How to use if elif else in bash

Mastering Conditional Logic: If, Elif, and Else in Bash Scripting

Learn how to implement robust conditional logic in your Bash scripts using if, elif, and else statements. This guide covers syntax, common comparisons, and best practices for effective decision-making.

Conditional statements are fundamental to any programming or scripting language, allowing your code to make decisions and execute different blocks of commands based on specific conditions. In Bash, the if, elif (else if), and else constructs provide powerful tools for controlling script flow. Understanding how to use these effectively is crucial for writing dynamic and responsive shell scripts.

The Basic if Statement

The simplest form of conditional logic in Bash is the if statement. It evaluates a condition, and if that condition is true, it executes a block of commands. If the condition is false, the block is skipped. The condition is typically an expression that returns an exit status of 0 (true) or non-zero (false).

# Basic if statement syntax
if [ condition ]; then
    # Commands to execute if condition is true
    echo "Condition is true!"
fi

# Example: Check if a number is greater than 10
NUM=15
if [ "$NUM" -gt 10 ]; then
    echo "$NUM is greater than 10."
fi

# Example: Check if a file exists
FILE="/etc/passwd"
if [ -f "$FILE" ]; then
    echo "File '$FILE' exists."
fi

Basic if statement syntax and examples.

Adding Alternatives with else

The else statement provides an alternative block of commands to execute when the if condition evaluates to false. This allows you to define behavior for both outcomes of a single condition.

# if-else statement syntax
if [ condition ]; then
    # Commands if condition is true
    echo "Condition is true."
else
    # Commands if condition is false
    echo "Condition is false."
fi

# Example: Check if a number is even or odd
NUM=7
if (( NUM % 2 == 0 )); then
    echo "$NUM is an even number."
else
    echo "$NUM is an odd number."
fi

Using if-else for two-way decision making.

Handling Multiple Conditions with elif

When you need to check multiple distinct conditions sequentially, elif (short for 'else if') comes into play. It allows you to chain several conditions together. The script will execute the command block of the first if or elif condition that evaluates to true, and then exit the entire if block. If none of the if or elif conditions are true, the else block (if present) will be executed.

# if-elif-else statement syntax
if [ condition1 ]; then
    # Commands if condition1 is true
elif [ condition2 ]; then
    # Commands if condition2 is true
elif [ condition3 ]; then
    # Commands if condition3 is true
else
    # Commands if none of the above conditions are true
fi

# Example: Grade calculator
SCORE=85
if [ "$SCORE" -ge 90 ]; then
    echo "Grade: A"
elif [ "$SCORE" -ge 80 ]; then
    echo "Grade: B"
elif [ "$SCORE" -ge 70 ]; then
    echo "Grade: C"
else
    echo "Grade: F"
fi

Using if-elif-else for multi-way decision making.

flowchart TD
    A[Start]
    A --> B{Condition 1?}
    B -- True --> C[Execute Block 1]
    B -- False --> D{Condition 2?}
    D -- True --> E[Execute Block 2]
    D -- False --> F{Condition 3?}
    F -- True --> G[Execute Block 3]
    F -- False --> H[Execute Else Block]
    C --> I[End]
    E --> I
    G --> I
    H --> I

Flowchart illustrating the logic of an if-elif-else structure.

Common Test Operators and Best Practices

Bash offers various operators for testing conditions within if statements. You can use [ ] (test command), [[ ]] (new test command), or (( )) (arithmetic evaluation).

File Test Operators:

• -f FILE: True if FILE exists and is a regular file.
• -d DIR: True if DIR exists and is a directory.
• -e PATH: True if PATH exists (file or directory).
• -r FILE: True if FILE exists and is readable.
• -w FILE: True if FILE exists and is writable.
• -x FILE: True if FILE exists and is executable.

String Test Operators:

• "STRING1" = "STRING2": True if the strings are equal.
• "STRING1" != "STRING2": True if the strings are not equal.
• -z "STRING": True if the string is empty.
• -n "STRING": True if the string is not empty.

Integer Test Operators:

• NUM1 -eq NUM2: True if NUM1 is equal to NUM2.
• NUM1 -ne NUM2: True if NUM1 is not equal to NUM2.
• NUM1 -gt NUM2: True if NUM1 is greater than NUM2.
• NUM1 -ge NUM2: True if NUM1 is greater than or equal to NUM2.
• NUM1 -lt NUM2: True if NUM1 is less than NUM2.
• NUM1 -le NUM2: True if NUM1 is less than or equal to NUM2.

For arithmetic comparisons, (( )) is generally preferred as it supports standard C-style operators (==, !=, >, <, >=, <=) and doesn't require quoting variables.

# Using [[ ... ]] for string and pattern matching
NAME="Alice"
if [[ "$NAME" == "Alice" ]]; then
    echo "Hello, Alice!"
fi

# Using (( ... )) for arithmetic comparisons
AGE=25
if (( AGE >= 18 && AGE <= 65 )); then
    echo "You are in the working age range."
fi

# Combining conditions with logical operators
# -a for AND, -o for OR within [ ]
# && for AND, || for OR within [[ ]] or (( ))
USER="admin"
if [ -f "/var/log/syslog" -a "$USER" == "admin" ]; then
    echo "Admin user and syslog exists."
fi

if [[ -d "/tmp" && "$USER" != "guest" ]]; then
    echo "/tmp exists and user is not guest."
fi

Examples of different test operators and combining conditions.