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Logical Operators: Combine Conditions

Logical operators combine conditions. AND. OR. NOT.

Here's the thing: Combining conditions makes scripts powerful. Learn these. Use them.

AND (&&)​

# Both conditions must be true
if [ condition1 ] && [ condition2 ]; then
    echo "Both true"
fi

# Short-circuit: run command2 only if command1 succeeds
command1 && command2

My take: && means both must be true. Use it when you need multiple conditions.

Real example:

if [ -f "$file" ] && [ -r "$file" ]; then
    process_file "$file"
fi

OR (||)​

# Either condition can be true
if [ condition1 ] || [ condition2 ]; then
    echo "At least one true"
fi

# Short-circuit: run command2 only if command1 fails
command1 || command2

My take: || means either can be true. Use it for alternatives.

Real example:

if [ "$USER" = "root" ] || [ "$USER" = "admin" ]; then
    echo "Admin access"
fi

NOT (!)​

# Invert condition
if [ ! condition ]; then
    echo "Condition is false"
fi

if ! command; then
    echo "Command failed"
fi

My take: ! inverts. Use it to check for false conditions.

Real example:

if [ ! -f "$file" ]; then
    echo "File not found"
fi

Combining Operators​

# Complex conditions
if [ condition1 ] && [ condition2 ] || [ condition3 ]; then
    # Logic
fi

# With parentheses (bash)
if [[ (condition1 && condition2) || condition3 ]]; then
    # Logic
fi

My take: Combine operators for complex logic. Use parentheses for clarity.

Common Patterns​

Check Multiple Conditions​

if [ -f "$file" ] && [ -r "$file" ] && [ -s "$file" ]; then
    process_file "$file"
fi

Try Command, Fallback​

command1 || command2 || command3

My take: Try first command. If it fails, try second. If that fails, try third.

Check if Not​

if [ ! -d "$dir" ]; then
    mkdir -p "$dir"
fi

Common Mistakes (I've Made These)​

  1. Wrong syntax: [ condition1 && condition2 ] doesn't work. Use [ condition1 ] && [ condition2 ].

  2. Not using short-circuit: command1 && command2 is useful. Use it.

  3. Too many conditions: Keep it simple. Too many conditions are hard to read.

  4. Wrong operator: && for AND, || for OR. Don't mix them up.

  5. Not using !: ! inverts conditions. Useful for "if not" checks.

Real-World Examples​

Validate Multiple Conditions​

if [ -f "$file" ] && [ -r "$file" ] && [ -s "$file" ]; then
    process_file "$file"
else
    echo "File is invalid"
fi

Try Multiple Commands​

ping -c 1 server1 || ping -c 1 server2 || echo "Both servers down"

Check if Not​

if [ ! -d "$backup_dir" ]; then
    mkdir -p "$backup_dir"
fi

What's Next?​

Now that you can combine conditions, scripts become more powerful. Or learn about File Operations to work with files.

Personal note: When I started, I'd write nested if statements. Then I learned logical operators. Now I combine conditions. Scripts are cleaner. More readable. Learn logical operators. They're worth it.