ocli

A simple, opinionated logger for command line tools

ocli aims at a very simple thing: logging for CLI tools done right. It uses the log crate and the ansi_term crate for colors. It provides very few configuration — at this time, just the expected log level.

Features

logs everything to stderr

CLI tools are expected to be usable in a pipe. In that context, the messages addressed to the user must be written on stderr to have a chance to be read by the user, independently of the log level. The program outputs that are meant to be used with a pipe shouldn't go through the logging system, but instead be printed to stdout, for example with println!.

shows the Info message as plain uncolored text

Info is expected to be the normal log level to display messages that are not highlighting a problem and that are not too verbose for a standard usage of the tool. Because it is intended for messages that are related to a normal situation, the messages of that level are not prefixed with the log level.

prefix the messages with their colored log level for any level other than Info

The color depends on the log level, allowing to quickly locate a message at a specific log level

displays the module path and line when configured at the Trace log level

for all the messages, even if they are not at the Trace log level. The Trace log level is used to help the developer understand where a message comes from, in addition to display a larger amount of messages.

disable all colorization in case the stderr is not a tty or when requested by NO_COLOR

so the output is not polluted with unreadable characters when stderr is redirected to a file. This crate uses the termcolor crate to color text, which disables colorization when NO_COLOR is set.

Example with Info log level

#[macro_use] extern crate log;

fn main() {
     ocli::init(log::Level::Info).unwrap();

     error!("This is printed to stderr, with the 'error: ' prefix colored in red");
     warn!("This is printed to stderr, with the 'warn: ' prefix colored in yellow");
     info!("This is printed to stderr, without prefix or color");
     debug!("This is not printed");
     trace!("This is not printed");
}

Example with Trace log level

#[macro_use] extern crate log;

fn main() {
     ocli::init(log::Level::Trace).unwrap();

     error!("This is printed to stderr, with the 'path(line): error: ' prefix colored in red");
     warn!("This is printed to stderr, with the 'path(line): warn: ' prefix colored in yellow");
     info!("This is printed to stderr, with the 'path(line): info: ' prefix");
     debug!("This is printed to stderr, with the 'path(line): debug: ' prefix colored in blue");
     trace!("This is printed to stderr, with the 'path(line): trace: ' prefix colored in magenta");
}

and a more realistic example with hld:

Example with clap integration

The log level can be configured with a command line argument, using the clap crate. Try running it with cargo run --example cli -- --log-level trace and change the log level to see the difference.

#[macro_use]
extern crate log;

use clap::Parser;

/// A demo of ocli with clap
#[derive(Parser, Debug)]
#[command(author, version, about, long_about = None)]
pub struct Config {
    /// Log level
    #[arg(short, long, default_value_t = log::Level::Info)]
    pub log_level: log::Level,
}

fn main() {
    let config = Config::parse();
    ocli::init(config.log_level).unwrap();

    println!("this is on stdout — try to pipe it to another command like `grep` or `wc`");
    error!("log at error level on stderr");
    warn!("log at warn level on stderr");
    info!("log at info level on stderr");
    debug!("log at debug level on stderr");
    trace!("log at trace level on stderr");
    info!("the logs at any level are meant to inform the user");
    info!("while still being able to pipe stdout");
}

Example with clap-verbosity-flag integration

The log level can be configured with a command line argument, using the clap-verbosity-flag crate. Try running it with cargo run --example verbosity -- -vv or cargo run --example verbosity -- -q. Changing the number of -v or -q changes the log level.

#[macro_use]
extern crate log;

use clap::Parser;
use clap_verbosity_flag::{InfoLevel, Verbosity};

/// A demo of ocli with clap
#[derive(Parser, Debug)]
#[command(author, version, about, long_about = None)]
pub struct Config {
    /// Log level
    #[command(flatten)]
    pub verbose: Verbosity<InfoLevel>,
}

fn main() {
    let config = Config::parse();
    if let Some(level) = config.verbose.log_level() {
        ocli::init(level).unwrap();
    }
    println!("this is on stdout — try to pipe it to another command like `grep` or `wc`");
    error!("log at error level on stderr");
    warn!("log at warn level on stderr");
    info!("log at info level on stderr");
    debug!("log at debug level on stderr");
    trace!("log at trace level on stderr");
    info!("the logs at any level are meant to inform the user");
    info!("while still being able to pipe stdout");
}

License

ocli is distributed under the terms of the MIT license.

See LICENSE for details.