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

// 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):

#!/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:

    chmod +x compile_and_run.sh
    ./compile_and_run.sh
   

   This will result in a compilation error with the specified message:

    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:

    ./compile_and_run.sh
   

   Now, the compilation should succeed, and the program can be executed:

    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.