file file.txt

cat -n file.txt
nl file.txt

tree -F -L 1 /

-F - show file type
-L - level (how nested should be), 1 - show only files under first directory level.

ln -s lab02/.Hulk hulk

This will create file hulk as a shortcut for the (original) file lab02/.Hulk

tar -cvf archive.tar decrypted_file.txt encrypted_file.enc dir2

c - create
v - verbose
f - file name of the archive

tar -uf documente.tar raport_nou.txt
tar -rf documente.tar raport_nou.txt

-u - update
-r - add (does not check if the file is newer)

tar -tf archive.tar

t - table (view archive content as a 'table')
f - file name (archive name)

tar -xvf archive.tar

x - extract
v - verbose
f - file

tar -czvf archive.tar.gz decrypted_file.txt encrypted_file.enc dir2

z - compressed gzip
c - create
v - verbose
f - file name of the archive

This cannot be done directly using tar tool, but we will use gunzip to extract the tar archive, add a new file to it and then gzip it again.

gunzip archive.tar.gz && tar -rf archive.tar newfile.txt && gzip archive.tar

tar -tzf archive.tar.gz

t - table (view archive content as a 'table')
z - compressed gzip
f - file name (archive name)

tar -xzvf archive.tar.gz

x - extract
v - verbose
f - file

zip archive.zip file1.txt file2.txt

zip -u archive.zip new_file

-u - update (or add if not already in the archive)

zip -sf archive.zip
zip --show-files archive.zip

unzip archive.zip -d dir

-d - optional destination directory, if missing the current directory will be used.

find ~ -name "*.txt"

sudo find / -iname "*ssh*conf"

find ~ -type f -size +100M

find /tmp -type f -name "*.tmp" -exec rm {} \;

{} - placeholder for each file, it will contain the full path name
\; - marks the end of the command

find . -type f -executable

find . -type d -empty

find /usr/include -name '*[0-9]*'

sudo apt install mlocate
sudo updatedb

locate "*ssh*conf"

locate -i "Example.txt"

locate example.txt | head -10

ps -ef

e - 'every' - we want to see every process
f - 'full' - get full details

ps aux

a - all users
u - user oriented format output (display user who owns the process, CPU, memory, etc.)
x - also includes the processes that are not attached to a terminal.

ps -u username

Change 2638 with the actual process pid.

ps -p 2638

ps -aux --sort=-%cpu

ps -aux --sort=-%mem

ps -e -ouser,uid,pid,%mem,%cpu,rss,cmd --sort=%mem

Just add a - in the –sort.

ps -e -ouser,uid,pid,%mem,%cpu,rss,cmd --sort=-%mem | less

Replace gnome with your keyword.

ps -ef | grep gnome

pstree
pstree -u
pstree -p

u - show users p - show pid

Multiple methods:

pgrep nginx

ps aux | grep nginx | grep -v grep

pidof nginx

sudo apt install nginx
sudo apt search nginx
sudo apt remove nginx
sudo apt purge nginx
sudo apt update
sudo apt upgrade

sudo ip link show

Display only the names and states (UP / DOWN) of the network interfaces on the system, using the format interface_name:state.

ip link show | awk '/^[0-9]+:/ {print $2 " " $9}' | sed 's/://g' | awk '{print $1 ":" ($2=="UP"?"UP":"DOWN")}'

Output:

lo:DOWN
ens33:UP
ens34:UP
docker0:DOWN
br1:DOWN
br2:DOWN

ip link set dev eth1 up

sudo ip address show

sudo ip address flush eth1

ping -c 4 10.125.50.50

c - number of packets to send (by default is infinite)

ip route show

Default gateway (192.168.112.2) is listed under the 'default via' section:

To extract just the default gateway ip:

ip route show | awk 'NR==1 {print $3}'

nmcli dev show | grep DNS

resolvectl status | grep 'Current DNS'

To print only IPs:

nmcli dev show | grep DNS | cut -d ':' -f2 | tr -d ' '

host elf.cs.pub.ro

Change the /etc/resolv.conf: nameserver 8.8.8.8 nameserver 8.8.4.4

sudo netstat -ulpn

u - udp ports only
l - listening state ports only
p - program name / PID that has opened the port
n - shows the actual IP address on which the system is listening for connections, instead of resolving the host name.

sudo ss -ulpn

sudo netstat -tlpn

t - TCP

sudo ss -tlpn

sudo netstat -tulpn

t - TCP
u - UDP

sudo ss -tulpn

nc google.com 80

wget elf.cs.pub.ro

Download in background with a custom file name:

wget -b -O google.html http://google.com

b - run in background
O - output file name.

curl -I -L http://ocw.cs.pub.ro

I (or –head) - send only HTTP HEAD request instead of HTTP GET
L - follow any redirectrion.

ssh user@192.168.1.8

ssh root@10.10.10.3 ip address show

scp /home/file.txt student@10.10.10.3:~/

scp student@10.10.10.3:/home/file.txt ~/

scp -r /home/dir student@10.10.10.3:~/

scp -r student@10.10.10.3:/home/dir ~/

ssh-keygen -t rsa -b 4096
ssh-copy-id root@10.10.10.3

What was generated:

• ~/.ssh/id_rsa –> private key
• ~/.ssh/id_rsa.pub –> public key

Just enclose the string in “”

ls support-globbing/"[HOME]"*

find . -name "*search*"

cat /proc/meminfo | grep "Mem"

cat /proc/cpuinfo

cat workspace/C/searching/binary_search.c | grep search

grep search workspace/C/searching/binary_search.c

grep 'error' log.txt

grep -i 'string_to_search' file.txt

grep -c 'cache' /proc/cpuinfo

grep -o 'searched_string' file.txt | wc -l

o - print each occurrence on a separate line
l (from wc) - count lines

grep -r 'cache' /proc 2>/dev/null

2>/dev/null - don't show errors

grep -nri search workspace/C/ | less

n - show line number r - recursive i - case insensitive

grep -nri -w "search" workspace/C/ | less

Show all lines that does not contain error string:

grep -v 'error' log.txt

Search for lines that starts with error string:

grep '^error' log.txt

Displays each line that contains error, followed by 3 lines:

grep -A 3 'error' log.txt

Displays two lines before each line that contains error:

grep -B 2 'error' log.txt

Displays two lines before and after each line that contains error:

grep -C 2 'error' log.txt

grep -E -i "error(s)?|warning(s)?" example.txt

E - extended regular expression
i - case insensitive
error(s) - search for strings that contains error or errors
| - or
warning(s) - search for string that contains warning or warnings
? - makes that previous element (s) to be optional.

grep -E -i "\berror(s)?|warning(s)?\b" example.txt

\b - the match will be made at the start and at the end of a word.

Instead of running xdg-open https://google.com, we want to use go https://google.com.
For this we need to define an alias in ~/.bash_aliases (this is read from ~/.bashrc).
So we need to append line: alias go='xdg-open'

grep -qxF 'alias go='\''xdg-open'\''' ~/.bash_aliases || echo 'alias go='\''xdg-open'\''' >> ~/.bash_aliases

|| - the command after this OR operator will be executed if the command before it fails.
q - run in silent mode, without printing anything
x - search on the ENTIRE line
F - disable regular expression, the string is a simple text.
>> - append to the end.

Next, reload the ~/.bashrc file:

source ~/.bashrc

mkdir demo; cd demo; touch Hello; ls

mkdir operators/demo && cd operators/demo

ls -d operators || mkdir operators

find . -maxdepth 1 -type f -exec ls -l {} \;

find . -maxdepth 1 -type f -print0 | xargs -0 ls -l

-print0 - finish each string found with a null character instead of newline. This helps in manipulating files that contains spaces or newlines.
0 - treat received input as a list of args separated by the null character - used to correctly processed the output of print0.

find -L /lib -type f -not -name 'lib*.*'

L - follow symlinks (this is necessary because lib is a symlink for usr/lib)
not - negate the name

ls *.c | xargs -I {} -p mv {} ..

I {} - placeholder / replacer for each character
p - requires user confirmation

Safer version (handles better the files with spaces):

find . -maxdepth 1 -type f -name "*.c" -exec mv {} .. \;

Redirect both in firefox-ignore file.

firefox &> firefox-ignore

Another way:

firefox > firefox-ignore 2>&1

To redirect input in a file and output to another file:

firefox > firefox-output.txt 2> firefox-error.txt

OR

firefox 1> firefox-output.txt 2> firefox-error.txt

> is equivalent to 1>

dd if=/dev/zero of=null-100mb count=100 bs=1M

if=/dev/zero - input file will get data from /dev/zero
of=null-100mb - name of the output file
count=100 - number of blocks that will be copied
bs=1M - size of each block

dd if=/dev/urandom of=rand-100mb count=100 bs=1M

Search for lines that starts with Hello:

grep '^Hello' message.txt

Search for lines that ends with ending:

grep 'ending$' message.txt

Search any lines that contains e followed by any character and then r:

grep 'e.r' message.txt

Search lines that contains sc followed by h or y and the o:

grep 'sc[hy]o' wordlist.txt

Search lines that contains sc followed any character except h or y and the o:

grep 'sc[^hy]o' wordlist.txt

Lines with the character o at least once:

grep -E 'o+' wordlist.txt

This means o with zero or more apparitions. So it will match everything:

grep -E 'o*' wordlist.txt

Search for one or another

grep 'succes|error' mesaje.txt

Students whose name starts with G:

grep '^G' students.txt

grep '[ -]F[a-z]\+[^\.]' students.txt

[ -]F - space or - followed by F
[a-z]\+ - any a-z characters at least once. In standard grep (not extended) + should be prefixed with \
[^\.] - following character is not .

ls ?.txt

? - any unique character

ls [abc]*.txt

[abc] - characters a, b or c * - any number of characters (including 0) .txt - ends with txt

grep -E ' [A-Z]\.? ' students.txt

\.? - optional dot

cat students.txt | tr " " "," | tr "\t" ","

OR

cat students.txt | tr " \t" ","

OR (much more cleaner)

tr " \t" "," < students.txt

echo "hello world" | tr 'a-z' 'A-Z'

echo "hello world" | tr -d 'a-e'

echo "helloooo world" | tr -s 'o'

echo "hello world" | sed 's/world/universe/'

echo "apa de apa" | sed 's/apa/apa minerala/'

s - init substitution command

echo "apa de apa" | sed 's/apa/apa minerala/g'

g - substitution will be applied globally (for all occurrences on that line)

echo -e "hello\nworld" | sed '/world/d'

If line1 is found, it will insert line1.5 after it.

echo -e "line1\nline2" | sed '/line1/a\line1.5'

Replaces all occurrences of “old” with “new” in file.txt directly, modifying the file in place.
Use with caution and ideally with a backup.

sed -i 's/old/new/g' file.txt

i - in place

cut -d ' ' -f1,2 students.txt

-d ' ' - delimiter is space
-f1,2 - column numbers to extract (starts with index 1)

cut -b 1,2,3 students.txt

cut -b 1-5 students.txt

cut -c 2,5,7 state.txt

cut -c 1-7 students.txt

awk -F: '{print $1}' /etc/passwd

F - field separator

Another way to set delimiter is in the BEGIN block.

awk 'BEGIN{FS="[ \t]+"} {print $1}' students.txt

BEGIN - special block used to adjust the initial settings before executing awk.
FS - field separator.
[ \t]+ - one or more spaces or tabs.
$1 - first column

w | awk 'FNR>2 {print $1}'

FNR - file number record (internal awk variable holding current line number).

history | awk '{$1=""; print $0}'

{$1=""} - sets the first column (represented by $1) to an empty string, effectively removing it from the output. \\ 
print $0 - prints the entire line to the output.

history | awk '{$1=""; $2=""; print $0}'

awk 'BEGIN {print "The File Contents:"} {print $0}' students.txt

awk 'BEGIN {print "The File Contents:"} {print $0} END {print "This is the end my friends."}' students.txt

awk '{print $1}' students.txt

$1 - first column

awk '{print $NF}' students.txt

NF - number of fields (last column number)

echo "Hello Tom" | awk '{$2="Adam"; print $0}'

$0 - entire line

awk '$5 > 5 {print $1, $2, $5}' students.txt

awk '{sum += $5} END {print "Average of final grades:", sum/NR}' students.txt

sum - custom variable to count the sum (it is not predefined).
$5 - fifth column.
END - block which will be executed after all lines from the file were processed.
NR - internal awk variable which stores the total number of lines.

Print - between the first and second column:

awk '{print $1, "-", $2}' students.txt

awk '{ if ($5>=5) { nr=$5*10} else { nr=$5 } print nr}' students.txt

cat /etc/passwd | awk 'NR==5 || NR == 7 {print $0}'

seq 1 5

seq 1 2 10

seq 0.5 0.5 2.5

seq -f "%.2f" 1 3

f - format

seq -w 1 10

w - equal width for all generated numbers

echo -e "5\n3\n8\n1" | sort -n

e - interpret escape sequences n - numbers (sort as numbers)

echo -e "5\n3\n8\n1" | sort -nr

r - reverse (descending) order

Sort numerically by second column.

echo -e "a 2\nc 1\nb 3" | sort -n -k2

k2 - sort by second column

Sort lexicographically by first column.
(k1 can be omitted here, sort will automatically sort lexicographically by the first column.)

echo -e "a 2\nc 1\nb 3" | sort -k1

cat multiple.txt | sort -k1,1 -k2,2gr

k1,1 - sort entire first column
k2,2 - sort entire second column
g - general numeric sort (sort better different kind of numbers for ex 2, 2.5, 3.55)
r - reverse

cat multiple.txt | sort -k1,1 -u

u - unique

Default separator is tab (\t). We want to change it to :.

echo -e "a:2\nb:1\nc:3" | sort -t: -k2

t - change separator

echo -e "a 2\nc 1\nb 3" | sort -c

c - check

sort file.txt | uniq

To match the lines case insensitive:

sort file.txt | uniq -i

sort file.txt | uniq -c

c - count

sort file.txt | uniq -d

d - duplicated

sort file.txt | uniq -u

u - unique

history | awk '{$1=""; print $0}' | sort | uniq -c | sort -nr -k1 | head -10 | awk '{$1=""; print $0}'

for i in {1..10}; do sleep 3000 & done; pkill sleep

Using a one-liner, create 15 files with names ranging from final-exam-00.txt to final-exam-15.txt.

for i in {0..15}; do touch final-exam-$(printf "%02d" $i).txt; done

arg.sh

#!/bin/bash
 
echo "All args"
 
echo "Using \$*:"
for arg in "$*"; do
    echo "$arg"
done
 
echo "Using \$@:"
for arg in "$@"; do
    echo "$arg"
done
 
echo "Using \$1, \$2, ..."
echo "Arg 1: $1", "Arg 2: $2"
 
echo "Script name is $0"
 
echo "Number of args received is $#"
 
echo "Printing all args using \$# and a for loop"
 
nr_args=$#
 
for (( i=1; i<=$nr_args; i++ ))
do
	eval arg=\${$i}
	echo "Arg $i is: $arg"
done

Extract from position 7 with a length of 5 characters:

Execute a command based on the condition of another command:

if [[ $(grep -c "searched_word" fisier.txt) -gt 0 ]]; then
    echo "Word was found in the file."
else
    echo "Word was not found, sorry."
fi

Iterate through files:

for file in $(ls *.txt); do
    echo "Processing txt file: $file"
done

Nesting commands (find kernel version):

while-read.sh

#!/bin/bash
 
IFS=$'\t'
while read name group final_grade test_grade practical_grade; do
    echo "$name --> $group --> $final_grade --> $test_grade --> $practical_grade"
done < students.txt

IFS - defines the separator, here tab (\t).

while-read-cond.sh

#!/bin/bash
 
IFS=$'\t'
while read name group final_grade test_grade practical_grade; do
	if test "$final_grade" -gt 5; then
        	echo "$name --> $group --> Good job, final grade is $final_grade --> $test_grade --> $practical_grade"
    	fi
	echo "$name --> $group --> $final_grade --> $test_grade --> $practical_grade"
done < students.txt

manip.sh

#!/bin/bash
 
for file in "$1"/*
do
    if test -f "$file"; then
        stat -c "%a %F %n" "$file"
        cp "$file" "$file.bkp"
    fi
done

%a - file permission in numeric form.
%F - file type.
%n - complete file name.

nothing.sh

#!/bin/bash
 
rm -- "$0"

– - end of command line options.
“$0” - the name of the script.

dumb-sum.sh

#!/bin/bash
 
nr_args=$#
 
if [ "$nr_args" -eq 0 ]; then
    echo "Error: No arguments provided. Usage: $0 <arg1> <arg2> ..."
    exit 1
fi
 
sum=0
 
for (( i=1; i<=$nr_args; i++ ))
do
	eval arg=\${$i}
	 sum=$((sum + arg))
done
 
echo "Sum is: $sum"

$((sum + arg)) is to evaluate arithmetic expression.

Script to download the wbritish package (contains dictionaries) and which should validate the words from in.txt file located at in.txt with the words from the dictionary.
Example:

./lookup.sh in.txt
apple in dictionary
noapple not in dictionary

Lookup script with the in.txt file downloaded locally (using wget http://elf.cs.pub.ro/uso/res/final/146/in.txt)

lookup.sh

#!/bin/bash
 
if [ "$#" -ne 1 ]; then
    echo "Usage: $0 <input_file>"
    exit 1
fi
 
input_file=$1
 
if ! dpkg -l | grep -qw wbritish; then
    echo "Installing wbritish package."
    sudo apt-get update && sudo apt-get install -y wbritish
fi
 
if [ ! -f "$input_file" ]; then
    echo "File does not exist: $input_file"
    exit 2
fi
 
while IFS= read -r word; do
    if look "$word" /usr/share/dict/british-english > /dev/null; then
        echo "$word in dictionary"
    else
        echo "$word not in dictionary"
    fi
done < "$input_file"

Output:

./lookup.sh in.txt
mine in dictionary
yours not in dictionary
apples in dictionary
notSoApples not in dictionary

Lookup script that will download the in.txt file from the remote location:

lookup.sh

#!/bin/bash
 
if [ "$#" -ne 1 ]; then
    echo "usage: $0 <input_file>"
    exit 1
fi
 
input_file_url=$1
input_file="in.txt"
 
wget -O "$input_file" "$input_file_url" || { echo "Eroare downloading input file."; exit 2; }
 
if ! dpkg -l | grep -qw wbritish; then
    echo "wbritissh not installed, installing"
    sudo apt-get update && sudo apt-get install -y wbritish
fi
 
if [ ! -f "$input_file" ]; then
    echo "input file does not exist: $input_file"
    exit 2
fi
 
#read each word from input file and check if it is present in dictionary
 
while IFS= read -r word; do
    if look "$word" /usr/share/dict/british-english > /dev/null; then
        echo "$word in dictionary"
    else
        echo "$word not in dictionary"
    fi
done < "$input_file"

Create a script named disk-usage.sh that reads a number from stdin and displays the partition that has a disk usage percentage greater than that number.
If the input received is not a number, use a default threshold value chosen by you.

Usage example:

$ ./disk-usage.sh
Threshold = 12
WARNING: Partition "/dev/nvme0n1p6" used 13%
WARNING: Partition "/dev/nvme0n1p1" used 27%

$ ./disk-usage.sh
Threshold = ceva
WARNING: Partition "/dev/nvme0n1p6" used 30%
WARNING: Partition "/dev/nvme0n1p1" used 31%

disk-usage.sh

#!/bin/bash
 
# User should input the threshold
echo "Enter the threshold (percent):"
read threshold
 
# Check if the threshold is a number
if ! [[ "$threshold" =~ ^[0-9]+$ ]]; then
    echo "Input is not a number, using default threshold: 20."
    threshold=20
else
    echo "Threshold = $threshold"
fi
 
df -h | awk -v threshold="$threshold" 'NR>1 {gsub(/%/,"",$5); if($5 > threshold) print "WARNING: Partition \""$1"\" used "$5"%"}'

Create a script that takes two arguments (strings). The script hash_it.sh concatenates the two arguments and displays their SHA256 hash sum.

hash_it.sh

#!/bin/bash
 
nr_args=$#
 
if [ "$nr_args" -ne 2 ]; then
    echo "Error: two args should be present. Usage: $0 <arg1> <arg2>"
    exit 1
fi
 
result="$1$2"
echo -n $result | openssl dgst -sha256

Suppose the input file is called parole.txt.

hash_it_all.sh

#!/bin/bash
 
IFS=$'\n'
while read password; do
    echo -n $password | openssl dgst -sha256 | cut -d '=' -f2 | awk '{print $1}'
done < parole.txt

top.sh

#!/bin/bash
while true; do
    clear
    #echo "Update on: $(date)"
    echo "Process with highest CPU usage:"
    ps -eo %cpu,pid,command --sort=-%cpu | head -n 2
    sleep 1
done

git config --global user.name "Joe Black"
git config --global user.email "joe.black@github.com"
git config --global init.defaultBranch main

git init

git remote add origin https://github.com/odefta/array-sorting-algorithms.git

The sync between the local repo and the remote (github) repo is made with pull (get) and push (put) commands.

git remote -v

git fetch origin
git pull origin main

git branch --show-current

git branch

git status

git log

Show only the last 4 commits:

git log -n 4

Show only the last 4 commits - more compact form:

git log -n 4 --oneline

Create a file, and run git status:

echo "Hello World" > hello.txt
git status

We have an untracked file we can add it (in staging area - which is a pre-commit step):

git add hello.txt

Use . to add all files (from current dir - which includes sub-dirs).

Now we can commit the hello.txt to the local repo:

git commit -m "Add hello.txt with hello world"

m - add a comment

After modifying a file (hello.txt) we can issue again a commit.

git push origin main

git branch add-gitignore

Check if the new branch was added:

git branch

git checkout add-gitignore

Check

git branch

First, let's commit something to our current branch add-gitignore and sync with the remote:

echo "*.o" > .gitignore
git add .gitignore
git commit -m "Add .gitignore file"
git push origin add-gitignore

Now we want to merge our branch main with the add-gitignore branch (to get the .gitignore file).

First, switch to the main branch and get the changes from the remote:

git checkout main
git pull origin main

Now we are ready to merge our current branch main with add-gitignore:

git merge add-gitignore

Push the changes to the remote:

git push origin main

We want to delete our secondary branch add-gitignore.

First, we need to be on another branch (main).

git checkout main

Remove local branch.

git branch -d add-gitignore

Remove remote branch.

git push origin --delete add-gitignore

Remote branch is test.

git fetch origin
git branch -r (verify available remote branches)
git checkout -t origin/test

whoami

id

sudo id root

who

To display only the user name:

who | cut -d ' ' -f1

To display additional info:

w

To filter only the logged-in users:

w | awk 'FNR>2 {print $1}'

who | awk '$2 ~ /^tty/ {print $1}'

~ - equals for regular expressions
/ / - delimitates a regular expression
^tty - starts with tty.

This will print all info.

id syslog

This will print the first column.

id syslog | awk '{print $1}'

This will get the uid value after =.

id syslog | awk '{print $1}' | cut -d= -f2

Finally, this will get the actual uid value.

id syslog | awk '{print $1}' | cut -d= -f2 | cut -d'(' -f1

Switch to another user, you should know only your password. User should be present in sudoers file.

sudo su <user_name>

Switch to another user, you should know only your password.

sudo su - <user_name>

- will use the environment variables of that new user.

Switch to another user, you should know the password of the new user.

su <user_name>

Switch to another user, simulate initial login (-i).

sudo -u <user_name> -i

Start a new login session.

sudo login <user_name>

Simplest and interactive way:

sudo adduser joe

We can add the existing user joe to the joegroup:

sudo adduser joe joegroup

Advanced, with more options (specify home directory, default shell, default group).

sudo groupadd joegroup
sudo useradd -m -d /home/joe -s /bin/bash -g joegroup joe

m - create home directory.
d - path to the home directory.
s - set implicit shell.
g - set primary group.

To add also secondary groups:

sudo groupadd joesecond
sudo groupadd joethird
sudo useradd -m -d /home/joe -s /bin/bash -g joegroup -G joesecond,joethird joe

G - secondary groups separated by comma.

Add / change the joe's password:

sudo passwd joe

Add secondary groups to the user:

sudo groupadd joefourth
sudo groupadd joefive
sudo usermod -a -G joefourth,joefive joe

a - add (important!), without it, the user will be eliminated from the other secondary groups.

Modify the home directory:

sudo usermod -d /home/newjoe joe

d - dir name.

Change the joe's default shell (First, inspect the /etc/shells file for a list of all available shells)

sudo usermod -s /bin/sh joe

Same effect can be obtained with

sudo chsh -s /bin/sh joe

Modify the joe's primary group:

sudo usermod -g joesecond joe

Remove joe, but keep its files.

sudo userdel joe

Remove joe with all its files (from home dir and other places).

sudo userdel -r joe

r - recursive

sudo usermod -s /usr/sbin/nologin joe

finger joe

Owner read and write, group read, other no permission:

chmod 640 file.txt
chmod u=rw,g=r,o= file.txt

Add execute right for others:

chmod 641 file.txt
chmod o+x file.txt

Eliminare read permission for group:

chmod 601 file.txt
chmod g-r file.txt

Eliminare write permission for all:

chmod ugo-w file.txt

Change the owner to joe user and joegroup group.

sudo chown joe:joegroup file.txt

Change only the owner (let the group intact).

sudo chown joe file.txt

Change the owner of all files in a directory (recursive) - let the group intact.

sudo chown -R joe dir

Change the owner of all files in a directory (recursive) - change also the group.

sudo chown -R joe:joegroup dir

Suppose those 2 users are uso and so. Folder path is /uso.

sudo groupadd usogroup
sudo usermod -aG usogroup uso
sudo usermod -aG usogroup so
sudo chgrp -R usogroup /uso
sudo chmod -R 770 /uso

Generate 5 password of length 10, one per line, each containing at least one uppercase letter.

for i in {1..5}; do
    echo $(openssl rand -base64 9 | tr -dc 'a-zA-Z0-9' | cut -c 1-9)$(tr -dc 'A-Z' < /dev/urandom | head -c 1)
done

Same thing using pwgen tool.

sudo apt install pwgen

pwgen -c -n -B 10 5 | tr ' ' '\n'

c (–capitalize) - at least one capital letter will be included.
n (–numerals) - at least one number will be included.
B (–ambiguous) - eliminate ambiguous characters (like 0 and O).
10 - length of the password.
5 - generate 5 password.

Encoding.

echo -n "Base64 is not encryption!" | base64

QmFzZTY0IGlzIG5vdCBlbmNyeXB0aW9uIQ==

Decoding.

echo -n "QmFzZTY0IGlzIG5vdCBlbmNyeXB0aW9uIQ==" | base64 -d

d - decoding
Base64 is not encryption!

Encoding.

base64 file.txt > file64.txt

Decoding.

base64 -d file64.txt > file_dec.txt

Encrypt the file file.txt to file file.enc

echo -n "plaintext content" > file.txt
openssl aes-256-cbc -in file.txt -out file.enc -pass pass:"my secret" -pbkdf2 -iter 10000

aes-256-cbc - encrypt the file using AES-256 in CBC mode
in - input file (plain text)
out - output file (encrypted)
pass - sets the passphrase (will be used for decryption)
pbkdf2 - selects PBKDF2 for key derivation, enhancing security against brute-force attacks
iter 10000 - sets the number of iterations in the key derivation, increasing the difficulty of cracking the encryption.

Decrypt the file file.enc to decrypted_file.txt.

openssl aes-256-cbc -d -in file.enc -out decrypted_file.txt -pass pass:"my secret" -pbkdf2 -iter 10000

d - decrypt

Using md5 (less secure) - two ways of achieving the same result.

echo -n "plaintext content" | md5sum

f88d6c9eab9c6a2ef3cfd3c59832b4d6 -

echo -n "plaintext content" | openssl dgst -md5

MD5(stdin)= f88d6c9eab9c6a2ef3cfd3c59832b4d6

Using SHA256 (much more secure - less collision issues).

echo -n "plaintext content" | openssl dgst -sha256

SHA2-256(stdin)= d800efe86c80b6054cff4850f3044d73e979429809f28029e26cd9c4f6d6cea8

file.txt content: plaintext content

Using md5.

md5sum file.txt

88d6c9eab9c6a2ef3cfd3c59832b4d6 file.txt

Using SHA256.

sha256sum file.txt

d800efe86c80b6054cff4850f3044d73e979429809f28029e26cd9c4f6d6cea8 file.txt

Install John The Ripper tool.

sudo apt install john

Merge the Linux system files /etc/passwd and /etc/shadow to create a single file (mypasswd.txt) containing user accounts and their hashed passwords.

sudo unshadow /etc/passwd /etc/shadow > mypasswd.txt

Create a word list used to guess the password.

echo -ne "school\nuniversity\nstudent" > wordlist.txt

n - avoid printing the newline
e - interpret backslash escapes.

Now we are ready to run John The Ripper.

john --wordlist=wordlist.txt mypasswd.txt

Finally, crack the passwords:

john --format=crypt mypasswd.txt

View the actual passwords (right after username):

john --format=crypt mypasswd.txt --show

hashfile contains extracted password hashes.
This will use the default dictionary (word list) file.

john --format=zip hashfile

To use a custom dictionary file:

john --format=zip hashfile --wordlist=path/to/your/wordlist.txt

Install the fcrackzip tool.

sudo apt install fcrackzip 

Brute-force guessing the password. We know it has 4 characters.

fcrackzip -v -l 1-4 -u secret_brute_force.zip

v - verbose
l 1-4 - pwd length from 1 to 4
u - extract the zip archive when the password is found.

Use a dictionary to crack the password.

fcrackzip -u -D -p wordlist.txt secret.zip

u - extract the zip archive when the password is found.
D - enables dictionary mode.
p - path to the dictionary file.

Suppose we have my-hashes.txt:

F5D6C6C424929713C297514199F7CA41
AAD24C09BF816BBC6EA29C35DF52F896
F95B70FDC3088560732A5AC135644506
6EF5FA162DA08580BF5D1F88520F7388
CBB184DD8E05C9709E5DCAEDAA0495CF
1BC29B36F623BA82AAF6724FD3B16718

We'll take each line, put it in crackstation.net
The decrypted info from each line will form the final message.