如何实现SCons项目中动态头文件每日仅重建一次？

Great question—this is a common pain point with SCons when you need a generated file to update on a schedule (like daily) instead of every build or never. Let’s walk through how to make your current_time.h regenerate only once per day, without the heavy overhead of AlwaysBuild().

The Problem Recap #

Your current setup uses a Command() with no dependencies, so SCons has no way to detect when the date changes. Adding AlwaysBuild() forces a rebuild every time, which is too costly for real projects.

Here’s your original code for reference:

SConstruct #

current_time = Command("current_time.h", None, "echo '#define CURRENT_TIME' `date +%H%M` > $TARGET")
test = Program("test", "test.cpp", current_time)
# AlwaysBuild(current_time)

test.cpp #

#include <iostream>
#include "current_time.h"
int main() {
    std::cout << "the time is: " << CURRENT_TIME << std::endl;
}

The Solution: Use a Daily Unique Dependency #

The key is to give SCons a dependency that only changes once per day. We’ll use SCons’ Value() function to create a dependency based on the current date, so SCons will only regenerate current_time.h when the date rolls over.

Step 1: Create a Function to Get the Daily Identifier #

First, write a small function that returns a string representing the current date (not time—since we only care about daily changes). This will be our trigger for rebuilds:

def get_daily_identifier():
    # Get current date in YYYYMMDD format (unique per day)
    import subprocess
    result = subprocess.run(["date", "+%Y%m%d"], capture_output=True, text=True)
    return result.stdout.strip()

Step 2: Update the Command with the Daily Dependency #

Replace the None dependency with Value(get_daily_identifier()). This tells SCons: "Only run this command if the daily identifier has changed since the last build."

Here’s the updated SConstruct:

def get_daily_identifier():
    import subprocess
    result = subprocess.run(["date", "+%Y%m%d"], capture_output=True, text=True)
    return result.stdout.strip()

# Use the daily identifier as a dependency
daily_trigger = Value(get_daily_identifier())
current_time = Command(
    "current_time.h",
    daily_trigger,
    "echo '#define CURRENT_TIME' `date +%H%M` > $TARGET"
)
test = Program("test", "test.cpp", current_time)

Why This Works #

• Value(get_daily_identifier()) creates a virtual dependency whose "content" is the current date string. SCons checks this value every build.
• When the date changes (e.g., from 20240520 to 20240521), SCons detects the dependency has changed and runs the Command() to regenerate current_time.h.
• On subsequent builds the same day, the identifier stays the same, so SCons skips regenerating the header entirely—no unnecessary build overhead.

Optional: Simplify with Inline Command (If Preferred) #

If you prefer a more concise version, you can inline the date command directly in Value() (though the function approach is cleaner for readability/maintenance):

import subprocess

current_time = Command(
    "current_time.h",
    Value(subprocess.run(["date", "+%Y%m%d"], capture_output=True, text=True).stdout.strip()),
    "echo '#define CURRENT_TIME' `date +%H%M` > $TARGET"
)
test = Program("test", "test.cpp", current_time)

内容的提问来源于stack exchange，提问作者hochl