Shell Cheat Sheet

#!/bin/bash
echo "Hello, World!"

#!/bin/bash   # Bash
#!/bin/sh     # POSIX shell
#!/usr/bin/env bash  # Portable

# Single line comment

: '
Multi-line comment
using colon operator
'

chmod +x script.sh
./script.sh

bash script.sh
sh script.sh

name="John"          # No spaces around =
count=42
path="/usr/local/bin"

echo $name           # Basic usage
echo "${name}"       # Explicit braces
echo "Hello, $name"  # In strings

today=$(date +%Y-%m-%d)
files=$(ls -l)
count=$(wc -l < file.txt)

read -p "Enter name: " username
read -sp "Password: " password  # Silent
read -t 5 -p "Quick: " answer   # Timeout

export PATH="/usr/local/bin:$PATH"
export DB_HOST="localhost"
env | grep PATH  # View all

$0  # Script name
$1, $2  # Arguments
$#  # Argument count
$@  # All arguments
$?  # Last exit status
$$  # Process ID

result=$((5 + 3))
count=$((count + 1))
value=$((10 * 2 - 5))

if [ "$str1" = "$str2" ]; then
  echo "Equal"
fi

if [ "$str1" != "$str2" ]; then
  echo "Not equal"
fi

if [ $a -eq $b ]; then echo "Equal"; fi
if [ $a -ne $b ]; then echo "Not equal"; fi
if [ $a -gt $b ]; then echo "Greater"; fi
if [ $a -lt $b ]; then echo "Less"; fi

[ -f file.txt ]  # File exists
[ -d /path ]     # Directory exists
[ -r file ]      # Readable
[ -w file ]      # Writable
[ -x file ]      # Executable

if [ $a -gt 0 ] && [ $b -gt 0 ]; then
  echo "Both positive"
fi

if [ $a -gt 0 ] || [ $b -gt 0 ]; then
  echo "At least one positive"
fi

if [ $age -ge 18 ]; then
  echo "Adult"
fi

if [ $score -ge 60 ]; then
  echo "Pass"
else
  echo "Fail"
fi

if [ $grade -ge 90 ]; then
  echo "A"
elif [ $grade -ge 80 ]; then
  echo "B"
else
  echo "C"
fi

case $fruit in
  apple)
    echo "Red"
    ;;
  banana)
    echo "Yellow"
    ;;
  *)
    echo "Unknown"
    ;;
esac

if [[ $name == "John" ]]; then
  echo "Hello John"
fi

if [[ $str =~ ^[0-9]+$ ]]; then
  echo "Number"
fi

for item in apple banana cherry; do
  echo $item
done

for i in {1..10}; do
  echo "Number: $i"
done

for ((i=0; i<10; i++)); do
  echo $i
done

count=0
while [ $count -lt 5 ]; do
  echo $count
  count=$((count + 1))
done

count=0
until [ $count -ge 5 ]; do
  echo $count
  count=$((count + 1))
done

for file in *.txt; do
  echo "Processing: $file"
  wc -l "$file"
done

while IFS= read -r line; do
  echo "Line: $line"
done < file.txt

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

greet "World"

add() {
  local sum=$(($1 + $2))
  echo $sum
}

result=$(add 5 3)
echo "Result: $result"

check_file() {
  if [ -f "$1" ]; then
    return 0  # Success
  else
    return 1  # Failure
  fi
}

if check_file "test.txt"; then
  echo "File exists"
fi

calculate() {
  local x=10
  local y=20
  echo $((x + y))
}

fruits=("apple" "banana" "cherry")
numbers=(1 2 3 4 5)

echo ${fruits[0]}     # First element
echo ${fruits[2]}     # Third element
echo ${fruits[-1]}    # Last element

echo ${#fruits[@]}    # Number of elements
echo ${#fruits[0]}    # Length of first element

for fruit in "${fruits[@]}"; do
  echo $fruit
done

fruits+=("orange")       # Append
unset fruits[1]          # Remove element

echo ${fruits[@]:1:2}   # Elements 1-2
echo ${fruits[@]:2}     # From index 2 to end

echo "Hello" > file.txt       # Overwrite
echo "World" >> file.txt      # Append

content=$(cat file.txt)
while read line; do
  echo $line
done < file.txt

cat <<EOF > config.txt
Name: John
Age: 30
EOF

command 2> error.log          # Stderr to file
command 2>&1                  # Stderr to stdout
command &> output.log         # Both to file

ls -l | grep ".txt"
cat file.txt | wc -l
ps aux | grep nginx

if [ -f "file.txt" ]; then
  cat file.txt
else
  echo "File not found"
fi