Basic Bash Bulletpoints

This page have some useful Bash notes I took while learning bash these days. They are mostly for reference as I always forget how to make whiles, for cycles and other small constructs and I always need to DuckDuckGo them on StackOverflow. Also with the search engine and AI vibe coded sludge that these days plague the web, this is my little page to take code that I made myself while trying to genuinely learn this language.
Hopeful you will find this code reference page useful as well.

Make file Executable

chmod +x filename.sh

#!/bin/bash
echo "I am Nijkstral";

#!/bin/bash
decorator=🐀
global_var="I am Global"                   # This is a global variable
echo "Print Variable, $global_var!";

my_function() {
  local local_var="I am Local"; # This is a local variable
  echo "printing local Variable:  $local_var$decorator";
  echo "printing global Variable: $global_var$decorator";
  
}
my_function

#!/bin/bash
num1=5; num2=10
sum=$((num1 + num2 + 10))   # It can also be difference (-), Product (*), Quotient (/), Modulus (%)
echo "The sum is $sum"      # 25

Simple IF and Logical Operators

# Basic if statement for Numbers
num=15

# -eq: Equal to
# -ne: Not equal to
# -lt: Less than
# -le: Less than or equal to
# -gt: Greater than
# -ge: Greater than or equal to
if [ $num -ne 15 ]; then
  echo "Number is not 15"
elif ( [ $num -gt 10 ] && [ $num -lt 20 ] ) || [ $num -eq 15 ]; then
  echo "Number is greater than 10"
else
  echo "Number is a Rat number"
fi

# Check if variable is not setted
if [ -z "$animal" ]; then
    $animal="Default Rat"
fi
echo $animal

While

# While loop example
count=1
while [ $count -le 5 ]; do #instead of "while", you can use "until" to iterate until a condition is meet
  echo "Count is $count"
  ((count++))
done

Break and Continue

# Break and continue example
for i in {1..5}; do
  if [ $i -eq 3 ]; then
    continue
  fi
  echo "Number $i"
  if [ $i -eq 4 ]; then
    break
  fi
done

Indexed

#!/bin/bash
fruits=("apple" "banana" "cherry")
for fruit in "${fruits[@]}"; do
  echo $fruit
done

echo "Print first element: ${fruits[0]}"

Associative

#!/bin/bash
# Associative array example
declare -A colors
colors[apple]="red"
colors[banana]="yellow"
colors[grape]="purple"
unset colors[banana]
echo ${colors[apple]} # red
echo ${colors[grape]} # purple

Array Manipulation

#!/bin/bash
# Indexed Array
animals=("rabbit" "kangaroo" "elephant")
echo "this is the whole Array: ${animals[@]}. Array lenght: ${#animals[@]}"; # "*" Can aslo be used instead of "@"
echo "Print first element is: ${animals[0]}. Element lenght: ${#animals[0]}"

# Modify Array
orignal=("${animals[@]}")     # Copy array
animals+=(donkey horse)       # Add Element
animals[0]=bunny              # Change Element
animals=(${animals[@]/o/owo}) # Find and reaplce on every element
unset animals[0]              # Delete the first Element (can be used to delete the whole variable)
#unset animals[-1]            # Delete the last element

## Merge Arrays
additionalAnimals=("dolphin" "camel" "zebra")
animals=("${animals[@]}" "${additionalAnimals[@]}")

## Sort the list
IFS=$'\n' animals=($(sort <<<"${animals[*]}")); unset IFS

## Filter the list
animals=($(printf "%s\n" "${animals[@]}" | grep 'do'))

# Cycle through
echo -e "\nCycling with \e[1;32m'for in'\e[0m"
for animal in "${animals[@]}"; do
    echo $animal
done
 
# C-style for loop 
echo -e "\nCycling with \e[1;32m'C for loop'\e[0m"
for ((i = 0; i <= ${#animals[@]}; i++))
do
    echo "${animals[i]}";
done

printf "%s" "Enter fullname: "
read fullname
echo $fullname

fullname="Nijkstral"
echo $(echo $fullname | grep "Ni")

add_function() {
  local first_var=$1
  local second_var=$2
  local sum=$(($first_var + $second_var))
  echo $sum
}
result=$(add_function 5 3)
echo "The sum is $result"

To replace IFs

#Run the second command only if the First SUCCEED (&&)
cat non_existent_file.txt && echo "operation succeeded";

#Run the second command only if the First FAIL ❌(&&)
cat non_existent_file.txt || echo "operation failed";

#This can be used to turn 
if [ -z "$animal" ]; then
    $animal="Default Rat"
fi
#Into this
[ -z "$animal" ] && animal="Default Rat";

Don't wait for a command to finish

sleep 5 & echo "Command is already done";

Directories and Files as Array

files=(*)
for filename in "${files[@]}"; do
  echo $filename
done

• grep
  • -x Match only Whole line
  • -E Use Extended RegExp
  • -o Show only matched word

sed -i 's/"//g' Download.txt

# Source - https://stackoverflow.com/a/16844327
# Posted by demure, modified by community. See post 'Timeline' for change history
# Retrieved 2026-04-15, License - CC BY-SA 3.0

RCol='\e[0m'    # Text Reset
# Regular           Bold                Underline           High Intensity      BoldHigh Intens     Background          High Intensity Backgrounds
Bla='\e[0;30m';     BBla='\e[1;30m';    UBla='\e[4;30m';    IBla='\e[0;90m';    BIBla='\e[1;90m';   On_Bla='\e[40m';    On_IBla='\e[0;100m'; # Black
Red='\e[0;31m';     BRed='\e[1;31m';    URed='\e[4;31m';    IRed='\e[0;91m';    BIRed='\e[1;91m';   On_Red='\e[41m';    On_IRed='\e[0;101m'; # Red
Gre='\e[0;32m';     BGre='\e[1;32m';    UGre='\e[4;32m';    IGre='\e[0;92m';    BIGre='\e[1;92m';   On_Gre='\e[42m';    On_IGre='\e[0;102m'; # Green
Yel='\e[0;33m';     BYel='\e[1;33m';    UYel='\e[4;33m';    IYel='\e[0;93m';    BIYel='\e[1;93m';   On_Yel='\e[43m';    On_IYel='\e[0;103m'; # Yellow
Blu='\e[0;34m';     BBlu='\e[1;34m';    UBlu='\e[4;34m';    IBlu='\e[0;94m';    BIBlu='\e[1;94m';   On_Blu='\e[44m';    On_IBlu='\e[0;104m'; # Blue
Pur='\e[0;35m';     BPur='\e[1;35m';    UPur='\e[4;35m';    IPur='\e[0;95m';    BIPur='\e[1;95m';   On_Pur='\e[45m';    On_IPur='\e[0;105m'; # Purple
Cya='\e[0;36m';     BCya='\e[1;36m';    UCya='\e[4;36m';    ICya='\e[0;96m';    BICya='\e[1;96m';   On_Cya='\e[46m';    On_ICya='\e[0;106m'; # Cyan
Whi='\e[0;37m';     BWhi='\e[1;37m';    UWhi='\e[4;37m';    IWhi='\e[0;97m';    BIWhi='\e[1;97m';   On_Whi='\e[47m';    On_IWhi='\e[0;107m'; # White

echo -e "${Blu}blue ${Red}red ${RCol}etc...."

Reset: \e[0m

	Regular	Bold	Underline	High Intensity	BoldHight Intens	Background	High Intensity Backgrounds
Black	\e[0;30m	\e[1;30m	\e[4;30m	\e[0;90m	\e[1;90m	\e[40m	\e[0;100m
Red	\e[0;31m	\e[1;31m	\e[4;31m	\e[0;91m	\e[1;91m	\e[41m	\e[0;101m
Green	\e[0;32m	\e[1;32m	\e[4;32m	\e[0;92m	\e[1;92m	\e[42m	\e[0;102m
Yellow	\e[0;33m	\e[1;33m	\e[4;33m	\e[0;93m	\e[1;93m	\e[43m	\e[0;103m
Blue	\e[0;34m	\e[1;34m	\e[4;34m	\e[0;94m	\e[1;94m	\e[44m	\e[0;104m
Purple	\e[0;35m	\e[1;35m	\e[4;35m	\e[0;95m	\e[1;95m	\e[45m	\e[0;105m
Cyan	\e[0;36m	\e[1;36m	\e[4;36m	\e[0;96m	\e[1;96m	\e[46m	\e[0;106m
White	\e[0;37m	\e[1;37m	\e[4;37m	\e[0;97m	\e[1;97m	\e[47m	\e[0;107m