path: root/USAGE-SQFSTAR-4.6

	SQFSTAR - A tool to create a Squashfs filesystem from a TAR archive

Sqfstar will read an uncompressed TAR archive from standard in, and it will
create a Squashfs filesystem from it.  If a TAR archive is compressed it
should be piped through a decompressor utility and then input into Sqfstar.

Sqfstar supports V7, ustar, bsdtar (libarchive), GNU tar and PAX extensions.
Sparse file extensions are supported, including the "old GNU format, type S",
and PAX formats, Versions 0.0, 0.1 and the current 1.0.

Sqfstar supports extended attributes, and recognises the SCHILY xattr
PAX extension (used by GNU tar), and the LIBARCHIVE xattr PAX extension
(used by bsdtar).

The Sqfstar usage info is:

SYNTAX:sqfstar [OPTIONS] FILESYSTEM [list of exclude dirs/files]

Filesystem compression options:
-b <block_size>		set data block to <block_size>.  Default 128 Kbytes.
			Optionally a suffix of K or M can be given to specify
			Kbytes or Mbytes respectively
-comp <comp>		select <comp> compression
			Compressors available:
				gzip (default)
				lzo
				lz4
				xz
				zstd
-noI			do not compress inode table
-noId			do not compress the uid/gid table (implied by -noI)
-noD			do not compress data blocks
-noF			do not compress fragment blocks
-noX			do not compress extended attributes
-no-compression		do not compress any of the data or metadata.  This is
			equivalent to specifying -noI -noD -noF and -noX

Filesystem build options:
-reproducible		build filesystems that are reproducible (default)
-not-reproducible	build filesystems that are not reproducible
-mkfs-time <time>	set filesystem creation timestamp to <time>. <time> can
			be an unsigned 32-bit int indicating seconds since the
			epoch (1970-01-01) or a string value which is passed to
			the "date" command to parse. Any string value which the
			date command recognises can be used such as "now",
			"last week", or "Wed Feb 15 21:02:39 GMT 2023"
-all-time <time>	set all file timestamps to <time>. <time> can be an
			unsigned 32-bit int indicating seconds since the epoch
			(1970-01-01) or a string value which is passed to the
			"date" command to parse. Any string value which the date
			command recognises can be used such as "now", "last
			week", or "Wed Feb 15 21:02:39 GMT 2023"
-root-time <time>	set root directory time to <time>. <time> can be an
			unsigned 32-bit int indicating seconds since the epoch
			(1970-01-01) or a string value which is passed to the
			"date" command to parse. Any string value which the date
			command recognises can be used such as "now", "last
			week", or "Wed Feb 15 21:02:39 GMT 2023"
-root-mode <mode>	set root directory permissions to octal <mode>
-root-uid <value>	set root directory owner to specified <value>,
			<value> can be either an integer uid or user name
-root-gid <value>	set root directory group to specified <value>,
			<value> can be either an integer gid or group name
-all-root		make all files owned by root
-force-uid <value>	set all file uids to specified <value>, <value> can be
			either an integer uid or user name
-force-gid <value>	set all file gids to specified <value>, <value> can be
			either an integer gid or group name
-default-mode <mode>	tar files often do not store permissions for
			intermediate directories.  This option sets the default
			directory permissions to octal <mode>, rather than 0755.
			This also sets the root inode mode
-default-uid <uid>	tar files often do not store uids for intermediate
			directories.  This option sets the default directory
			owner to <uid>, rather than the user running Sqfstar.
			This also sets the root inode uid
-default-gid <gid>	tar files often do not store gids for intermediate
			directories.  This option sets the default directory
			group to <gid>, rather than the group of the user
			running Sqfstar.  This also sets the root inode gid
-pseudo-override	make pseudo file uids and gids override -all-root,
			-force-uid and -force-gid options
-exports		make the filesystem exportable via NFS
-no-sparse		do not detect sparse files
-no-fragments		do not use fragments
-no-tailends		do not pack tail ends into fragments
-no-duplicates		do not perform duplicate checking
-no-hardlinks		do not hardlink files, instead store duplicates

Filesystem filter options:
-p <pseudo-definition>	add pseudo file definition.  The definition should
			be quoted
-pf <pseudo-file>	add list of pseudo file definitions.  Pseudo file
			definitions in pseudo-files should not be quoted
-ef <exclude_file>	list of exclude dirs/files.  One per line
-regex			allow POSIX regular expressions to be used in exclude
			dirs/files
-ignore-zeros		allow tar files to be concatenated together and fed to
			Sqfstar.  Normally a tarfile has two consecutive 512
			byte blocks filled with zeros which means EOF and
			Sqfstar will stop reading after the first tar file on
			encountering them. This option makes Sqfstar ignore the
			zero filled blocks

Filesystem extended attribute (xattrs) options:
-no-xattrs		do not store extended attributes
-xattrs			store extended attributes (default)
-xattrs-exclude <regex>	exclude any xattr names matching <regex>.  <regex> is a
			POSIX regular expression, e.g. -xattrs-exclude '^user.'
			excludes xattrs from the user namespace
-xattrs-include <regex>	include any xattr names matching <regex>.  <regex> is a
			POSIX regular expression, e.g. -xattrs-include '^user.'
			includes xattrs from the user namespace
-xattrs-add <name=val>	add the xattr <name> with <val> to files.  If an
			user xattr it will be added to regular files and
			directories (see man 7 xattr).  Otherwise it will be
			added to all files.  <val> by default will be treated as
			binary (i.e. an uninterpreted byte sequence), but it can
			be prefixed with 0s, where it will be treated as base64
			encoded, or prefixed with 0x, where val will be treated
			as hexidecimal.  Additionally it can be prefixed with
			0t where this encoding is similar to binary encoding,
			except backslashes are specially treated, and a
			backslash followed by 3 octal digits can be used to
			encode any ASCII character, which obviously can be used
			to encode control codes.  The option can be repeated
			multiple times to add multiple xattrs

Sqfstar runtime options:
-version		print version, licence and copyright message
-force			force Sqfstar to write to block device or file
-exit-on-error		treat normally ignored errors as fatal
-quiet			no verbose output
-info			print files written to filesystem
-no-progress		do not display the progress bar
-progress		display progress bar when using the -info option
-percentage		display a percentage rather than the full progress bar.
			Can be used with dialog --gauge etc.
-throttle <percentage>	throttle the I/O input rate by the given percentage.
			This can be used to reduce the I/O and CPU consumption
			of Sqfstar
-limit <percentage>	limit the I/O input rate to the given percentage.
			This can be used to reduce the I/O and CPU consumption
			of Sqfstar (alternative to -throttle)
-processors <number>	use <number> processors.  By default will use number of
			processors available
-mem <size>		use <size> physical memory for caches.  Use K, M or G to
			specify Kbytes, Mbytes or Gbytes respectively
-mem-percent <percent>	use <percent> physical memory for caches.  Default 25%
-mem-default		print default memory usage in Mbytes

Expert options (these may make the filesystem unmountable):
-nopad			do not pad filesystem to a multiple of 4K
-offset <offset>	skip <offset> bytes at the beginning of FILESYSTEM.
			Optionally a suffix of K, M or G can be given to specify
			Kbytes, Mbytes or Gbytes respectively.
			Default 0 bytes
-o <offset>		synonym for -offset

Miscellaneous options:
-fstime <time>		alternative name for mkfs-time
-root-owned		alternative name for -all-root
-noInodeCompression	alternative name for -noI
-noIdTableCompression	alternative name for -noId
-noDataCompression	alternative name for -noD
-noFragmentCompression	alternative name for -noF
-noXattrCompression	alternative name for -noX

-help			output this options text to stdout
-h			output this options text to stdout

-Xhelp			print compressor options for selected compressor

Pseudo file definition format:
"filename d mode uid gid"		create a directory
"filename m mode uid gid"		modify filename
"filename b mode uid gid major minor"	create a block device
"filename c mode uid gid major minor"	create a character device
"filename f mode uid gid command"	create file from stdout of command
"filename s mode uid gid symlink"	create a symbolic link
"filename i mode uid gid [s|f]"		create a socket (s) or FIFO (f)
"filename x name=val"			create an extended attribute
"filename l linkname"			create a hard-link to linkname
"filename L pseudo_filename"		same, but link to pseudo file
"filename D time mode uid gid"		create a directory with timestamp time
"filename M time mode uid gid"		modify a file with timestamp time
"filename B time mode uid gid major minor"
					create block device with timestamp time
"filename C time mode uid gid major minor"
					create char device with timestamp time
"filename F time mode uid gid command"	create file with timestamp time
"filename S time mode uid gid symlink"	create symlink with timestamp time
"filename I time mode uid gid [s|f]"	create socket/fifo with timestamp time


Files and directories in the TAR archive may be excluded, and both anchored
and non-anchored exclude files are supported (see section 6 and examples later).
Wildcards (globbing) is supported by default.

Sqfstar can also by invoked by running "mksquashfs - image.sqfs -tar", if
the Sqfstar link isn't available.

1. USAGE EXAMPLES
-----------------

% sqfstar image.sqfs < archive.tar

    Create a Squashfs image from archive.tar, using defaults (gzip compression,
    128K blocks).

% sqfstar -comp xz -b 1M image.sqfs < archive.tar

    As previous, but use XZ compression and 1Mbyte block sizes.

% zcat archive.tgz | sqfstar image.sqfs

    Create a Squashfs image from a gzip compressed tar archive.

% sqfstar -root-uid 0 -root-gid 0 image.sqfs < archive.tar

    Tar files do not supply a definition for the root directory, and the
    default is to make the directory owned/group owned by the user running
    Sqfstar.  The above command sets the ownership/group ownership to root.

% sqfstar -root-mode 0755 image.sqfs < archive.tar

   The default permissions for the root directory is 0777 (rwxrwxrwx).  The
   above command sets the permissions to 0755 (rwxr-xr-x).

% sqfstar image.sqsh dir1/file1 dir2/file2 < archive.tar

   Create a Squashfs image but exclude the files "file1" and "file2".

% sqfstar image.sqsh "... *.[ch]" < archive.tar

   Create a Squashfs image but exclude any file matching "*.[ch]" anywhere
   in the archive.


2. CHANGING COMPRESSION ALGORITHM AND COMPRESSION SPECIFIC OPTIONS
------------------------------------------------------------------

By default Sqfstar will compress using the GZIP compression algorithm.  This
algorithm offers a good trade-off between compression ratio, and memory and time
taken to decompress.

Squashfs also supports LZ4, LZO, XZ and ZSTD compression.  LZO offers worse
compression ratio than GZIP, but is faster to decompress.  XZ offers better
compression ratio than GZIP, but at the expense of greater memory and time
to decompress (and significantly more time to compress).  LZ4 is similar
to LZO.  ZSTD has been developed by Facebook, and aims to compress well and
be fast to decompress.  You should experiment with the compressors to
see which is best for you.

If you're not building the squashfs-tools and kernel from source, then
the tools and kernel may or may not have been built with support for LZ4, LZO,
XZ or ZSTD compression.  The compression algorithms supported by the build of
Sqfstar can be found by typing sqfstar -help.

The full list of compressors available and their compression specific options
are:

Compressors available and compressor specific options:
	gzip (default)
	  -Xcompression-level <compression-level>
		<compression-level> should be 1 .. 9 (default 9)
	  -Xwindow-size <window-size>
		<window-size> should be 8 .. 15 (default 15)
	  -Xstrategy strategy1,strategy2,...,strategyN
		Compress using strategy1,strategy2,...,strategyN in turn
		and choose the best compression.
		Available strategies: default, filtered, huffman_only,
		run_length_encoded and fixed
	lzo
	  -Xalgorithm <algorithm>
		Where <algorithm> is one of:
			lzo1x_1
			lzo1x_1_11
			lzo1x_1_12
			lzo1x_1_15
			lzo1x_999 (default)
	  -Xcompression-level <compression-level>
		<compression-level> should be 1 .. 9 (default 8)
		Only applies to lzo1x_999 algorithm
	lz4
	  -Xhc
		Compress using LZ4 High Compression
	xz
	  -Xbcj filter1,filter2,...,filterN
		Compress using filter1,filter2,...,filterN in turn
		(in addition to no filter), and choose the best compression.
		Available filters: x86, arm, armthumb, powerpc, sparc, ia64
	  -Xdict-size <dict-size>
		Use <dict-size> as the XZ dictionary size.  The dictionary size
		can be specified as a percentage of the block size, or as an
		absolute value.  The dictionary size must be less than or equal
		to the block size and 8192 bytes or larger.  It must also be
		storable in the xz header as either 2^n or as 2^n+2^(n+1).
		Example dict-sizes are 75%, 50%, 37.5%, 25%, or 32K, 16K, 8K
		etc.
	zstd
	  -Xcompression-level <compression-level>
		<compression-level> should be 1 .. 22 (default 15)

If the compressor offers compression specific options (all the compressors now
have compression specific options except the deprecated lzma1 compressor)
then these options are also displayed (.i.e. in the above XZ is shown with two
compression specific options).  The compression specific options are, obviously,
specific to the compressor in question, and the compressor documentation and
web sites should be consulted to understand their behaviour.  In general
the Sqfstar compression defaults for each compressor are optimised to
give the best performance for each compressor, where what constitutes
best depends on the compressor.  For GZIP/XZ best means highest compression,
for LZO/LZ4 best means a tradeoff between compression and (de)-compression
overhead (LZO/LZ4 by definition are intended for weaker processors).

3. REDUCING CPU AND I/O USAGE
-----------------------------

By default Sqfstar will use all the CPUs available to compress and create the
filesystem, and will read from the TAR archive and write to the output
filesystem as fast as possible.  This maximises both CPU usage and I/O.

Sometimes you don't want Sqfstar to use all CPU and I/O bandwidth.  For those
cases Sqfstar supports two complementary options, -processors and -throttle.

The -processors option can be used to reduce the number of parallel compression
threads used by Sqfstar.  Reducing this to 1 will create the minimum number of
threads, and this will reduce CPU usage, and that in turn will reduce I/O
(because CPUs are normally the bottleneck).

The -throttle option reduces the speed Sqfstar reads from the TAR archive.
The value is a percentage (obviously from 1 - 100), and 50 will reduce the
read rate by half (the read thread will spend half its time idling), and 75
by three quarters.  Reducing the speed of I/O will also reduce the CPU
usage as there is insufficient data rate to use all cores.

Which option should you use?  Both will effectively reduce CPU and I/O in
normal cases where intensive use is being made of both I/O and CPUs.  But
in edge cases there can be an imbalance where reducing one has no effect, or
it can't be reduced any further.  For instance when there is only 1 or 2 cores
available, setting -processors to the minimum of 1 may still use too much
CPU.  Additionally if your input source is slow Sqfstar may still max it out
with -processors set to the minimum of 1.  In this case you can use -throttle
in addition to -processors or on its own.

4. CHANGING GLOBAL COMPRESSION DEFAULTS USED IN SQFSTAR
-------------------------------------------------------

There are a large number of options that can be used to control the compression
in Sqfstar.  By and large the defaults are the most optimum settings and should
rarely need to be changed.

Note, this does not apply to the block size, increasing the block size from
the default of 128 Kbytes will increase compression (especially for the XZ and
ZSTD compressors) and should increase I/O performance too.  However, a block
size of greater than 128 Kbytes may increase latency in certain cases (where the
filesystem contains lots of fragments, and no locality of reference is
observed).  For this reason the block size default is configured to the less
optimal 128 Kbytes.  Users should experiment with 256 Kbyte sizes or above.

The -b option allows the block size to be selected, both "K" and "M" postfixes
are supported, this can be either 4K, 8K, 16K, 32K, 64K, 128K, 256K, 512K or
1M bytes.

The -noI, -noD, -noF and -noX options can be used to force Sqfstar to not
compress inodes/directories, data, fragments and extended attributes
respectively.  Giving all options generates an uncompressed filesystem.

The -no-fragments option tells Sqfstar to not generate fragment blocks.  A
fragment block consists of multiple small files (all smaller than the block
size) packed and compressed together.  This produces much better compression
than storing and compressing these files separately.  It also typically
improves I/O as multiple files in the same directory are read at the same time.
You don't want to enable this option unless you fully understand the effects.

The -no-tailends option tells Sqfstar to not pack file tailends into fragment
blocks.  Normally a file will not be a multiple of the block size, and so
there were always be a tail which doesn't fit fully into a data block.  This
tailend is by default packed into fragment blocks.  Enabling this option will
reduce compression.
 
The -no-duplicates option tells Sqfstar to not check the files being added to
the filesystem for duplicates.  This can result in quicker filesystem generation
although obviously compression will suffer badly if there is a lot of duplicate
files.

5. SPECIFYING THE UIDs/GIDs USED IN THE FILESYSTEM
--------------------------------------------------

By default files in the generated filesystem use the ownership of the file
stored in the TAR archive.  TAR archives depending on the TAR format stores
ownership in two ways.  The early V7 format only stored a numeric UID and GID.
If Sqfstar is reading a V7 archive these are used.  Later ustar and PAX
archives can also store a string user name and group name.  If these are
present and are recognised by the system (i.e. can be mapped to a UID and GID),
their UID and GID is used.  If a user name or group name is not recognised
by the system, the numeric UID or GID is used.

The -all-root option forces all file UIDs/GIDs in the generated Squashfs
filesystem to be root.  This allows root owned filesystems to be built without
root access on the host machine.

The "-force-uid value" option forces all files in the generated Squashfs
filesystem to be owned by value, where value can either be a user name or a
numeric UID.

The "-force-gid value" option forces all files in the generated Squashfs
filesystem to be group owned by value, where value can be either a group name or
a numeric GID.

6. EXCLUDING FILES FROM THE FILESYSTEM
--------------------------------------

Sqfstar can exclude files from the TAR archive, so that they don't appear in
the Squashfs filesystem.  Exclude files can be directly specified on
the command line (immediately after the FILESYSTEM argument), or the -ef
option can be used to specify an exclude file, with one exclude file per line.

Exclude files by default use wildcard matching (globbing) and can match on
more than one file (if wildcards are used).  Regular expression matching
can be used instead by specifying the -regex option.  In most cases wildcards
should be used rather than regular expressions because wildcard matching
behaviour is significantly easier to understand!

In addition to wildcards/regex expressions, exclude files can be "anchored" or
"non-anchored".  An anchored exclude is one which matches from the root of the
directory and nowhere else, a non-anchored exclude matches anywhere.  For
example given the directory hierarchy "a/b/c/a/b", the anchored exclude "a/b"
will match "a/b" at the root of the directory hierarchy, but it will not match
the "/a/b" sub-directory within directory "c", whereas a non-anchored exclude
would.

A couple of examples should make this clearer.
 
Anchored excludes

  1. sqfstar image.sqsh 'test/*.gz' < archive.tar

     Exclude all files matching "*.gz" in the top level directory "test".

  2. sqfstar image.sqsh '*/[Tt]est/example*' < archive.tar

     Exclude all files beginning with "example" inside directories called
     "Test" or "test", that occur inside any top level directory.

  Using extended wildcards, negative matching is also possible.

  3. sqfstar image.sqsh 'test/!(*data*).gz' < archive.tar

     Exclude all files matching "*.gz" in top level directory "test",
     except those with "data" in the name.

Non-anchored excludes

  By default excludes match from the top level directory, but it is
  often useful to exclude a file matching anywhere in the source directories.
  For this non-anchored excludes can be used, specified by pre-fixing the
  exclude with "...".

  Examples:

  1. sqfstar image.sqsh '... *.gz' < archive.tar

     Exclude files matching "*.gz" anywhere in the source directories.
     For example this will match "example.gz", "test/example.gz", and
     "test/test/example.gz".

  2. sqfstar image.sqsh '... [Tt]est/*.gz' < archive.tar

     Exclude files matching "*.gz" inside directories called "Test" or
     "test" that occur anywhere in the source directories.

  Again, using extended wildcards, negative matching is also possible.

  3. sqfstar image.sqsh '... !(*data*).gz' < archive.tar

     Exclude all files matching "*.gz" anywhere in the source directories,
     except those with "data" in the name.


7. FILTERING AND ADDING EXTENDED ATTRIBUTES (XATTRs)
----------------------------------------------------

Sqfstar has a number of options which allow extended attributes (xattrs) to be
filtered from the TAR archive or added to the created Squashfs filesystem.

The -no-xattrs option removes any extended attributes which may exist in the
TAR archive, and creates a filesystem without any extended attributes.

The -xattrs-exclude option specifies a regular expression (regex), which
removes any extended attribute that matches the regular expression from all
files.  For example the regex '^user.' will remove all User extended attributes.

The -xattrs-include option instead specifies a regular expression (regex)
which includes any extended attribute that matches, and removes anything
that does't match.  For example the regex '^user.' will only keep User
extended attributes and anything else will be removed.

Sqfstar also allows you to add extended attributes to files in the Squashfs
filesystem using the -xattrs-add option.  This option takes an xattr name and
value pair separated by the '=' character.

The extended attribute name can be any valid name and can be in the namespaces
security, system, trusted, or user.  User extended attributes are added to files
and directories (see man 7 xattr for explanation), and the others are added to
all files.

The extended attribute value by default will be treated as binary (i.e. an
uninterpreted byte sequence), but it can be prefixed with 0s, where it will be
treated as base64 encoded, or prefixed with 0x, where it will be treated as
hexidecimal.

Obviously using base64 or hexidecimal allows values to be used that cannot be
entered on the command line such as non-printable characters etc.  But it
renders the string non-human readable.  To keep readability and to allow
non-printable characters to be entered, the 0t prefix is supported.  This
encoding is similar to binary encoding, except backslashes are specially
treated, and a backslash followed by three octal digits can be used to encode
any ASCII character, which obviously can be used to encode non-printable values.

The following four command lines are equivalent

sqfstar -xattrs-add "user.comment=hello world" image.sqfs
sqfstar -xattrs-add "user.comment=0saGVsbG8gd29ybGQ=" image.sqfs
sqfstar -xattrs-add "user.comment=0x68656c6c6f20776f726c64"
sqfstar -xattrs-add "user.comment=0thello world" image.sqfs

Obviously in the above example there are no non-printable characters and so
the 0t prefixed string is identical to the first line.  The following three
command lines are identical, but where the space has been replaced by the
non-printable NUL '\0' (null character).

sqfstar -xattrs-add "user.comment=0thello\000world" image.sqfs
sqfstar -xattrs-add "user.comment=0saGVsbG8Ad29ybGQ=" image.sqfs
sqfstar -xattrs-add "user.comment=0x68656c6c6f00776f726c64" image.sqsh

8. PSEUDO FILE SUPPORT
----------------------

Sqfstar supports pseudo files, these allow files, directories, character
devices, block devices, fifos, symbolic links, hard links and extended
attributes to be specified and added to the Squashfs filesystem being built,
rather than requiring them to be present in the TAR archive.  This, for example,
allows device nodes to be added to the filesystem without requiring root access.

Pseudo files also support "dynamic pseudo files" and a modify operation.
Dynamic pseudo files allow files to be dynamically created when Sqfstar
is run, their contents being the result of running a command or piece of
shell script.  The modifiy operation allows the mode/uid/gid of an existing
file in the source filesystem to be modified.

Two Sqfstar options are supported, -p allows one pseudo file to be specified
on the command line, and -pf allows a pseudo file to be specified containing a
list of pseduo definitions, one per line.

8.1 CREATING A DYNAMIC FILE
---------------------------

Pseudo definition

Filename f mode uid gid command

mode is the octal mode specifier, similar to that expected by chmod.

uid and gid can be either specified as a decimal number, or by name.

command can be an executable or a piece of shell script, and it is executed
by running "/bin/sh -c command".   The stdout becomes the contents of
"Filename".

Examples:

Running a basic command
-----------------------

/somedir/dmesg f 444 root root dmesg

creates a file "/somedir/dmesg" containing the output from dmesg.

Executing shell script
----------------------

RELEASE f 444 root root \
		if [ ! -e /tmp/ver ]; then \
			echo 0 > /tmp/ver; \
		fi; \
                ver=`cat /tmp/ver`; \
                ver=$((ver +1)); \
                echo $ver > /tmp/ver; \
                echo -n `cat /tmp/release`; \
                echo "-dev #"$ver `date` "Build host" `hostname`

Creates a file RELEASE containing the release name, date, build host, and
an incrementing version number.  The incrementing version is a side-effect
of executing the shell script, and ensures every time Sqfstar is run a
new version number is used without requiring any other shell scripting.

The above example also shows that commands can be split across multiple lines
using "\".  Obviously as the script will be presented to the shell as a single
line, a semicolon is need to separate individual shell commands within the
shell script.

Reading from a device (or fifo/named socket)
--------------------------------------------

input f 444 root root dd if=/dev/sda1 bs=1024 count=10

Copies 10K from the device /dev/sda1 into the file input.  Ordinarily Sqfstar
given a device, fifo, or named socket will place that special file within the
Squashfs filesystem, the above allows input from these special files to be
captured and placed in the Squashfs filesystem.

8.2 CREATING A BLOCK OR CHARACTER DEVICE
----------------------------------------

Pseudo definition

Filename type mode uid gid major minor

Where type is either
	b - for block devices, and
	c - for character devices

mode is the octal mode specifier, similar to that expected by chmod.

uid and gid can be either specified as a decimal number, or by name.

For example:

/dev/chr_dev c 666 root root 100 1
/dev/blk_dev b 666 0 0 200 200

creates a character device "/dev/chr_dev" with major:minor 100:1 and
a block device "/dev/blk_dev" with major:minor 200:200, both with root
uid/gid and a mode of rw-rw-rw.

8.3 CREATING A DIRECTORY
------------------------

Pseudo definition

Filename d mode uid gid

mode is the octal mode specifier, similar to that expected by chmod.

uid and gid can be either specified as a decimal number, or by name.

For example:

/pseudo_dir d 666 root root

creates a directory "/pseudo_dir" with root uid/gid and mode of rw-rw-rw.

8.4 CREATING A SYMBOLIC LINK
----------------------------

Pseudo definition

Filename s mode uid gid symlink

uid and gid can be either specified as a decimal number, or by name.

Note mode is ignored, as symlinks always have "rwxrwxrwx" permissions.

For example:

symlink s 0 root root example

Creates a symlink "symlink" to file "example" with root uid/gid.

8.5 CREATING HARD LINKS (FILE REFERENCES)
-----------------------------------------

The "f" Pseudo definition allows a regular file to be created from the output of
a command (or shell).  Often this is used to reference a file outside the source
directories by executing "cat", e.g.

README f 0555 0 0 cat /home/phillip/latest-version/README

Because this is a quite frequent use of the definition, an alternative faster
"File reference" or Hard Link Pseudo definition exists:

README l /home/phillip/latest-version/README

Will create a reference to "/home/phillip/latest-version/README",
and obviously the timestamp/mode and owership will be used.

The definition also can be used to create additional references to files
within the source directories.  For instance if "phillip/latest/README" was a
file being added to the filesystem, then

README l phillip/latest/README

Will create a Hard Link (and increment the nlink count on the inode).

In both cases, the path to the file being referenced is the system
filesystem path, and can be absolute (prefixed with /), or relative
to the current working directory.

There is an additional 'L' Pseudo definition, which closes a loophole in
the above 'l' definition.  The 'l' Pseudo definition cannot create references
or Hard Links to files created by Pseudo definitions, because by
definition they do not exist in the system filesystem.

with 'L' the referenced file is expected to be a Pseudo file, and in this case
the path is taken to be from the root of the Squashfs filesystem being created,
e.g.

char-dev c 0555 0 0 1 2

link L char-dev

Will create a Hard Link named "link" to the character device called "char-dev"
created by the previous Pseudo definition.

8.6 CREATING SOCKETS/FIFOS
--------------------------

Pseudo definition

filename i mode uid gid [s|f]

To create a Unix domain socket, 's' should be used, i.e.

filename i 0777 root root s

and to create a FIFO, 'f' should be used, i.e.

filename i 0777 root root f

8.7 ADDING EXTENDED ATTRIBUTES TO FILES
---------------------------------------

Pseudo definition

filename x name=val

Will add the extended attribute <name> to <filename> with <val > contents.  See
Section 7 for a description of the <val> formats supported.

8.8 MODIFYING ATTRIBUTES OF AN EXISTING FILE
--------------------------------------------

Pseudo definition

Filename m mode uid gid

mode is the octal mode specifier, similar to that expected by chmod.

uid and gid can be either specified as a decimal number, or by name.

For example:

dmesg m 666 root root

Changes the attributes of the file "dmesg" in the filesystem to have
root uid/gid and a mode of rw-rw-rw, overriding the attributes obtained
from the TAR archive.


9. EXTENDED PSEUDO FILE DEFINITIONS WITH TIMESTAMPS
---------------------------------------------------

The Pseudo file definitions described above do not allow the timestamp
of the created file to be specified, and the files will be timestamped
with the current time.

Extended versions of the Pseudo file definitions are supported which
take a <time> timestamp.  These are distinquished from the previous
definitions by using an upper-case type character.  For example the "D"
definition is identical to the "d" definition, but it takes a <time>
timestamp.

The list of extended definitions are:

	filename F time mode uid gid command
	filename D time mode uid gid
	filename B time mode uid gid major minor
	filename C time mode uid gid major minor
	filename S time mode uid gid symlink
	filename I time mode uid gid [s|f]
	filename M time mode uid gid

<time> can be either an unsigned decimal integer (which represents the
seconds since the epoch of 1970-01-01 00:00 UTC), or a "date string"
which is parsed and converted into an integer since the epoch, by calling
the "date" command.

Because most date strings have spaces, they will need to be quoted, and if
entered on the command line, these quotes will need to be protected from the
shell by backslashes, i.e.

% sqfstar img.sqfs -p "file D \"1 jan 1980\" 0777 phillip phillip" < archive.tar

Obviously anything "date" accepts as a valid string can be used, such as
"yesterday", "last week" etc.


10. MISCELLANEOUS OPTIONS
------------------------

The -info option displays the files/directories as they are compressed and
added to the filesystem.  The original uncompressed size of each file
is printed, along with DUPLICATE if the file is a duplicate of a
file in the filesystem.

The -nopad option informs Sqfstar to not pad the filesystem to a 4K multiple.
This is performed by default to enable the output filesystem file to be mounted
by loopback, which requires files to be a 4K multiple.  If the filesystem is
being written to a block device, or is to be stored in a bootimage, the extra
pad bytes are not needed.