In bash, exit codes indicate the success or failure of a command or script.

• 0 → success
• anything else → failure

Bash primarily deals with strings, but can handle numbers and arrays. Example:

name="John"
age=30

Allows you to use the output of a command as part of another command. Example:

current_date=$(date +%Y-%m-%d)

Allows you to control where input comes from and where output goes. Example:

echo "Hello" > output.txt  # Redirect output to a file
cat < input.txt            # Read input from a file

Used for conditional execution of code. Example:

if [[ condition ]]; then
  echo "condition met"
elif [[ condition_2 ]]; then
  echo "condition_2 met"
else
  echo "no condition met"
fi

Used for multiple conditional branches. Example:

case "$variable" in
  pattern1) command1 ;;
  pattern2) command2 ;;
  *) default_command ;;
esac

Used to iterate over a list of items. Examples:

# Regular
my_array=(1 2 3 4 5)
for item in ${my_array[@]}
do
  echo $item
done
 
# C-style
for ((i = 0; i < 10; i++)); do
  echo "$i"
done
 
# Range
for i in {1..10}; do
  echo "$i"
done
 
# Pattern matching
for item in ./content/*.md; do
  echo "$item"
done
 
# Command result
for item in $(ls ~/Notes/); do
  echo $item
done

Executes a block of code as long as a condition is true. Example:

counter=0
while [[ $counter -lt 5 ]]; do
  echo $counter
  ((counter++))
done

Reusable blocks of code. They operate like mini-scripts. Example:

greet() {
  echo "Hello, $1!"
}
greet "World"

val="a"
[[ "$val" == "a" ]]
[[ "$val" != "b" ]]

num=1
[[ "$num" -eq 1 ]]  # equal
[[ "$num" -ne 2 ]]  # not equal
[[ "$num" -lt 2 ]]  # less than
[[ "$num" -le 2 ]]  # less than or equal
[[ "$num" -gt 1 ]]  # greater than
[[ "$num" -ge 1 ]]  # greater than or equal

val=""
[[ -z $val ]]  # var is null
[[ -n $val ]]  # var is not null

file="./hello"
[[ -f $file ]]  # file exists
[[ -d $file ]]  # directory exists
[[ -e $file ]]  # file or directory exists

file="./hello"
[[ -r $file ]]  # readable
[[ -w $file ]]  # writable
[[ -x $file ]]  # executable

[[ $val -gt 5 -a $val -lt 10 ]]  # -a -> AND
[[ $val -gt 5 -o $val -lt 3 ]]   # -o -> OR

[[ $val -gt 5 ]] && [[ $val -lt 10 ]]  # AND
[[ $val -gt 5 ]] || [[ $val -lt 3 ]]   # OR

command1 && command2  # Execute command2 if command1 was successful
command1 || command2  # Execute command2 if command1 failed

Pauses the script for a specified amount of time.

Reads input from the user. Example:

read -p "Do you want to continue (Y/n) " resp
if [[ $resp != "Y" ]]; then
  exit 1
fi
echo "Continuing..."

Activates strict mode in bash:

set -euo pipefail

• set -e: exit on error
• set -u: exit on unset var
• set -o pipefail: exit on pipe fail

Creates a temporary file or directory.

Sets up a function to be called when the script receives specific signals.

Store multiple values in a single variable. Example:

fruits=("apple" "banana" "cherry")
echo ${fruits[0]}  # Outputs: apple

• Use meaningful variable names
• Comment your code
• Use functions for repeated code
• Always quote variables when using them
• Use set -euo pipefail for safer scripts