This is an advanced topic for users interested in Puppy Linux "under the hood".
Updated 1-September-2023
Written by dimkr this documentation is the most comprehensive description of the inner workings of a Puppy Linux to date.
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/init
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The init script has been called the heart of Puppy Linux.
This is not because it implements a lot of Puppy stuff, but because this is where Puppy starts.
When init starts, the full-blown Puppy is still just a number of files on a storage device.
The job of init is to use those files to build a Puppy based in RAM and then hand over control to it.
Some significant files in the init world:
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vmlinuz:
This is the Linux part of Puppy Linux, usually referred to as the Linux kernel.
The boot process has already loaded this into memory and started running it before init starts.
initrd.gz:
This contains the Puppy files that form the RAM based filesystem that is in place when init runs.
The init script is one of these files.
The puppy sfs files, puppy...sfs, zdrv...sfs, fdrv...sfs, ydrv...sfs, adrv...sfs:
(Where ... is a particular puppy name and version, e.g. zdrv_slacko64_6.9.5.sfs)
puppy...sfs:
This is the main Puppy file, containing most, if not all, the software that is in the current Puppy.
This is the only sfs file that is required. If the init script cannot load it for any reason, the boot is abandoned.
zdrv...sfs:
This contains kernel modules (device drivers), and optionally firmware files matching the kernel in vmlinuz.
Without this file, Puppy will usually still boot, but some devices will either not work or not work properly.
fdrv...sfs:
This usually contains firmware files. It can be used to supplement / augment the contents of zdrv...sfs.
This file is present in only some Puppies.
adrv...sfs:
Notionally an application file for applications such as web browsers and/or office automation.
It overrides the contents of all other sfs files. It may not be present.
bdrv...sfs:
Another application file. In some builds, contains a basic installation or skeleton of the compatible distro.
With this file, more binary packages from the compatible distro repos are likely to work.
It is only present in some builds.
ydrv...sfs:
Notionally a patch file. It can be used to override the contents of puppy...sfs.
It is usually not present.
Overview of how it works:
=========================
A typical frugal install of Puppy is a directory containing the above files.
So init begins by establishing the location of this directory,
by looking for the puppy...sfs file.-
In the absence of any indication as to it's location, init searches throughout
the partitions of the system until it finds it. -
If it cannot locate the puppy...sfs file, it abandons the boot by dropping out to
a console with several error messages on the screen. -
Having located the puppy...sfs it proceeds to create a layered file system
from the sfs files in the same directory, or specified directories
if these have been identified with boot parameters - see below.
A layered filesystem consists of a stack of directories, most of these layers can be read-only,
but the top one is always read-write.
The directory that contains the stack, appears to contain all the files from every layer.
But if a file exists in more than one layer the one in the top-most layer is the one that is seen.
So the order of layers is significant.
The layered filesystem (aufs or overlayfs) stack is constructed as follows:
The tmpfs read-write layer is at the top of the stack and takes precedence over everything else. It is populated from the personal save area if present - either save folder or save file (but see below for save areas on 'flash' drives).
-
The zdrv read-only layer is at the bottom of the stack and contains kernel modules (device drivers), and optionally firmware files.
The puppy read-only layer is in the middle of the stack and contains the majority of software that constitutes Puppy Linux.
-
The fdrv read-only layer is located above the zdrv, and takes precedence over the zdrv, but is below the Puppy layer.
-
The adrv, bdrv and ydrv read-only layers are located between the read-write layer and the Puppy layer and all take precedence over all the layers below them. The only difference between these 3 layers is their position, and hence precedence, in the stack - the order being adrv -> ydrv -> bdrv -> Puppy
By convention, typical usage is:
The fdrv contains firmware files needed by the device drivers in the zdrv and may not be needed if the firmware files are included in the zdrv.
-
The adrv contains large, personal choice dependent, applications (such as a web browser or office automation suite).
-
The bdrv is a 2nd application file which in some builds can contain a basic installation or skeleton of the compatible distro to improve compatibility with the distro.
-
The ydrv can be used as a patch file to make small, possibly experimental, changes to the system.
The stack from top to bottom is therefore:
Name |
Properties |
|---|---|
tmpfs |
read-write |
adrv.sfs |
read-only, optional |
ydrv.sfs |
read-only, optional |
bdrv.sfs |
read-only, optional |
puppy.sfs |
read-only |
fdrv.sfs |
read-only, optional |
zdrv.sfs |
read-only |
-
If this is a first boot, the stack is ready to be made into the running system.
-
But, this first boot stack contains no persistent storage.
-
If you change any files in the running system they are written to the
read-write layer which only exists in RAM. -
The first time you reboot or shutdown, Puppy asks if you want to save the session.
-
If you save the session you will be guided through a process to create a save layer,
to which Puppy will then copy any changed files. -
So, if this is not a first boot, init has to setup any save layer and insert it into the stack.
-
Init attempts to sort out what type of save layer mechanism is being used, and make it available as a directory.
-
If this attempt fails at any point, nothing extra will be done and the boot proceeds with the first boot stack.
-
If the boot is considered to be from a "flash" device, the directory containing the save layer
is inserted as a read-only layer immediately below the read-write layer. -
Otherwise the tmpfs read-write layer is replaced with the directory
containing the save layer as the read-write layer. -
The stack is then made into the running system and init exits.
-
If non-critical errors are detected by init it usually writes them to a file called bootinit.log
-
bootinit.log also stores debug messages you might want to read.
-
This file can be accessed in a running puppy as /initrd/tmp/bootinit.log
Things that provide input to init and change the things it does:
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DISTRO_SPECS:
This is a file in initrd.gz that is created by the Puppy builder.
It contains definitions of various information about a particular Puppy.
DISTRO_FILE_PREFIX defines the name that occurs frequently in files that belong to it. e.g. 'slacko64'.
DISTRO_VERSION defines the version number. e.g. '6.9.5'
Very significant for init are DISTRO_PUPPYSFS, DISTRO_ZDRVSFS, DISTRO_FDRVSFS, DISTRO_YDRVSFS,
DISTRO_ADRVSFS, which define the default filenames for each of the Puppy sfs files.
Boot parameters:
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pmedia=<atahd|ataflash|usbhd|usbflash|cd>
Indicates the type of boot device.
If it's "cd" then the partitions are searched for a save layer file, the only situation that triggers such a search.
If the first 3 characters are "usb", then any searching is restricted to only usb devices.
If the last 5 characters are "flash" the top layer in the stack remains the tmpfs in memory, otherwise any found save layer becomes the top layer in the stack.
This boot parameter should always be provided.
psubdir=</path/to/install/directory>
If the Puppy files are not in the root of a partition, but in a sub-directory, the path of this directory, relative to the partition root, must be specified with this parameter.
e.g. If the sdb2 partition is mounted as /mnt/sdb2 and the Puppy files are in /mnt/sdb2/tahr64, then "psubdir=tahr64" or "psubdir=/tahr64" must be specified.
This parameter can specify subdirectories at more that a single level, e.g. "psubdir=puppy/tahr64" or "psubdir=/puppy/tahr64".
If a leading "/" is not provided, init will add it.
This is the default path for locating any puppy file and any partition.
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pupsfs=<partition>:<path>/<filename> Specifies the puppy...sfs file.
zdrv=<partition>:<path>/<filename> Specifies the zdrv...sfs file.
fdrv=<partition>:<path>/<filename> Specifies the fdrv...sfs file.
adrv=<partition>:<path>/<filename> Specifies the adrv...sfs file.
ydrv=<partition>:<path>/<filename> Specifies the ydrv...sfs file.
psave=<partition>:<path>/<filename> Specifies the save layer file.
Where <partition> can be the name e.g sdb2, or a label e.g. Work, or a uuid e.g. 0db94719-cdf1-44b7-9766-23db62fb85a5
When a label or uuid is used, only the beginning is required, enough to be unique on your system, e.g "pupsfs=0db94719-cdf1"
Where <path> is the sub-directory within the partition. e.g. "pupsfs=sdb2:/path/to/" or "psave=:/path/to/"
Any specified <path> is relative to the root of the partition, the same as "psubdir=".
If <path> does not start with a "/" then a "/" is prepended to it.
If no <path> is specified, the directory defined by "psubdir=" is used.
Where <filename> is just a filename, e.g. "pupsfs=sdb2:/path/to/my-improved-puppy.sfs" or "psave=sdc2:my-improved-savefolder"
If no <filename> is specified the default filename, as determined by the DISTRO_SPECS file, is used.
For the purposes of the "psave=" specification a savefolder is considered to be just a file.
The <path> specification defines the directory containing the savefolder, and the <filename> specification defines it's name.
So, "psave=sdb4:/lxpupsc/mysave" says that the savefolder is on the sdb4 partition in the "/lxpupsc" directory, named "mysave".
Whereas "psave=sdb4:/lxpupsc/mysave/" says that the savefolder is on the sdb4 partition in the "/lxpupsc/mysave" directory, with the default savefolder name for the puppy.
It is not necessary to specify all elements, but if there is no ":" it is assumed to be a <partition> specification.
e.g. "pupsfs=sdb2", specifies that the puppy...sfs file is on sdb2 in the default path with the default filename.
"fdrv=:alternate-firmware.sfs", specifies that it's a file called alternate-firmware.sfs on the default partition in the default directory i.e. where the puppy...sfs is located.
It is recommended that a pupsfs=<partition> always be specified.
This enables init to go straight to that partition to find the Puppy files instead of searching through all partitions looking for puppy...sfs.
ex: psave=sdc1:/path/to/tahrsave.4fs
ex: psave=sdc1:tahrsave.4fs
ex: zdrv=sdc1:/zz/myzz.sfs
ex: adrv=sdd6:/puppy/drvs/custom/adrv.sfs
ex: pupsfs=sdb2:/puppy/precise/puppy_precise_5.7.1.sfs
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pkeys=<keyboard layout specification> e.g de
Used to setup the keyboard layout to be used by Puppy.
plang=<language specification> e.g. de_DE.UTF-8
Specifies the language to be used by Puppy, including any messages displayed by init.
If no pkeys parameter is provided the first 2 letters of this specification are used to set the keyboard layout.
pimod=<, separated list of kernel module names>
On some computers the keyboard requires a kernel module to be loaded before they will work.
The normal loading of kernel modules does not happen until after init has finished.
But sometimes init needs to request input from the user via the keyboard.
Specifying kernel modules in this parameter will cause init to load them before any possible keyboard interaction.
pfix=<ram, nox, trim, nocopy, fsck, fsckp, rdsh, <number>>
The pfix parameter is a ',' separated list of 1 or more of the above sub-parameters.
ram: run in ram only (do not load ${DISTRO_FILE_PREFIX}save).
nox: do not start X.
trim: add "discard" to mount options if SSD.
copy: copy .sfs files into ram
nocopy: do not copy .sfs files into ram (default is copy if ram <= 1024 MB).
fsck: do fsck of ${DISTRO_FILE_PREFIX}save.?fs file.
fsckp: do fsck before first mount of supported partitions.
rdsh: exit to shell in initial ramdisk.
<number>: blacklist last <number> folders (multisession). e.g. pfix=3
Parameter files:
================
If they exist in the frugal install directory their contents are used to set some variables that otherwise could be set by boot parameters.
SAVEMARK
Provides a means of specifying that the save layer file is on a different partition on the same
device. It contains a single number. If the puppy...sfs is located in sdb2 and SAVEMARK
contains 4, a save layer file is expected to be in sdb4.
initmodules.txt
Contains a list of kernel modules that init loads before any keyboard interaction.
Usually these are modules needed for the keyboard to work.
BOOT_SPECS
This is a file that sets variables, like DISTRO_SPECS. But it is meant to be for the user to override the variables normally set by boot parameters.
It can also be used to set other variables in init, e.g. "TZ='XXX-10'" sets the timezone in init to Queensland, Australia.
The idea is that there is a copy of this file in user space, the user edits this file and then stores a copy of it in initrd.gz.
This file could also be used instead of specific parameter files like initmodules.txt and even SAVEMARK.
Part of this concept is to move the complication out of init into the running system.
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MORE TECHNICAL NOTES
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How the script determines what pupsave to use
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If you haven't specified psave=<partition>:<filename> then init looks
for a file with this base name:
/DISTRO_SPECS -> DISTRO_FILE_PREFIX='...'
${DISTRO_FILE_PREFIX}save - is the fixed base name for all pupsave folders
${DISTRO_FILE_PREFIX}save.?fs - is the fixed base name for all pupsave files