Snakemake新手求助：作业重运行规则含义解析

Hey there! I totally get why this line from the Snakemake docs might feel confusing when you're just starting out—let's break it down into simple, actionable terms with examples so it clicks.

First, let's restate the official rule for clarity:

Snakemake only re-runs jobs if one of the input files is newer than one of the output files or one of the input files will be updated by another job

This is Snakemake's core logic for deciding whether a rule needs to run—it's all about keeping your outputs in sync with their dependencies. Let's unpack each scenario:

1. When an input file is newer than an output file #

Snakemake relies heavily on file modification timestamps to track freshness. Here's how this works:

• Suppose you have a rule that takes raw_data.csv as input and produces cleaned_data.csv as output.
• If you edit raw_data.csv (making its modification timestamp later than cleaned_data.csv), Snakemake will detect this mismatch. It knows your output is outdated because the source data has changed, so it will re-run the rule to generate a fresh cleaned_data.csv.
• This also applies if you delete the output file entirely—since there's no output to compare, Snakemake will run the rule to create it from scratch.

2. When an input file will be updated by another job #

This is about Snakemake handling dependency chains between rules. Let's use a two-rule example:

• rule prepare_data generates intermediate_data.txt from raw_data.txt
• rule analyze_data takes intermediate_data.txt as input to produce final_report.pdf

If intermediate_data.txt doesn't exist, or if raw_data.txt is newer than intermediate_data.txt, Snakemake first knows it needs to run prepare_data to update intermediate_data.txt. Since analyze_data's input (intermediate_data.txt) is going to be updated by another job, Snakemake will mark analyze_data to re-run after prepare_data finishes. This ensures your final report always uses the latest intermediate data.

The key takeaway here is that Snakemake doesn't run rules unnecessarily—it only does work when it detects that outputs are either outdated or dependent on something that's about to be updated.

内容的提问来源于stack exchange，提问作者Anlin Li