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

Unary Expressions [ * {data1}] / [ ! * {data1} ] (To do the inverse)
Files

Operator	TRUE if ${data} is
-a -e	FILE
-d	DIRECTORY
-b	BLOCK SPECIAL FILE
-c	CHARACTER SPECIAL FILE
-f	REGULAR FILE
-g	FILE with SET-GROUP-ID bit
-g	FILE with SET-USER-ID bit
-h -L	SYMBOLIC LINK
-k	FILE with STICKY bit
-p	NAMED PIPE
-r	READABLE FILE
-w	WRITABLE FILE
-x	EXECUTABLE FILE
-s	FILE with non-zero size
-G	FILE owned by GROUP ID
-O	FILE owned by USER ID
-N	FILE modified since last access
-S	SOCKET
-t	FILE DESCRIPTORS

• -o: TRUE if optname is anabled
• -z: TRUE if string is empty
• -v: TRUE if ${data1} has been declared

Binary Expressions: [ {data1} * {data2} ];

• -nt: if {data1} is a file that is newer than {data2}
• -ot: if {data1} is a file that is older than {data2}
• -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

Simple IF and Logical Operators

# Basic if statement for Numbers
num=15

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

Switch

echo -n "Enter an animal!"
read ANIMAL
echo -n "The official language of ${ANIMAL} is "

case ${ANIMAL} in

  Rat | Mouse)
    echo "🐀 You choosed rat or mouse!"
    ;;

  Rabbit)
    echo "🐇 You choosed Rabbit!"
    ;;

  Horse | "Pony" | Donkey | "Jenny")
    echo "🐎 You choosed an equine!"
    ;;

  *)
    echo "unknown"
    ;;
esac

Combo example

#!/bin/bash
VALUE="admin"

case $VALUE in

  admin)
    echo "Has admin access"
    ;;&

  admin | editor)
    echo "Can edit content"
    ;;&

  admin | editor | viewer)
    echo "Can view content"
    ;;
esac

Menu

#!/bin/bash
options=("Rat" "Deer" "Donkey" "Rabbit" "Slug")
PS3="Select an Animal: "
select opt in "${options[@]}"
echo "choosen animal: ${opt}"

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"

# Function definition
function_name(){
    local parameter_1=$1
    local parameter_2=$2
    ...
    local parameter_n=$n

    # commands
    return 7 # Can only return integers, No way to return Strings
}

# Function call
function_name p1 p2 ... pn
echo "The return value is ${?}"

String Lenght

parameter="This is a long string made of rats"
echo ${#parameter}

Take a specified section of a string

parameter="Only take the first letter then the last!"
echo ${parameter:0:1}
echo ${parameter:${#parameter}-1:${#parameter}}

Split

STR="ubuntu:debian:fedora:arch"
IFS=":" 
read -r first second third fourth <<< "$STR"

echo "$first"
echo "$second"
echo "$third"
echo "$fourth"

Split All

IN="Rabbit.it;Rat.com"
arrIN=(${IN//;/ })
echo ${arrIN[1]}                  # Output: Rat.com

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";

See if a directory exists

if [ -d "$DIRECTORY" ]; then # if [ ! -d "$DIRECTORY" ]; If a directory doesn't
  echo "$DIRECTORY does exist."
fi

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

• Options
  • -x Match only Whole line
  • -E Use Extended RegExp
  • -P Use Extended RegExp (Perl)
  • -o Show only matched word
• Preset
  • echo "Test String" | grep -E "Test"
  • grep -E "Test" fileToSearch.txt

Sed [TODO]

• Options -i 's/"//g' Download.txt

Tar

• Options
  • -c --Create archive
  • -x --Extract
  • -t Read all file in the archive
  • -f --file=archive-NAME
  • -v --verbose Show the process
  • --add-file=FILE
• Preset
  • tar -cv --file=${ArchiveName} [List] # Create an archive
  • tar -xv --file=${ArchiveName} # Extract an archive
  • tar -t --file=test.tar # See archive content

XZ Useful for files

• Options
  • -1...9, -1e...9e # Levels of compression (Will require more RAM the more compressed it is)
• Preset
  • xz [List] # This will leave only the archive
  • unxz ${archive} # Uncompress an XZ archive

Zip Useful for protected archives

• Options
  • -r --recurisve # Include directories
  • -e # Enable (weak) Encryption
  • -P # Insert a password for encryption
• Preset
  • zip [List] # Simple Archive
  • zip -Pe {password} -r {ArchiveName} [List] # Archive with password
  • unzip ${archive} # Decompress the archive

# 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