1# Logging in the `wasmtime` CLI 2 3Wasmtime's libraries use Rust's [`log`] crate to log diagnostic 4information, and the `wasmtime` CLI executable uses [`pretty_env_logger`] 5by default for logging this information to the console. 6 7Basic logging is controlled by the `WASMTIME_LOG` environment variable. For example, 8To enable logging of WASI system calls, similar to the `strace` command on Linux, 9set `WASMTIME_LOG=wasmtime_wasi=trace`. 10 11```sh 12$ WASMTIME_LOG=wasmtime_wasi=trace hello.wasm 13[...] 14TRACE wiggle abi{module="wasi_snapshot_preview1" function="fd_write"} wasmtime_wasi::preview1::wasi_snapshot_preview1 > fd=Fd(1) iovs=*guest 0x14/1 15Hello, world! 16TRACE wiggle abi{module="wasi_snapshot_preview1" function="fd_write"}: wasmtime_wasi::preview1::wasi_snapshot_preview1: result=Ok(14) 17TRACE wiggle abi{module="wasi_snapshot_preview1" function="proc_exit"}: wasmtime_wasi::preview1::wasi_snapshot_preview1: rval=1 18TRACE wiggle abi{module="wasi_snapshot_preview1" function="proc_exit"}: wasmtime_wasi::preview1::wasi_snapshot_preview1: result=Exited with i32 exit status 1 19``` 20 21Wasmtime can also redirect the log messages into log files, with the 22`-D log-to-files` option. It creates one file per thread within Wasmtime, with 23the files named `wasmtime.dbg.*`. 24 25[`log`]: https://crates.io/crates/log 26[`pretty_env_logger`]: https://crates.io/crates/pretty_env_logger 27