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#!/bin/bash  # Shebang line
# This is a comment

# Basic script example
echo "Hello, World!"

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# Running commands
command [options] [arguments]

# Example
ls -la /home/user

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# Strings
name="John Doe"
greeting='Hello, world!'

# Integers
age=30
count=$((10 + 5))

# Boolean equivalents
if [ $? -eq 0 ]; then
    echo "Previous command succeeded"
fi

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# Local variables
local_var="local value"

# Global variables
GLOBAL_VAR="global value"

# Read-only (constant)
readonly CONSTANT_VAR="cannot be changed"

# Environment variables
export PATH=$PATH:/new/path

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# Default values
name=${USER:-"Anonymous"}

# Substring extraction
string="Hello, World!"
echo ${string:0:5}  # Outputs: Hello

# Length of string
echo $     # Outputs: 13

# Replace substring
echo ${string/World/Bash}

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# Basic arithmetic
a=10
b=20
echo $((a + b))  # Addition
echo $((a - b))  # Subtraction
echo $((a * b))  # Multiplication
echo $((b / a))  # Division
echo $((b % a))  # Modulus

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# Numeric comparisons
[ 10 -eq 10 ]  # Equal
[ 10 -ne 20 ]  # Not equal
[ 10 -lt 20 ]  # Less than
[ 10 -le 20 ]  # Less than or equal
[ 20 -gt 10 ]  # Greater than
[ 20 -ge 10 ]  # Greater than or equal

# String comparisons
[ "$str1" = "$str2" ]    # Equal
[ "$str1" != "$str2" ]   # Not equal
[ -z "$str" ]            # Zero length
[ -n "$str" ]            # Non-zero length

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# If-Else
if [ condition ]; then
    # commands
elif [ another_condition ]; then
    # commands
else
    # commands
fi

# Compact if
[ condition ] && command1 || command2

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# For loop
for item in {1..5}; do
    echo $item
done

# C-style for loop
for ((i=0; i<5; i++)); do
    echo $i
done

# While loop
while [ condition ]; do
    # commands
done

# Until loop
until [ condition ]; do
    # commands
done

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# Break and continue
for i in {1..10}; do
    if [ $i -eq 5 ]; then
        break    # Exit loop
    fi
    if [ $i -eq 3 ]; then
        continue # Skip iteration
    fi
    echo $i
done

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

# Function with return value
add() {
    return $((${1} + ${2}))
}
add 5 3
echo $?  # Prints result

# Function with local variables
calculate() {
    local result=$((${1} * ${2}))
    echo $result
}

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# Declaration
fruits=("apple" "banana" "cherry")

# Access elements
echo ${fruits[0]}         # First element
echo ${fruits[@]}         # All elements
echo $        # Array length

# Modifying arrays
fruits+=("date")          # Append
unset fruits[1]           # Remove element

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# Declare associative array
declare -A person=(
    [name]="John"
    [age]=30
    [city]="New York"
)

# Access
echo ${person[name]}

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

# Input redirection
sort < input.txt > sorted.txt

# Error redirection
command 2> error.log

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# Pipe output between commands
cat file.txt | grep "pattern" | sort

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# Grep (search)
grep "pattern" file.txt

# Sed (stream editor)
sed 's/old/new/g' file.txt

# Awk (text processing)
awk '{print $1}' file.txt

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# Background jobs
command &               # Run in background
jobs                    # List background jobs
fg %1                   # Bring job to foreground

# Process control
ps aux                  # List processes
kill PID                # Terminate process
killall process_name    # Kill all processes with name

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# Environment variables
printenv               # Show all variables
export VARNAME=value   # Set environment variable

# Shell configuration
source ~/.bashrc       # Reload configuration

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# Exit codes
command
echo $?  # Check exit status (0 = success)

# Error checking
if ! command; then
    echo "Command failed"
    exit 1
fi

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# File tests
[ -f file.txt ]    # Regular file exists
[ -d directory ]   # Directory exists
[ -r file ]        # File is readable
[ -w file ]        # File is writable
[ -x file ]        # File is executable

# Permissions
chmod 755 script.sh    # Change file permissions
chown user:group file  # Change ownership