journalctl Trình tạo lệnh

## What is journalctl?

`journalctl` queries the systemd journal — the centralized log store for the system and services. It is the standard way to read service logs, filter by unit, priority, boot, or time range.

## How to Use

1. **Filter**: Narrow by unit (`-u`), priority (`-p`), boot (`-b`), or kernel (`-k`). 2. **Time Window**: Use `--since` and `--until` with values like `today`, `yesterday`, or `2026-06-25 09:00:00`. 3. **Output**: Tail with `-f`, limit lines with `-n`, format with `-o`. 4. **Copy & Run**: Copy the generated command into your terminal.

### `-u` Unit Show messages from a specific systemd unit (service), e.g. `-u nginx` or `-u ssh.service`.

### `-p` Priority Filter by syslog priority 0–7 (0 emerg, 1 alert, 2 crit, 3 err, 4 warning, 5 notice, 6 info, 7 debug). Use `-p 3` to show errors and above.

### `-b` Current boot Show messages from the current boot. Combine with an index like `-b -1` for the previous boot.

### `-k` Kernel Show only kernel messages (equivalent to `dmesg`).

### `-n` Lines Show the last N lines, e.g. `-n 100`.

### `-f` Follow Continuously print new entries as they arrive (like `tail -f`).

### `-a` Show all Show all fields, including very long ones normally truncated.

### `--no-pager` Disable the pager for piping into other commands or scripts.

### `--since` / `--until` Filter by time, e.g. `--since today`, `--since "1 hour ago"`, or `--since "2026-06-25 09:00:00"`.

### `-x` Catalog Add explanatory catalog entries for many log messages.

### `-r` Reverse Show newest entries first.

### `-o` Output format Change the output format: `cat` (message only), `short` (default), `json`, `json-pretty`, etc.