the system calls used in the program
ltrace objFile	Lists all the library calls used in the program
gcc -o objFile source -v	Lists all the path of libraries and headers used in creation of objFile
du -sh folder	Folder size
df -h	Disk space usage
free -h	Memory usage
uname -a	Kernel and system info
uptime	System uptime
hostname	Hostname

Network commands

Command	Description
netstat -t	Lists all the active TCP connections showing domain names.
netstat -u	Lists all the active UDP connections showing domain names.
netstat -n	Lists all the active, showing IP and port numbers.<
nslookup domain	Shows the IP address related to the domain (E.g. IP of www.google.it)
dig @server name type	DNS lookup utility: server name or IP address of the name server to query; name name of the resource record that is to be looked up; type type of query is required (ANY, A, MX, SIG, etc.). If no type is specified, A is performed by default.
wc [file]	Prints in order newlines, words, and bytes (characters) counts for file. If file not specified or equal to -, counts from stdin.
route -n	Show numerical addresses instead of trying to determine symbolic hostnames in routing table.
arp -a	List all the MAC addresses stored after some ARP requests and replies made by our ethernet interfaces.
ifconfig	List network interfaces.

Search strings in files

grep [options] STRING file

Option	Description
-e pattern	Specify pattern
-i	Case insensitive
-v	Invert match (List all string without matches)
-c	Output count of matching lines only
-l	Output matching files only
-n	Precede each matching line with a line number
-b	A historical curiosity: precede each matching line with a block number
-h	Output matching lines without preceding them by file names
-s	Suppress error messages about nonexistent or unreadable files
-w	Force PATTERN to match only whole words
-x	Force PATTERN to match only whole lines
-f file	Take regexes from a file
-o	Output the matched parts of a matching line

Files and directory management

Option	Description
cd	Take regexes from a file
cp X PATH	Make a copy of file/folder X into the path PATH
mv X PATH	Move/rename the file/folder X in PATH
cat X	Concatenate file content of X
pwd	Print working directory
touch FILE	Create a file with name FILE
rm FILE	Delete a file with name FILE
mkdir DIR	Create a directory with name DIR
rmdir DIR	Delete an empty directory with name DIR
rm -r DIR	Delete recursively a directory with name DIR

Process management

Option	Description
CMD &	If specified at the end of the command, run it in background.
CMD1 && CMD2	Combine multiple commands together in one line of your terminal.
CMD1; CMD2	Execute several command in order (one after the other).

Option	Description
ps -f	See background processes
ps -e	More info about processes
ps -a	Select all processes except both session leaders and processes not associated with a terminal
ps -u USERLIST	Select by effective user ID (EUID) or name
fg PID	It continues the stopped job with id PID by running it in the foreground
bg	A job control command that resumes suspended jobs (CTRL+Z)
kill PID	It terminates the job with id PID
kill -9 PID	It forces kill of the job with id PID
top	It shows all the processes running in Linux working environment
htop	It shows all the processes running in Linux working environment

UNIX common Files

File path	Description
/etc/hosts	Local resolution table.
/etc/services	List all the applications with their port
/etc/protocols	Internet protocols.
/usr/include/x86\_64-linux-gnu/bits/socket.h	List all the protocol type possible for socket.
/usr/include/x86\_64-linux-gnu/sys/socket.h	Definition of struct sockaddr and specific ones.

Shell operators

Operator	Description
CMD1 | CMD2	Redirect command output to input of the next command
!!	Previous launched command
CMD2 < CMD1	Redirection of the command input (overwriting text)
CMD1 > CMD2	Redirection of the command output (overwriting text)
CMD1 >> CMD2	Redirection of the of the command output (appending text)

Download files

Command	Description
wget URL	Download to the current folder
wget -O <new_filename> URL	Download with a different name
wget -c URL	Resume download
curl -O URL	Download to the current folder
curl -o <new_filename> URL	Download with a different name
curl -C - -O URL	Resume download

SCP

scp user@server:/path/file.txt .

SFTP

sftp user@server
get file.txt

SMB

smbclient //SERVER_IP/share -U username

Then, inside the prompt:

ls
get file.txt
get bigfile.zip
exit

Transfer files

Transfer a file from the SOURCE path to the DEST path, using SSH channel:

scp SOURCE DEST -P <not_standard_port>

SOURCE and DEST have the following format:

<IP>:<path>

or

<domain>:<path>

Automation

• Open the crontab by running the following command:

crontab -e

A crontab is simply a special file with formatting that is recognised by the cron process to execute each line step-by-step.\ The crontab will execute each line as a cronjob.\

Crontab line fomat

Each line of the crontab has the following format:

MIN HOUR DOM MON DOW cmd

Keyword	Description
MIN	What minute to execute at
HOUR	What hour to execute at
DOM	What day of the month to execute at
MON	What month of the year to execute at
DOW	What day of the week to execute at
cmd	The actual command that will be executed

Special time interval statements

Statement	Description
@yearly	Run the task every year at 12:00am on the 1st of January
@annually	"
@daily	Run the cronjob every day at 12:00am
@midnight	"
@monthly	Run the job once a month, on the 1st, at 12:00am
@weekly	Run the job once a week at 12:00am on Sunday
@hourly	Run the job at the top of every hour
@reboot	Run the task once, at startup

Open crontab with different user

sudo crontab -u www-data -e

Time management

Current time

timedatectl

Available timezone

timedatectl list-timezones

Set new timezone

sudo timedatectl set-timezone America/New_York

Users management

• Create the user username

sudo useradd [options] <username>

Option	Description
-m	Creates the home directory for the user (by default /home/username) Copies default files from /etc/skel (like .bashrc, .profile) Sets correct ownership and permissions
-G <group>	Automatically add the user to the group <group> (the group must exists before the execution)
-m -d <new_home_path>	Creates the home directory for the user at <new_home_path>

If -m is not specified, the user <username> is created without home directory.

• Set a password for the user username

sudo passwd <username>

• Change user

sudo su <username>

• Delete a user

deluser <username>

• Get user id

id <username>

• Get user's Groups

groups <username>

Groups management

• Create a group

sudo groupadd [options] <groupname>

Option	Description
-g <gid>	Create the group with a specific group ID <gid>

• Add users to a group

sudo usermod -aG <groupname> <username>

• Delete a group

sudo groupdel groupname

Permissions

List file and directions

ls [options] [path]

Command	Description
ls -a	List all files, including hidden ones (filename starting with ".")
ls -color	Colored list [=always/never/auto]
ls -l	List with long format (show permissions)
ls -h	List with readable file size
ls -r	List in reverse order
ls -R	List recursively directory tree
ls -s	List file size
ls -S	Sort by file size
ls -t	Sort by time & date
ls -X	Sort by extension name

Permission	Directory meaning
r	list contents
w	create/delete files
x	enter/access directory

Change permissions

chmod [-R] [permission_options] <file_folder>

If -R specified with chmod on a folder, the operation is applied recursively.

Examples:

sudo chmod 752 test.txt # Owner -> rwx, Group -> rx, Others -> w
sudo chmod -R +x folder_test

chmod permission options

Targets (who)

Symbol	Meaning
u	user / owner
g	group
o	others
a	all (u+g+o)

Operators

Symbol	Meaning
+	add permission
-	remove permission
=	set exactly

Permissions

Symbol	Meaning
r	read
w	write
x	execute

Common combinations

Command	Effect
chmod u+x file	add execute for user
chmod g+x file	add execute for group
chmod o+x file	add execute for others
chmod a+x file or chmod +x file	add execute for all
chmod ug+x file	add execute for user & group
chmod go+x file	add execute for group & others
chmod u-x file	remove execute from user
chmod g-x file	remove execute from group
chmod o-x file	remove execute from others
chmod a-x file	remove execute from all

Set exact permissions (=)

Command	Result
chmod u=x file	user: execute only
chmod g=rx file	group: read + execute
chmod o= file	others: no permissions
chmod a=rx file	all: read + execute

Multiple permissions at once

Command	Effect
chmod u+rwx file	full access for user
chmod g+rx file	read & execute for group
chmod o+r file	read only for others

File/Folder ownership

chown [OPTIONS] USER[:GROUP] FILE

Common chown usages

Command	Effect
chown user file	change owner only
chown user:group file	change owner and group
chown :group file	change group only
chown user: file	change owner, keep group
chown user.group file	same as user:group

Useful options

Option	Meaning
-R	recursive
-v	verbose (show changes)
-c	report only when changes occur
--reference=<file_path>	copy owner/group from another file