# Pros use Conditional Compilation

Let's explore conditional compilation in C with detailed examples, including the use of `#ifdef`, `#ifndef`, `#else`, `#endif`, and `#undef`. We'll also incorporate bash scripting and environment variables to demonstrate dynamic control over compilation.

### Example: Simple Conditional Compilation

```c
// main.c

#include <stdio.h>

// Uncomment the following line to enable debugging
#define DEBUG

int main() {
#ifdef DEBUG
    printf("Debugging is enabled.\n");
#else
    printf("Debugging is disabled.\n");
#endif

    return 0;
}
```

In this example, the `#ifdef DEBUG` checks if the symbol `DEBUG` is defined. If defined, the code within the `#ifdef` block is included during compilation; otherwise, the code within the `#else` block is included. If the `DEBUG` line is uncommented, the output will be:

```bash
Debugging is enabled.
```

### Example: Using Bash and Environment Variables

```c
// main.c

#include <stdio.h>

#ifdef ENABLE_FEATURE
#define FEATURE "Feature is enabled."
#else
#define FEATURE "Feature is disabled."
#endif

int main() {
    printf("%s\n", FEATURE);
    return 0;
}
```

Bash script (`compile_and_`[`run.sh`](http://run.sh)):

```bash
#!/bin/bash

# Enable or disable the feature by setting the environment variable
export ENABLE_FEATURE=true

# Compile the C code
gcc main.c -o my_program

# Run the compiled program
./my_program
```

If you run the bash script with `ENABLE_FEATURE=true`, the output will be:

```bash
Feature is enabled.
```

If you run the bash script with `ENABLE_FEATURE=false`, the output will be:

```bash
Feature is disabled.
```

This demonstrates how you can control the compilation behavior using environment variables.

### Example: Conditional Compilation with #else and #undef

```c
// main.c

#include <stdio.h>

// Uncomment the following line to enable debugging
#define DEBUG

int main() {
#ifdef DEBUG
    printf("Debugging is enabled.\n");
#else
    printf("Debugging is disabled.\n");
#endif

#ifdef RELEASE
    printf("Release mode is enabled.\n");
#else
    printf("Release mode is disabled.\n");
#endif

// Undefine DEBUG for the rest of the program
#undef DEBUG

#ifdef DEBUG
    printf("This line won't be printed because DEBUG is undefined.\n");
#endif

    return 0;
}
```

In this example, `DEBUG` is initially defined, but then it's undefined using `#undef DEBUG`. After the `#undef` directive, attempting to use `DEBUG` will result in a compilation error.

### Compilation and Execution

To compile and run these examples, you can use the following bash commands:

```bash
# Compile and run Example 1
gcc main.c -o example1
./example1

# Compile and run Example 2
chmod +x compile_and_run.sh
./compile_and_run.sh

# Compile and run Example 3
gcc main.c -o example3
./example3
```

These examples illustrate how conditional compilation can be utilized in different scenarios, allowing you to control the compilation process based on preprocessor directives and external factors such as environment variables.