# Logging in the `wasmtime` CLI Wasmtime's libraries use Rust's [`log`] crate to log diagnostic information, and the `wasmtime` CLI executable uses [`pretty_env_logger`] by default for logging this information to the console. Basic logging is controlled by the `RUST_LOG` environment variable. For example, To enable logging of WASI system calls, similar to the `strace` command on Linux, set `RUST_LOG=wasi_common=trace`. ```sh $ RUST_LOG=wasi_common=trace wasmtime hello.wasm [...] TRACE wasi_common::hostcalls_impl::fs > fd_write(fd=1, iovs_ptr=0x10408, iovs_len=1, nwritten=0x10404) Hello, world! TRACE wasi_common::hostcalls_impl::fs > | *nwritten=14 TRACE wasi_common::hostcalls > | errno=ESUCCESS (No error occurred. System call completed successfully.) TRACE wasi_common::hostcalls_impl::misc > proc_exit(rval=1) ``` Wasmtime can also redirect the log messages into log files, with the `-D log-to-files` option. It creates one file per thread within Wasmtime, with the files named `wasmtime.dbg.*`. [`log`]: https://crates.io/crates/log [`pretty_env_logger`]: https://crates.io/crates/pretty_env_logger