The [`ffx log`](https://fuchsia.dev/reference/tools/sdk/ffx#log_2) commands can monitor and filter log messages from a Fuchsia device. ## Concepts The `ffx log` command connects directly to the target device to stream and display system logs. Logs are [symbolized](https://fuchsia.dev/docs/development/tools/ffx/workflows/symbolize-logs) as they are read from the device. By default, the `ffx log` command prints all device logs and leaves the connection open to continuously stream new logs from the target device. ## Monitor device logs To monitor device logs in real time, run the following command: ffx log This command prints output similar to the following: $ ffx log [2022-03-29 18:43:40.973][]: logger started. [0.000][klog][klog][I] printing enabled [0.000][klog][klog][I] INIT: cpu 0, calling hook 0xffffffff00177fc0 (elf_build_id) at level 0x1, flags 0x1 [0.000][klog][klog][I] ACPI LITE: Found valid RSDT table at physical address 0x7ffe21cc [0.000][klog][klog][I] PMM: boot reserve add [0x100000, 0x488fff] [0.000][klog][klog][I] PMM: boot reserve add [0x3502000, 0x46d7fff] [0.000][klog][klog][I] PMM: adding arena 0xffffffff00485128 name 'memory' base 0x100000 size 0x7fede000 [0.000][klog][klog][I] PMM: adding arena 0xffffffff00485128 name 'memory' base 0x100000000 size 0x180000000 [0.000][klog][klog][I] PMM: boot reserve marking WIRED [0x100000, 0x488fff] [0.000][klog][klog][I] PMM: boot reserve marking WIRED [0x3502000, 0x46d7fff] [0.000][klog][klog][I] UART: kernel serial enabled: port=0x3f8, irq=0x4 [0.000][klog][klog][I] UART: enabled with FIFO depth 64 [0.000][klog][klog][I] INIT: cpu 0, calling hook 0xffffffff00342380 (intel_rng_init) at level 0x20001, flags 0x1 [0.000][klog][klog][I] INIT: cpu 0, calling hook 0xffffffff0015dee0 (global_prng_seed) at level 0x20002, flags 0x1 [0.000][klog][klog][I] [0.000][klog][klog][I] welcome to Zircon [0.000][klog][klog][I] [0.000][klog][klog][I] KASLR: .text section at 0xffffffff00100000 [0.000][klog][klog][I] initializing arch pre-vm ... Press `CTRL+C` to exit. ## Filter device logs To filter device logs in real time, run the following command: > [!NOTE] > **Note:** The `--filter` flag also works with the [`dump`](https://fuchsia.dev/fuchsia-src/development/tools/ffx/workflows/view-device-logs#dump-device-logs) option. ffx log --filter Replace `STRING` with a string you want to use to filter the logs. Use quotation marks (`""`) if it contains spaces. The example below monitors device logs and only prints messages that contain the string `Hello, World!`: $ ffx log --filter "Hello, World!" [252.071][core/ffx-laboratory:hello_world][][I] Hello, World! Press `CTRL+C` to exit. A common use case for this feature is to select a component (for example, `hello_world` below) and monitor logs only from that component: $ ffx log --filter hello_world [177262.816][core/ffx-laboratory:hello_world][][I] Hello, World! ## Filter device logs using log levels To filter device logs based on their log level in real time, run the following command: > [!NOTE] > **Note:** The `--severity` flag also works with the [`dump`](https://fuchsia.dev/fuchsia-src/development/tools/ffx/workflows/view-device-logs#dump-device-logs) option. ffx log --severity Replace `LOG_LEVEL` with a log level you want to use to filter the logs. Supported log levels are `error`, `warn`, `info`, `debug`, and `trace`. The example below monitors device logs and only prints messages with the log level `WARN`: $ ffx log --severity warn ... [166547.984][core/network/netstack][netstack,DHCP][W] client.go(692): ethp0004: recv timeout waiting for dhcpOFFER; retransmitting dhcpDISCOVER [166564.788][core/network/netstack][netstack,DHCP][W] client.go(692): ethp0004: recv timeout waiting for dhcpOFFER; retransmitting dhcpDISCOVER [166567.571][core/wlancfg][wlancfg_lib::client::scan,wlan][W] Failed to get an SME proxy for scan: no client ifaces available [166567.571][core/session-manager/session:session/workstation_session/login_shell/ermine_shell][ermine][W] Error encountered during scan: MethodException: ScanErrorCode(1) [166568.746][core/detect][detect][W] Snapshot trigger was missing: ValueError: Division by zero [166568.749][core/detect][detect][W] Snapshot trigger was missing: ValueError: Division by zero Press `CTRL+C` to exit. ## Dump device logs The `ffx log dump` command prints all device logs in a session and exits immediately. To dump all device logs, run the following command: ffx log dump This command prints output similar to `ffx log`, but exits after printing the logs. ## Dump device logs from a previous session The `ffx log dump` command can print device logs from the target device's previous session. To dump device logs from a selected session, run the following command: ffx log dump Replace `SESSION_NUMBER` with a number with `~` as a prefix, for example, `~1`, `~2`, `~3,` and so on. The example below prints the device logs from the second previous session: $ ffx log dump ~2 A session starts on each booting of a device. For session numbers, consider `0` is reserved for the current, active session on the device. From there, `~1` is the most recent previous session (that is, the session before the last reboot), and `~2` is the second most recent previous session. ## Dump device logs from some time ago To dump device logs starting from some specific time in the past, run the following command: > [!NOTE] > **Note:** The `--since` flag also works without the `dump` option. ffx log --since