A Linux Cheat-sheet So Good, You'll Want to Save It

Over the years I have faced several linux challenges that I have solved by googling or thanks to beautiful people who have shared their knowledge.

This is a brief compilation of useful tricks grouped in different categories: Files management, searching, bash scripting, networking, compressed files management, packet management, server information and miscellaneous.

📝📁Files management

Delete

• Delete first line:

  sed -i '1d' file.txt

• Delete all empty lines:

  sed '/^$/d' file.txt

• Delete first two characters from each line:

  cat file.txt | sed 's/^..//'

• Delete comments and empty lines:

  sed '/ *#/d; /^$/d' file.txt

• Delete carriage return \r:

  cat input.txt|tr -d "\r" > output.txt

Replace

• Replace string1 with string2 sed -d 's/string1/string2/g' file.txt
• Replace space by - :

  sed ':a;N;$!ba;s/\n/\'\'\-\'/g''

• Replace replace line break with space:

  sed ':a;N;$!ba;s/\n/ /g'

Add

• Add - at the beginning of the line

  sed -d s/^/\-/ file.txt 

• Add - at the end of the line

  sed -d s/$/\-/ file.txt

Filter

• Filter lines starting with a number:

  cat file.txt |grep ^[0-9]

• Filter multiple words:

  grep -E 'string1|string2' file.txt

• Filter multiple words old shells:

  grep -e 'string1' -e 'string2' file.txt

• Print the third and tenth words from file:

  cat file.txt | awk '{print $3,$10}'

• Print the third word from file separated by - :

   cat file.txt| awk -F"-" '{print $3}'

Misc

• Split a large file every 2000 lines:

  split -l 2000 input.txt output.txt

• View only the lines that contain string1:

  sed -n '/string1/p' files.txt 

• Convert lowercase to uppercase:

  cat file.txt | tr '[:lower:]' '[:upper:] '

• Copy permissions from a reference file:

  chmod --reference=reference_file new_file

• Copy ownership from a reference file:

  chown --reference=reference_file new_file

🧮Let's count

• Files on the directory:

  ls | wc -l

• Lines in a file:

  wc -l file.txt

• Words in a file:

  wc -s file.txt

• Length of the longest line in a file:

  wc -L file.txt

• Number of files and directories in a directory:

  tree -a 

🔍Searching

• Files less than two days old

  find -mtime -2

• Files older than 5 days:

  find -mtime +5

• Files created today:

  find ./ -type f -daystart -ctime -1

• Directory by name: 

  find / -type d -name 

  directory_name

• Find files by content:

  find ./ -type f -exec grep -H '

  content

  ' {} \;

👨‍💻👩‍💻Bash scripting basics

For syntax

#!/bin/bash 
for i in `seq 1 90` do      
   echo $i 
done

Read file line per line

#!/bin/bash
while IFS='' read -r line || [[ -n "$line" ]]; do
     echo "Text read from file: $line"
done < "$1"

Read file word per word

#!/bin/bash
while IFS='' read -r line || [[ -n "$line" ]]; do
     for word in $line      
          do
               echo $word
     done
done < "$1"

Check if a file exists

#!/bin/bash
if [ -f $path/to/file.txt ];  then 
     echo "exist"
else 
     echo "no exist"

fi

Check ip reachability

#!/bin/bash 
ping -w1 -c3

$IP

 >/dev/null 2>&1 && echo "ON" || echo "OFF"

🌐Networking

• Ping from a defined interface:

  ping -I

  interface_name ip

• Rename interface:

  link set

  current_name

  name

  new_name

• Show IP addresses without infconfig :

  ip address show

Traffic

• Traffic capture with tshark:

  tshark -i 

  interface_name

  .151 -f "host

  ip

  and port

  port_number

  "

• Traffic capture with tcpdump:

  tcpdump -i

  interface_name

  dst port port

  port_number

  -w capture.pcap

• Store traffic output:

  tcpdump -ni eth0 -w capture.pcap

• Read traffic capture form the CLI:

  tcpdump -ttttnnr capture.pcap

Port scanning 👀

• Check if 5050 port is open:

  nc -zvn

  ip 5050

• Check if 5050 and 5055 are open:

  nc -zvn 

  ip 5050 5055

• Check if range of ports:

  nc -zvn 

  ip 5050-5055

• Connect to ip on port 5050 and send content of input.in:

  nc

  ip 5050 

  < input.in

Listening 👂👂

• Listen for TCP connections on port 5050:

  netcat -l 5050

• Listen for UDP connections on port 5050:

  netcat -ul 5050

• Listen for TCP connections on port 5050 and store data received in to "received.txt":

  netcat -l 5050 > received.txt

• Listen for TCP connections on port 5050 and continue listening after client has disconnected:

  netcat -k -l 5050

🗃️Compressed files management

.tar.gz files
Compress:

tar -czvf compressed.tar.gz /direcory/to/compress/

tar -xzvf compressed.tar.gz

.tar files
Compress:

tar -cvf compressed.tar /direcory/to/compress/

tar -xvf compressed.tar

.gz files
Compress:

gzip file.txt

 gzip -d file.txt.gz

.zip files
Compress:

zip compressed.zip /direcory/to/compress/

unzip compressed.zip

Bonus 🎉
Read compressed file without decompress:

zcat compressed.gz

zgrep -n 

string 

compressed.gz

split -b 10m file.tar.gz "file.tar.gz.part"

📦Packet management (YUM and DNF)

• What provides a package
  

  yum whatprovides package_name
  dnf provides package_name

• Package description:
  

  yum info package_name
  dnf info package_name

• Update security packages:

  
  yum update --security
  dnf update --security

• List dependencies from a package example httpd:
  

  yum deplist

  httpd
  

  dnf repoquery --deplist httpd

• Download all packages and dependencies needed to install something, example FTP:
  

  yumdownloader --resolve FTP
  dnfdownload --resolve FTP

• Remove orphan packages:
  

  package-cleanup --orphans

  and

  package-cleanup --oldkernels
  dnf list extras

  and

  dnf remove $( dnf repoquery --installonly --latest-limit -1 -q )

• List pending security updates:
  

  yum list-security
  dnf updateinfo list --security

ℹ️Server info

• Number of processors:

  grep processor /proc/cpuinfo | wc -l

• Number of cores:

  dmidecode -t processor | grep "Core Count"

• Number of enabled cores:

  dmidecode -t processor | grep "Core Enabled"

• RAM memory:

  free -m | head -2 | tail -1 | awk {'print $4'}

• Free space:

  df -h | awk {'print $3'} | head -3 | tail -1

• Release version:

  cat /etc/*release*

🍕Misc

• Execute a command as another user:

  sudo -u

  user command

• Clean cache:

  echo 3 > /proc/sys/vm/drop_caches

• Create symbolic link:

  ln -s

  /file_or_directory_path /direct_access_path

• Create FIFO files:

  mkfifo

  filename

• Know more information about and image from CLI:

  identify -verbose

  image.png

• Pretty print json files in CLI:

  cat file.json | jq

• Print only values of the key my_key from json file:

   jq '.[] | .my_key' file.json

Hope this was useful for you. If you want more don't hesitate visit my github