# Tell them if you cannot compile! #error

The `#error` directive in C is used to generate a compilation error with a custom error message. This can help enforce certain conditions during compilation. Let's go through a detailed example that includes the use of `#error`, bash scripting, and environment variables.

### Example: Using #error with Bash and Environment Variables

```c
// main.c

#include <stdio.h>

// Uncomment the following line to enable a feature
#define ENABLE_FEATURE

#ifdef ENABLE_FEATURE
    // Perform feature-specific configuration
#else
    // Generate a compilation error if the feature is not enabled
    #error "Feature is not enabled. Please define ENABLE_FEATURE."
#endif

int main() {
    printf("Program execution after feature configuration.\n");
    return 0;
}
```

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

```bash
#!/bin/bash

# Uncomment the following line to enable the feature
# export ENABLE_FEATURE=true

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

In this example, the code includes a feature configuration block that is supposed to be enabled only when the `ENABLE_FEATURE` macro is defined. If `ENABLE_FEATURE` is not defined, a compilation error is generated using `#error` with a custom error message.

### Compilation and Execution

1. Initially, try to compile without defining `ENABLE_FEATURE`:
    
    ```bash
    chmod +x compile_and_run.sh
    ./compile_and_run.sh
    ```
    
    This will result in a compilation error with the specified message:
    
    ```bash
    main.c:12:2: error: #error "Feature is not enabled. Please define ENABLE_FEATURE."
     #error "Feature is not enabled. Please define ENABLE_FEATURE."
      ^~~~~
    ```
    
2. Uncomment the line `# export ENABLE_FEATURE=true` in the bash script and then try to compile again:
    
    ```bash
    ./compile_and_run.sh
    ```
    
    Now, the compilation should succeed, and the program can be executed:
    
    ```bash
    Program execution after feature configuration.
    ```
    

This example demonstrates how `#error` can be used to enforce certain conditions during compilation, and how it can be integrated with bash scripting and environment variables to control the compilation process based on predefined conditions.