Modified

Sometimes it is helpful to compute a timestamp of when a file was modified. In Rust programs this could be used to determine if a file needs to be parsed again.

With fs metadata, we can access information from the filesystem. And we can access the modified time. In Rust, this is a SystemTime, which we can convert to seconds with duration_since.

Example

Here we introduce the get_modified_secs function. This receives a path to a file, and it returns a usize that indicates the seconds since it was modified on the disk.

Start In get_modified_secs we panic if we cannot get the metadata for the file. Then we call modified() in a function chain.

Info With duration_since, we compute the number of seconds since the start of the UNIX time. Then we call as_secs() to get a u64 number.

use std::fs;

pub fn get_modified_secs(file: &str) -> usize {
    // Get modification timestamp from file.
    let meta = fs::metadata(file).expect("Need metadata");
    let secs = meta
        .modified()
        .expect("Need modified date")
        .duration_since(std::time::UNIX_EPOCH)
        .expect("Need duration")
        .as_secs();
    secs.try_into().unwrap()
}

fn main() {
    let modified = get_modified_secs("programs/example.txt");
    println!("MODIFIED TIMESTAMP: {}", modified);
}
MODIFIED TIMESTAMP: 1692807807

Some notes

In Rust we have to specify some of our logic carefully—we must compute a duration to get a figure in seconds. The result is an accurate timestamp for the modified time for the file.

Summary

By computing a modified timestamp, we can more effectively determine the freshness of files on disk. And we can then update other files only as needed (as an optimization).