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@ -43,3 +43,41 @@ A typical Linux system has, among others, the following directories:
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| `/var` | This directory contains files which may change in size, such as spool and [log](../dev/Log.md) files. |
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| `/var/cache` | Data cached for programs. |
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| `/var/log` | Miscellaneous [log](../dev/Log.md) files. |
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## Kernel Commandline
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The kernel, the programs running in the initrd and in the host system may be configured at boot via kernel command line arguments.
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The current cmdline can be seen at `/proc/cmdline`.
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For setting the cmdline use `/etc/kernel/cmdline` if you use UKIs.
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**Common Kernel Cmdline Arguments:**
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| Argument | Description |
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| ------------------------------------------------------------------------------------------------------ | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ |
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| `quiet` | Parameter understood by both the kernel and the system and service manager to control console log verbosity. |
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| `splash` | Show a plymouth splash screen while booting. |
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| `init=` | This sets the initial command to be executed by the kernel. If this is not set, or cannot be found, the kernel will try `/sbin/init`, then `/etc/init`, then `/bin/init`, then `/bin/sh` and panic if all of this fails. |
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| `ro` and `rw` | The `ro` option tells the kernel to mount the root filesystem as 'read-only'. The `rw` option tells the kernel to mount the root filesystem read/write. This is the default. |
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| `resume=...` | This tells the kernel the location of the suspend-to-disk data that you want the machine to resume from after hibernation. Usually, it is the same as your swap partition or file. Example: `resume=/dev/hda2` |
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| `panic=N` | By default, the kernel will not reboot after a panic, but this option will cause a kernel reboot after `N` seconds (if `N` is greater than zero). This panic timeout can also be set by `echo N > /proc/sys/kernel/panic` |
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| `plymouth.enable=` | May be used to disable the Plymouth boot splash. For details, see plymouth. |
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| `vconsole.keymap=, vconsole.keymap_toggle=, vconsole.font=, vconsole.font_map=, vconsole.font_unimap=` | Parameters understood by the virtual console setup logic. For details, see `vconsole.conf` |
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| `luks=, rd.luks=` | Defaults to "yes". If "no", disables the crypt mount generator entirely. `rd.luks=` is honored only in the initrd while `luks=` is honored by both the main system and in the initrd. |
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| `luks.crypttab=, rd.luks.crypttab=` | Defaults to "yes". If "no", causes the generator to ignore any devices configured in `/etc/crypttab` (`luks.uuid=` will still work however). `rd.luks.crypttab=` is honored only in initrd while `luks.crypttab=` is honored by both the main system and in the initrd. |
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| `luks.uuid=, rd.luks.uuid=` | Takes a LUKS superblock UUID as argument. This will activate the specified device as part of the boot process as if it was listed in `/etc/crypttab`. This option may be specified more than once in order to set up multiple devices. `rd.luks.uuid=` is honored only in the initrd, while `luks.uuid=` is honored by both the main system and in the initrd. |
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| `luks.name=, rd.luks.name=` | Takes a LUKS super block UUID followed by an `=` and a name. This implies `rd.luks.uuid=` or `luks.uuid=` and will additionally make the LUKS device given by the UUID appear under the provided name. `rd.luks.name=` is honored only in the initrd, while `luks.name=` is honored by both the main system and in the initrd. |
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| `luks.options=, rd.luks.options=` | Takes a LUKS super block UUID followed by an `=` and a string of options separated by commas as argument. This will override the options for the given UUID. If only a list of options, without a UUID, is specified, they apply to any UUIDs not specified elsewhere, and without an entry in `/etc/crypttab`. `rd.luks.options=` is honored only by initial RAM disk (initrd) while `luks.options=` is honored by both the main system and in the initrd. |
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| `fstab=, rd.fstab=` | Defaults to "yes". If "no", causes the generator to ignore any mounts or swap devices configured in `/etc/fstab`. `rd.fstab=` is honored only in the initrd, while `fstab=` is honored by both the main system and the initrd. |
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| `root=` | Configures the operating system's root filesystem to mount when running in the initrd. This accepts a device node path (usually `/dev/disk/by-uuid/...` or similar), or the special values `gpt-auto`, `fstab`, and `tmpfs`. Use `gpt-auto` to explicitly request automatic root file system discovery via `systemd-gpt-auto-generator`. Use `fstab` to explicitly request automatic root file system discovery via the initrd `/etc/fstab` rather than via kernel command line. Use `tmpfs` in order to mount a tmpfs file system as root file system of the OS. This is useful in combination with `mount.usr=` in order to combine a volatile root file system with a separate, immutable `/usr/` file system. Also see `systemd.volatile=` below. |
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| `rootfstype=` | Takes the root filesystem type that will be passed to the mount command. `rootfstype=` is honored by the initrd. |
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| `mount.usr=` | Takes the `/usr/` filesystem to be mounted by the initrd. If `mount.usrfstype=` or `mount.usrflags=` is set, then `mount.usr=` will default to the value set in `root=`. Otherwise, this parameter defaults to the `/usr/` entry found in `/etc/fstab` on the root filesystem. |
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| `mount.usrfstype=` | Takes the `/usr` filesystem type that will be passed to the mount command. |
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| `systemd.volatile=` | Controls whether the system shall boot up in volatile mode. |
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| `systemd.swap=` | Takes a boolean argument or enables the option if specified without an argument. If disabled, causes the generator to ignore any swap devices configured in `/etc/fstab`. Defaults to enabled. |
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## Misc
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### Cause a kernel panic
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To manually cause a kernel panic run:
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```sh
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echo c > /proc/sysrq-trigger
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```
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