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man: add note that emergency.target inherits mount state

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Based on an internal discussion whether emergency.target should remount disks
ro, or maybe remount them rw, or do nothing. In some cases people want to boot
ro, and always remounting rw would break that. In other cases, remounting disks
ro after they have already been mounted rw is mostly pointless and might even
not be possible. So let's just document that we don't change the state.

Also: any→other, since emergency.service *is* pulled in.

Also: just advertise "emergency" as the way to boot into the target.
We are not going to remove this option, and it's way easier to type than
"systemd.unit=emergency.target".
This commit is contained in:
Zbigniew Jędrzejewski-Szmek 2020-06-02 14:26:26 +02:00
parent 41a7c3bf5d
commit 927b9b8f63
1 changed files with 11 additions and 8 deletions
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19
man/systemd.special.xml
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@ -228,22 +228,25 @@
<term><filename>emergency.target</filename></term>
<listitem>
<para>A special target unit that starts an emergency shell on the main console. This
target does not pull in any services or mounts. It is the most minimal version of
target does not pull in other services or mounts. It is the most minimal version of
starting the system in order to acquire an interactive shell; the only processes running
are usually just the system manager (PID 1) and the shell process. This unit is supposed
to be used with the kernel command line option <varname>systemd.unit=</varname>; it is
also used when a file system check on a required file system fails, and boot-up cannot
are usually just the system manager (PID 1) and the shell process. This unit may be used
by specifying <varname>emergency</varname> on the kernel command line; it is
also used when a file system check on a required file system fails and boot-up cannot
continue. Compare with <filename>rescue.target</filename>, which serves a similar
purpose, but also starts the most basic services and mounts all file systems.</para>
<para>Use the <literal>systemd.unit=emergency.target</literal> kernel command line
option to boot into this mode. A short alias for this kernel command line option is
<literal>emergency</literal>, for compatibility with SysV.</para>
<para>In many ways booting into <filename>emergency.target</filename> is similar to the
effect of booting with <literal>init=/bin/sh</literal> on the kernel command line,
except that emergency mode provides you with the full system and service manager, and
allows starting individual units in order to continue the boot process in steps.</para>
<para>Note that depending on how <filename>emergency.target</filename> is reached, the root file
system might be mounted read-only or read-write (no remounting is done specially for this
target). For example, the system may boot with root mounted read-only when <varname>ro</varname>
is used on the kernel command line and remain this way for <filename>emergency.target</filename>,
or the system may transition to <filename>emergency.target</filename> after the system has been
partially booted and disks have already been remounted read-write.</para>
</listitem>
</varlistentry>
<varlistentry>