# Basic if statement
if [ condition ]; then
    echo "Condition is true"
fi

# If-else
if [ condition ]; then
    echo "True"
else
    echo "False"
fi

# If-else-if-else
if [ condition1 ]; then
    echo "Condition 1 is true"
elif [ condition2 ]; then
    echo "Condition 2 is true"
else
    echo "All false"
fi

# String equality
[ "$str1" = "$str2" ] # equal
[ "$str1" != "$str2" ] # not equal
[ -z "$str" ] # string is empty
[ -n "$str" ] # string is not empty

# Numeric comparisons
[ $num1 -eq $num2 ] # equal
[ $num1 -ne $num2 ] # not equal
[ $num1 -lt $num2 ] # less than
[ $num1 -le $num2 ] # less than or equal
[ $num1 -gt $num2 ] # greater than
[ $num1 -ge $num2 ] # greater than or equal

# Example
age=25
if [ $age -ge 18 ]; then
    echo "You are an adult"
fi

# File existence and type
[ -e $file ] # file exists
[ -f $file ] # regular file
[ -d $file ] # directory
[ -L $file ] # symbolic link
[ -p $file ] # named pipe
[ -S $file ] # socket

# File permissions
[ -r $file ] # readable
[ -w $file ] # writable
[ -x $file ] # executable

# File size
[ -s $file ] # file has size > 0

# Example
if [ -f "/etc/config" ]; then
    echo "Config file exists"
fi

# AND operator
[ condition1 ] && [ condition2 ]
[ condition1 ] && [ condition2 ] && [ condition3 ]

# OR operator
[ condition1 ] || [ condition2 ]
[ condition1 ] || [ condition2 ] || [ condition3 ]

# NOT operator
[ ! condition ]

# Multiple conditions in single brackets
[ condition1 ] && [ condition2 ]

# Example
if [ -f "$file" ] && [ -r "$file" ]; then
    echo "File exists and is readable"
fi

# Using test command
test condition
[ condition ] # equivalent
[[ condition ]] # extended test (Bash only)

# Double brackets allow more features
[[ $string == pattern* ]] # pattern matching
[[ $num -eq 5 ]] # arithmetic

# Switch/case statement
case $variable in
    pattern1)
        echo "Matches pattern1"
        ;;
    pattern2|pattern3)
        echo "Matches pattern2 or pattern3"
        ;;
    *)
        echo "Default case"
        ;;
esac

# Example
case $grade in
    A)
        echo "Excellent"
        ;;
    B)
        echo "Good"
        ;;
    C)
        echo "Average"
        ;;
    *)
        echo "Invalid grade"
        ;;
esac

# Check if command succeeded
if command; then
    echo "Command succeeded"
else
    echo "Command failed"
fi

# Exit status stored in $?
command
if [ $? -eq 0 ]; then
    echo "Success"
else
    echo "Failed with code $?"
fi

# Using && and ||
command && echo "Success" || echo "Failed"

#!/bin/bash

# Check argument count
if [ $# -lt 1 ]; then
    echo "Usage: $0 <filename>"
    exit 1
fi

file=$1

# Validate file
if [ ! -f "$file" ]; then
    echo "Error: File not found: $file"
    exit 1
fi

# Check permissions
if [ ! -r "$file" ]; then
    echo "Error: File not readable"
    exit 1
fi

echo "File is valid and readable"