systemd 启动与关机故障:收集证据,并撤销调试入口

机器在启动时停住,首先要弄清楚:控制权是否已经交给 systemd?如果故障发生在更早的阶段,排查方向就不同;如果 systemd 已经运行,则应尽量取得 shell,读取它正在等待的任务和留下的日志。

本文完整翻译 systemd 官方诊断指南的主要流程与命令,并把涉及真实系统状态的操作标在上下文中。原文是滚动文档,仍含2012年的输出格式和发行版相关路径;这里保留这些信息,同时说明它们的适用边界。所有命令都只作静态审阅,本次未修改任何系统服务、安全设置或启动参数。

systemd诊断流程:判断故障阶段,读取journal和任务状态,必要时临时进入调试环境,最后撤销入口并恢复设置。
编辑原创技术示意图。诊断结束时的恢复工作也是流程的一部分。

先确认 systemd 是否已经运行

可在下一次启动时临时去掉内核命令行中的 rhgb 和 quiet,让启动信息显示出来。如果看到了类似“Welcome to Fedora VERSION (codename)!”、“Starting name…”或“[ OK ] Started name.”的消息,就说明 systemd 已经开始工作。rhgb 及 Fedora 提示属于原文所用发行版的示例,不是所有 Linux 系统的共同界面。原文还提供了一张历史启动截图供对照。

如果没有登录提示,试着按 Ctrl+Alt+相应功能键切换虚拟终端。显示服务器启动故障可能让 tty1 没有可用登录界面,但其他虚拟终端仍然可用。

当所有虚拟控制台都没有登录提示时,原文建议先等最多约5分钟,再判断是否确实卡死。某个启动失败的服务可能在超时后被终止,随后启动继续;某个重要挂载点所需设备一直不出现,也可能最终触发 emergency 模式。这是诊断等待建议,并不意味着所有服务或所有版本都使用5分钟超时。

完全拿不到 shell 时

如果普通登录和 emergency shell 都不可用,需要换一种方式把诊断信息导出。可以先观察 Ctrl+Alt+Del 是否能重启;若没有反应,将这一现象记入问题报告。原文还提到 SysRq 或硬复位作为最后手段。强制复位可能造成未落盘数据丢失或文件系统损坏,只应在受控维护场景中使用,不能把它当作日常排障的第一步。

下一次启动时,根据所选诊断方式临时添加相应内核命令行参数。修改前先保留原参数和可用的回退启动项。

把调试日志送到串口控制台

若有硬件串口,或在虚拟机中有可连接的串口控制台,可以让 systemd 输出详细日志。以 virt-manager 为例,可从 View → Text Consoles 切换,或使用 virsh console MACHINE。原文给出的参数为:

systemd.log_level=debug systemd.log_target=console console=ttyS0,38400 console=tty1

这组参数适合观察 PID 1 自身的失败。若出问题的是较晚启动但很关键的服务,例如网络服务,则可以让 journald 把日志转发到控制台:

systemd.journald.forward_to_console=1 console=ttyS0,38400 console=tty1

console= 可以出现多次;原文使用 ttyS0、38400波特率和 tty1。应按实际串口设备和终端配置调整,不能把这里的设备名与速率当作通用值。大量调试日志还可能改变时序,记录复现条件时应注明。

进入 rescue 或 emergency 目标

在内核命令行中加入 systemd.unit=rescue.target,或者简写 1,可以直接进入 rescue 目标。它适合基本系统已经建立、启动普通服务时才发生的问题;在那里可以调查并按需要禁用有故障的服务。如果 rescue 也启动不了,可以尝试更精简的 emergency 目标。

加入 systemd.unit=emergency.target 或 emergency 可进入 emergency shell。原文提醒,在这个环境中编辑文件之前,可能需要自行把根文件系统重新挂载为可写:

mount -o remount,rw /

编辑补充:这条命令会改变根文件系统的挂载状态,应先确认根文件系统确实适合写入;如果怀疑存在损坏,优先按文件系统及发行版恢复流程处理。常见可排查问题是 /etc/fstab 中的错误条目。确认并修复后,运行 systemctl daemon-reload,使 systemd 重新读取由此产生的配置。

如果连 emergency 目标都不可用,原文建议用 init=/bin/sh 直接启动 shell;当 systemd 本身或其依赖库因文件系统损坏而不可用时,这可能有帮助,并可能需要重新安装受影响的软件包。如果它也不起作用,就需要从其他介质启动。上述路径都属于系统恢复操作,需确保有受控的控制台和恢复介质。

临时启用早期调试 shell

调试 shell 可以在启动很早的阶段就提供访问入口,供人使用 systemctl 查询状态、读取日志、执行 systemctl list-jobs 等诊断操作。原文给出两种启用方式:

systemctl enable debug-shell.service

或者在内核命令行加入:

systemd.debug_shell=1

如果无法通过 systemctl 与运行中的管理器通信,例如正在另一个已启动的系统中准备恢复,可以使用 --root=,使操作针对指定文件系统树而无需联系该管理器:

systemctl --root=/ enable debug-shell.service

这里的 / 是原文示例。在救援系统中处理另一个已挂载的系统时,必须换成那个目标系统的挂载根目录;否则可能修改救援系统自身的配置。下一次启动后,通常可按 Ctrl+Alt+F9 切换到 tty9,使用早期就存在的 root shell。

此入口有明确的安全风险:它提供无需普通登录认证的 root shell,只能短时用于调试。完成后要停止并禁用实际启用的服务,还要移除临时内核参数。原文警告段写成了 systemd-debug-shell.service,但前面的启用命令为 debug-shell.service;这里按启用命令给出一致的撤销示例,并要求先核对本机单元名:

systemctl disable --now debug-shell.service
systemctl is-enabled debug-shell.service
systemctl is-active debug-shell.service

以上为编辑补充:检查结果应与已禁用、已停止的目标状态一致。若入口来自 systemd.debug_shell=1,还必须从启动项移除该参数,并在之后的正常启动中确认入口没有再次出现。原文还提到把 kbrequest.target 关联到 debug-shell.service,以按需启动;它能避免一直运行 shell,但并不会消除 root 入口的风险。

核对内核与设备前提

systemd 需要至少部分已建立的 /dev。在嵌入式等自行配置内核的环境中,原文建议检查 CONFIG_DEVTMPFS、CONFIG_DEVTMPFS_MOUNT,并检查 CONFIG_CGROUPS 与 CONFIG_FANOTIFY 支持。执行某些 systemctl 操作时出现“Failed to get D-Bus connection: No connection to service manager.”,可作为这些前提不满足的线索。

编辑补充:相同报错也可能来自没有运行 systemd 的环境、容器边界或通信问题,不能只凭这一行消息断言内核配置缺失。应结合实际内核、运行环境与日志判断。

能够取得 shell 时

当 systemd 已运行到能提供 shell 的程度,应优先把可读证据导出。为增加日志细节,原文建议使用以下临时启动参数:

systemd.log_level=debug systemd.log_target=kmsg log_buf_len=1M printk.devkmsg=on

这些参数依次提高 systemd 日志级别、把日志写入内核日志缓冲区、增大缓冲区,并避免内核丢弃相应消息。进入 shell 后读取本次启动日志:

journalctl -b

问题报告需要保留完整上下文,可把输出重定向到文件。要找可能卡住的任务,使用:

systemctl list-jobs

列表中标为 running 的任务还在执行;依赖它们的 waiting 任务,要等相应工作完成后才能继续。此时先记录任务名、等待关系和日志,不必立即重启服务或清理状态。

诊断关机卡顿

关机和启动一样,先留出时间区分永久卡住与服务超时。原文建议至少等待约5分钟,并观察 Ctrl+Alt+Del 是否有任何效果。

原文随后给出强制重启或关机的内核能力测试:

reboot -f
poweroff -f

这两条是二选一的高风险操作,不是应当顺序执行的采证命令。它们可能绕过正常服务退出与文件系统同步,造成数据丢失;只能在已保存证据、具备恢复条件的维护场景中考虑。如果强制方式也无效,故障更可能涉及内核层,但这仍只是缩小范围的线索,不能据此完成归因。

最终能够关机,但耗时异常

如果正常 reboot 或 poweroff 最终会完成,只是非常慢,原文建议用下面的参数启动:

systemd.log_level=debug systemd.log_target=kmsg log_buf_len=1M printk.devkmsg=on enforcing=0

编辑补充:末尾的 enforcing=0 会让启用 SELinux 的系统降低强制访问控制,应单独评估是否确有需要,限定在隔离诊断环境,并在完成后恢复。不能把整行不加判断地复制成生产默认启动参数。

原文接着要求把以下脚本保存为 /usr/lib/systemd/system-shutdown/debug.sh,赋予可执行权限,然后重启:

#!/bin/sh
mount -o remount,rw /
dmesg > /shutdown-log.txt
mount -o remount,ro /

该脚本在关机后期把根文件系统重挂载为可写,将 dmesg 写到 /shutdown-log.txt,再重挂载为只读。完成后可在日志中查找超时,或将其用于问题报告。实际的 system-shutdown 钩子目录、根文件系统布局和可写条件应按发行版核对;这段脚本不包含失败处理,不应当作适合所有主机的可靠采集器。诊断结束后移除临时钩子,并撤销调试启动参数。

关机始终不能结束

如果等待数分钟仍无法完成,系统可能根本到不了上述关机钩子的阶段,因此不能指望 shutdown-log.txt 出现。此时可改用串口控制台,或受控地使用调试 shell;它不但在早期启动阶段可用,也能存活到较晚的关机阶段。调试入口的撤销要求仍然适用。

服务状态与日志在哪里

启动服务失败时,systemctl 可能只给出笼统提示。以下是原文的示例会话,foo.service 是占位服务名;不是要求在本机启动这个服务:

# systemctl start foo.service
Job failed. See system journal and 'systemctl status' for details.

服务由 systemd 启动,并不属于当前登录会话,因此它打印的内容不会自然出现在当前终端。这不表示输出丢失了:在原文描述的默认安排下,标准输出、标准错误以及通过 syslog(3) 产生的日志进入 journal,systemd 还保留失败服务的退出码。

# systemctl status foo.service
foo.service - mmm service
Loaded: loaded (/etc/systemd/system/foo.service; static)
Active: failed (Result: exit-code) since Fri, 11 May 2012 20:26:23 +0200; 4s ago
Process: 1329 ExecStart=/usr/local/bin/foo (code=exited, status=1/FAILURE)
CGroup: name=systemd:/system/foo.service

May 11 20:26:23 scratch foo[1329]: Failed to parse config

这个历史示例中的进程 PID 为1329,退出码为1,日志说明配置解析失败。日期明确是2012年5月11日,输出格式和 cgroup 路径不代表今天所有系统的格式。原文说 root 或 adm 组用户可在状态输出中看到若干日志;实际读取权限需以本机日志访问控制为准。

journalctl 可以读取更多日志。如果系统还运行 rsyslog 一类 syslog 服务,且配置了相应转发,日志可能同时进入 /var/log/messages。是否转发、写入何处取决于本机 journald 与 rsyslog 配置,不能因为文件不存在就认为服务没有日志。

组织一份可用的问题报告

原文建议先向发行版的问题跟踪器报告。若已有证据表明是上游 systemd 问题,先搜索 现有报告,确认没有相同问题后再提交新报告。日志应保留完整上下文,按上游要求提供未压缩的文件,而不是只截取几行或打成压缩包。

尽可能提供实际内核命令行,可从启动加载器配置(原文举例 /boot/grub2/grub.cfg)或 /proc/cmdline 获取;同时附上当前启动 journal、systemd 状态转储和测试模式输出。原文命令为:

journalctl -b > journal.txt
systemd-analyze dump > systemd-dump.txt
/usr/lib/systemd/systemd --test --system --log-level=debug > systemd-test.txt 2>&1

最好在前述 debug 日志参数下复现后采集。/usr/lib/systemd/systemd 的位置及测试模式限制需按本机版本核对;输出重定向会创建或覆盖指定文件。提交之前在副本中删除凭据、私人路径、内部地址等敏感信息,并清楚标出删改位置,尽量保留时间、错误和依赖关系的可分析性。不要因为报告要求“完整”就公开秘密。

最后确认调试 shell 已停止且禁用,临时启动参数和关机采集脚本已撤销,安全设置已恢复。采证、报告和恢复状态一起完成,才算结束这次诊断。

来源与许可

原文:Diagnosing Boot Problems;文档源码。单页未署个人作者与发布日期,归属 systemd 项目文档贡献者;页脚标注 © systemd, 2026。HTML 与文档源码的 SPDX 为 LGPL-2.1-or-later,本中文改编延续该许可,修改日期2026年10月5日;完整许可文本另附于 LICENSE.txt。新增提示与原创示意图已在文内明确标识。

版权与许可全文

以下保留本页涉及的来源材料或示例代码的版权、许可条件与免责声明;各自适用范围依原声明。中文翻译及编辑标注:未完纪,2026-10-05。

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This section is intended to make thoroughly clear what is believed to be a consequence of the rest of this License.

12. If the distribution and/or use of the Library is restricted in certain countries either by patents or by copyrighted interfaces, the original copyright holder who places the Library under this License may add an explicit geographical distribution limitation excluding those countries, so that distribution is permitted only in or among countries not thus excluded. In such case, this License incorporates the limitation as if written in the body of this License.

13. The Free Software Foundation may publish revised and/or new versions of the Lesser General Public License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns.

Each version is given a distinguishing version number. If the Library specifies a version number of this License which applies to it and "any later version", you have the option of following the terms and conditions either of that version or of any later version published by the Free Software Foundation. If the Library does not specify a license version number, you may choose any version ever published by the Free Software Foundation.

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NO WARRANTY

15. BECAUSE THE LIBRARY IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY FOR THE LIBRARY, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES PROVIDE THE LIBRARY "AS IS" WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE LIBRARY IS WITH YOU. SHOULD THE LIBRARY PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING, REPAIR OR CORRECTION.

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END OF TERMS AND CONDITIONS

How to Apply These Terms to Your New Libraries

If you develop a new library, and you want it to be of the greatest possible use to the public, we recommend making it free software that everyone can redistribute and change. You can do so by permitting redistribution under these terms (or, alternatively, under the terms of the ordinary General Public License).

To apply these terms, attach the following notices to the library. It is safest to attach them to the start of each source file to most effectively convey the exclusion of warranty; and each file should have at least the "copyright" line and a pointer to where the full notice is found.

     one line to give the library's name and an idea of what it does.
     Copyright (C) year  name of author

     This library is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation; either version 2.1 of the License, or (at your option) any later version.

     This library is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public License for more details.

     You should have received a copy of the GNU Lesser General Public License along with this library; if not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA Also add information on how to contact you by electronic and paper mail.

You should also get your employer (if you work as a programmer) or your school, if any, to sign a "copyright disclaimer" for the library, if necessary. Here is a sample; alter the names:

Yoyodyne, Inc., hereby disclaims all copyright interest in
the library `Frob' (a library for tweaking knobs) written
by James Random Hacker.

signature of Ty Coon, 1 April 1990
Ty Coon, President of Vice
That's all there is to it!
© 版权声明
THE END
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