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 [`tracing-subscriber`] 5for displaying this information on 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`. For more information on specifying 10filters, see [tracing-subscriber's EnvFilter docs]. 11 12```sh 13$ WASMTIME_LOG=wasmtime_wasi=trace wasmtime hello.wasm 14[...] 15TRACE wiggle abi{module="wasi_snapshot_preview1" function="fd_write"} wasmtime_wasi::preview1::wasi_snapshot_preview1 > fd=Fd(1) iovs=*guest 0x14/1 16Hello, world! 17TRACE wiggle abi{module="wasi_snapshot_preview1" function="fd_write"}: wasmtime_wasi::preview1::wasi_snapshot_preview1: result=Ok(14) 18TRACE wiggle abi{module="wasi_snapshot_preview1" function="proc_exit"}: wasmtime_wasi::preview1::wasi_snapshot_preview1: rval=1 19TRACE wiggle abi{module="wasi_snapshot_preview1" function="proc_exit"}: wasmtime_wasi::preview1::wasi_snapshot_preview1: result=Exited with i32 exit status 1 20``` 21 22Wasmtime can also redirect the log messages into log files, with the 23`-D log-to-files` option. It creates one file per thread within Wasmtime, with 24the files named `wasmtime.dbg.*`. 25 26[`log`]: https://crates.io/crates/log 27[`tracing-subscriber`]: https://crates.io/crates/tracing-subscriber 28[tracing-subscriber's EnvFilter docs]: https://docs.rs/tracing-subscriber/latest/tracing_subscriber/filter/struct.EnvFilter.html#directives 29