Some useful Linux snippets

list installed software

apt list --installed

install from .deb file

sudo dpkg -i <n.deb>

configure firewall

# block all incoming
# allow all outgoing
# all/limit common ports
# enable ufw
# show ufw status

ufw default deny incoming
ufw default allow outgoing
ufw limit 22/tcp  
ufw allow 80/tcp  
ufw allow 443/tcp  
ufw enable
ufw status verbose

install fail2ban

sudo apt install fail2ban
sudo systemctl enable fail2ban
sudo systemctl start fail2ban

check listening ports

netstat -tunlp

move folder between users

mv /root/ghost ~vmadmin/

folder sizes

# disk usage with sizes but go to a depth of 1 and sort 
# this means i just want the cumulative value and not individual sizes per object recursively down through the folder hierarchy
du -h -d 1 | sort -h

install from url

# install mockoon
wget https://github.com/mockoon/mockoon/releases/download/v1.11.0/mockoon-1.11.0.deb -O kam.deb && sudo dpkg -i kam.deb

How to create a swapfile

Resolution

• allocate the size
  sudo fallocate -l 1G /swapfile

• zero out the file (takes about a minute)
  sudo dd if=/dev/zero of=/swapfile bs=1024 count=1048576

• make the file only accessible by root
sudo chmod 600 /swapfile

• create the swapfile
  sudo mkswap /swapfile

• turn the swapfile on
  sudo swapon /swapfile

• register the changes with the system
  sudo vi /etc/fstab
  (append line) /swapfile swap swap default 0 0

• validate the configuration with the file system
  sudo mount -a

How to display mapped drives

Resolution

Use the display filesystem command:

df -aTh

Constructing json

export SAMPLE_CONTAINER=test
export SAMPLE_FILE=test.jar
export SAMPLE_TARGET=/opt/test/test.jar

echo $( jq -n \
                  --arg container "$SAMPLE_CONTAINER" \
                  --arg file "$SAMPLE_FILE" \
                  --arg target "$SAMPLE_TARGET" \
                  '{mycontainer: $container, myfile: $file, mytarget: $target}' )

{ "mycontainer": "test", "myfile": "test.jar", "mytarget": "/opt/test/test.jar" }

If statement on function, disecting json return to determine if value exists

if [[ $(az rest --method GET --uri https://graph.microsoft.com/v1.0/servicePrincipals/5cf3c8e1-4e18-4114-9e4d-8c86577fd17f/appRoleAssignedTo -o json | jq '[.value[] | select(.principalId == "bada8227-4452-451d-88d5-647b669cea07")] | length'==0 | grep 'true') = 'true' ]]; then echo 'not here!'; fi

Get value from json snippet in file where attribute is something

projectID=$(jq -r --arg EMAILID "$EMAILID" '
        .resource[]
        | select(.username==$EMAILID) 
        | .id' file.json)

Create an alias with parameters

alias kam='function (){ls $1 && printenv | grep -i $2}'

Running a call: kam system arm will list the folder "system" and grep for environment variables containing the word "arm"

Get got branch name from bash

# get git branch name
git branch 2> /dev/null | sed -e '/^[^*]/d' -e 's/* \(.*\)/ (\1)/'

Creating named parameters on a bash script

#!/bin/bash
# allows for named params on the cli eg. myscript --param1 true --param2 58

myapi=${myapi:-false}
myapi2=${myapi2:-false}
myapi3=${myapi3:-false}

while [ $# -gt 0 ]; do
   if [[ $1 == *"--"* ]]; then
        param="${1/--/}"
        declare $param="$2"
        # echo $1 $2 # Optional to see the parameter:value result
   fi
  shift
done

if [ "$myapi" = "true" ]; then ./myapi/myapi_tests.sh; fi 

if  [ "$myapi" = "false" ] && 
    [ "$myapi2" = "false" ] &&
    [ "$myapi3" =  "false" ]; 
then echo "No tests were set to run"; fi

Set the current directory to be the working directory (useful in scripts)

# set working directory - useful in scripts where you want to call from somewhere else but the script usws relative paths

cd "$(dirname "$0")"

Find the last return code

# find last return code

if [ $? -eq 0 ]; then echo 'ok!'; else echo 'no ok!'; fi

Retry loop

max_retry_count=3
retry=1
seconds_to_sleep=5
sleep_increment=5
      
mycommand=" \
az vm show -g vm-test \
-n vm-test \
"

until [ $retry -gt $max_retry_count ] || eval $mycommand
do
   sleep $seconds_to_sleep
   ((sleeper=seconds_to_sleep+$sleep_increment))
   echo "Attempt ($retry)"
   ((retry++))
done

Upper and lowercase

firstname=KAM
lastname=lagan
echo "${firstname:l} ${lastname:u}"

outputs

kam LAGAN

Hide the output of a command

command > /dev/null 2>&1

Explanation:

1. command > /dev/null redirects the output of command (stdout) to /dev/null
2. 2>&1 redirects stderr to stdout, so errors (if any) also goes to /dev/null

Keyboard produces ABCD characters when using arrow keys in vi

The three operations I found that helped me:

1. Create a ~/.vimrc file
2. Add set nocompatible to the file
3. Install vim: sudo apt-get install vim

Now restart your terminal and things should hopefully work out for ya!