MKSQUASHFS(1)

NAME
mksquashfs - tool to create and append to squashfs filesystems

SYNOPSIS
mksquashfs source1 source2 ... FILESYSTEM [OPTIONS] [-e list of exclude dirs/files]

DESCRIPTION
Squashfs is a highly compressed read-only filesystem for Linux. It uses either gzip/xz/lzo/lz4/zstd compression to compress both files, inodes and directories. Inodes in the system are very small and all blocks are packed to minimise data overhead. Block sizes greater than 4K are supported up to a maximum of 1Mbytes (default block size 128K).

Squashfs is intended for general read-only filesystem use, for archival use (i.e. in cases where a .tar.gz file may be used), and in constrained block device/memory systems (e.g. embedded systems) where low overhead is needed.

OPTIONS

  Filesystem compression options:

-b BLOCK-SIZE
    set data block to BLOCK-SIZE. Default 128 Kbytes. Optionally a suffix of K, KB, Kbytes or M, MB, Mbytes can be given to specify Kbytes or Mbytes respectively.

-comp COMP
    select COMP compression. Run -help-comp COMP to get compressor options for COMP, or ALL for all the compressors. Compressors available: gzip (default), lzo, lz4, xz, zstd, lzma.

-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:

-tar
    read uncompressed tar file from standard in (stdin).

-no-strip
    act like tar, and do not strip leading directories from source files.

-tarstyle
    alternative name for -no-strip.

-cpiostyle
    act like cpio, and read file pathnames from standard in (stdin).

-cpiostyle0
    like -cpiostyle, but filenames are null terminated. Can be used with find -print0 action.

-no-exports
    do not make filesystem exportable via NFS (-tar default).

-exports
    make filesystem exportable via NFS (default).

-no-sparse
    do not detect sparse files.

-no-tailends
    do not pack tail ends into fragments (default).

-tailends
    pack tail ends into fragments.

-no-fragments
    do not use fragments.

-no-duplicates
    do not perform duplicate checking.

-no-hardlinks
    do not hardlink files, instead store duplicates.

-keep-as-directory
    if one source directory is specified, create a root directory containing that directory, rather than the contents of the directory.

  Filesystem time options:

-mkfs-time TIME
    set filesystem creation timestamp to TIME. TIME can be "inode", which means use the latest inode timestamp, 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".

-inode-time TIME
    set all file and directory timestamps to TIME. TIME can be "inode", which means use the latest inode timestamp, 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 2025". This option sets and overrides the -root-time option.

-root-time TIME
    set root directory time to TIME. TIME can be "inode", which means use the latest inode timestamp, 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 2025".

-repro
    build a reproducible filesystem image. This is equivalent to -mkfs-time inode, which achieves reproducibility by setting the filesystem build time to the latest inode timestamp. Obviously the image won't be reproducible if the timestamps or content changes.

-repro-time TIME
    build a reproducible filesystem image. This is equivalent to specifying -mkfs-time TIME and -inode-time TIME, which achieves reproducibility by setting all timestamps to TIME. This option can be used in cases where timestamps may change, and where -repro cannot be used for this reason.

  Filesystem permissions options:

-all-root
    make all files and directories owned by root.

-root-mode MODE
    set root directory permissions to MODE. MODE can be symbolic or octal (see section Symbolic mode specification). Default root mode is ugo=rwx or 0777 octal.

-root-uid USER
    set root directory owner to specified USER, USER can be either an integer uid or user name.

-root-gid GROUP
    set root directory group to specified GROUP, GROUP can be either an integer gid or group name.

-force-file-mode MODE
    set all file (non-directory) permissions to MODE. MODE can be symbolic or octal (see section Symbolic mode specification).

-force-dir-mode MODE
    set all directory permissions to MODE. MODE can be symbolic or octal (see section Symbolic mode specification).

-force-uid USER
    set all file and directory uids to specified USER, USER can be either an integer uid or user name.

-force-gid GROUP
    set all file and directory gids to specified GROUP, GROUP can be either an integer gid or group name.

-uid-gid-offset VALUE
    offset all uid and gids by specified VALUE. VALUE should be a positive integer.

  Filesystem pseudo options:

-p PSEUDO-DEFINITION
    add pseudo file definition. The definition should be quoted. See section "Pseudo file definition format" for format details.

-pd "d mode uid gid"
    specify a default pseudo directory which will be used in pseudo definitions if a directory in the pathname does not exist. This also allows pseudo definitions to be specified without specifying all the directories in the pathname. The definition should be quoted.

-pd "D time mode uid gid"
    specify a default pseudo directory which will be used in pseudo definitions if a directory in the pathname does not exist. The D type also allows a timestamp to be specified in addition to mode, uid and gid.

-pf PSEUDO-FILE
    add list of pseudo file definitions from PSEUDO-FILE, use - for stdin. Pseudo file definitions should not be quoted.

-pseudo-override
    make pseudo file uids and gids override -all-root, -force-uid and -force-gid options.

  Filesystem filter options:

-sort SORT-FILE
    sort files according to priorities in SORT-FILE. One file or dir with priority per line. Priority -32768 to 32767, default priority 0.

-ef EXCLUDE-FILE
    list of exclude dirs/files. One per line.

-wildcards
    allow extended shell wildcards (globbing) to be used in exclude dirs/files.

-regex
    allow POSIX regular expressions to be used in exclude dirs/files.

-max-depth LEVELS
    descend at most LEVELS of directories when scanning filesystem.

-one-file-system
    do not cross filesystem boundaries. If a directory crosses the boundary, create an empty directory for each mount point. If a file crosses the boundary ignore it.

-one-file-system-x
    do not cross filesystem boundaries. Like -one-file-system option except directories are also ignored if they cross the boundary.

  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.

  Mksquashfs runtime options:

-version
    print version, licence and copyright message.

-exit-on-error
    treat normally ignored errors as fatal.

-quiet
    no verbose output.

-info
    print files written to filesystem to stdout. This automatically disables the progress bar. See -info-file to output to file without disabling the progress bar.

-info-file FILE
    print files written to filesystem to file FILE. This does not disable the progress bar.

-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 Mksquashfs.

-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 Mksquashfs (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.

-single-reader
    use a single thread to read files. This reads files sequentially from the source(s).

-small-readers N
    use N threads to read small files (files less than a block size) in parallel from the source(s) (default 4).

-block-readers N
    use N threads to read block files (files a block or larger in size) in parallel from the source(s) (default 4).

-overcommit PERCENT
    allow PERCENT more threads to run in parallel than available processors. Doing this may increase CPU utilisation. Default is 0%, because normally overcommiting reduces performance due to trashing. The percentage value is at the granularity of the number of processors, e.g. 4 processors have a percentage granularity of 25%, and 20 processors have a percentage granularity of 5%.

  Filesystem append options:

-noappend
    do not append to existing filesystem.

-root-becomes NAME
    when appending source files/directories, make the original root become a subdirectory in the new root called NAME, rather than adding the new source items to the original root.

-no-recovery
    do not generate a recovery file.

-recovery-path NAME
    use NAME as the directory to store the recovery file.

-recover NAME
    recover filesystem data using recovery file NAME.

  Filesystem actions options:

-action ACTION@EXPRESSION
    evaluate EXPRESSION on every file and directory, and execute ACTION if it returns TRUE.

-log-action ACTION@EXPRESSION
    as -action, but log expression evaluation results and actions performed.

-true-action ACTION@EXPRESSION
    as -action, but only log expressions which return TRUE.

-false-action ACTION@EXPRESSION
    as -action, but only log expressions which return FALSE.

-action-file FILE
    as action, but read actions from FILE.

-log-action-file FILE
    as -log-action, but read actions from FILE.

-true-action-file FILE
    as -true-action, but read actions from FILE.

-false-action-file FILE
    as -false-action, but read actions from FILE.

  Tar file only options:

-default-mode MODE
    tar files often do not store permissions for intermediate directories. This option sets the default directory permissions to MODE. MODE can be symbolic or octal (see section Symbolic mode specification). Default mode is u=rwx,go=rx or 0755 octal. This also sets the root directory mode.

-default-uid VALUE
    tar files often do not store uids for intermediate directories. This option sets the default directory owner to VALUE, rather than the user running Mksquashfs. VALUE can be either an integer uid or user name. This also sets the root directory uid.

-default-gid VALUE
    tar files often do not store gids for intermediate directories. This option sets the default directory group to VALUE, rather than the group of the user running Mksquashfs. VALUE can be either an integer uid or group name. This also sets the root directory gid.

-ignore-zeros
    allow tar files to be concatenated together and fed to Mksquashfs. Normally a tarfile has two consecutive 512 byte blocks filled with zeros which means EOF and Mksquashfs will stop reading after the first tar file on encountering them. This option makes Mksquashfs ignore the zero filled blocks.

  Expert options (these may make the filesystem unmountable):

-stream
    output the filesystem to STDOUT rather than to a file. This allows the output to be piped to another program or elsewhere with ssh. The resultant streamed Squashfs filesystem will not be a normal Squashfs filesystem because the super block will be at the end of the filesystem. But it can be fixed up afterwards with the -fix option and Unsquashfs 4.7.3 and later can read streamed unfixed up filesystems directly. Use - for the output FILESYSTEM on the command line. Duplicate detection is disabled using the -stream option.

-fix FILESYSTEM
    fix FILESYSTEM generated using the -stream option.

-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.

  Help options:

-help
    print help summary information to pager (or stdout if not a terminal).

-help-option REGEX
    print the help information for options matching REGEX to pager (or stdout if not a terminal).

-help-section SECTION
    print the help information for section SECTION to pager (or stdout if not a terminal). If SECTION does not exactly match a section name, it is treated as a regular expression, and all section names that match are displayed. Use "list" as section name to get a list of sections and their names.

-help-comp COMP
    print compressor options for compressor COMP. Use "list" to get a list of available compressors, and "all" to get the compressor options for all the compressors.

-help-all
    print help information for all Mksquashfs options and sections to pager (or stdout if not a terminal).

-Xhelp
    print compressor options for selected compressor.

-h  shorthand alternative to -help.

-ho REGEX
    shorthand alternative to -help-option.

-hs SECTION
    shorthand alternative to -help-section.

-ha shorthand alternative to -help-all.

-no-pager
    do not use a pager to output help information.

-cols WIDTH
    use WIDTH columns to output help information. Useful if output is not to a terminal.

  Miscellaneous options:

-fstime TIME
    alternative name for -mkfs-time.

-always-use-fragments
    alternative name for -tailends.

-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.

-pseudo-dir
    alternative name for -pd.

PSEUDO FILE DEFINITION FORMAT

-p "filename d mode uid gid"
    create a directory.

-p "filename m mode uid gid"
    modify filename.

-p "filename b mode uid gid major minor"
    create a block device.

-p "filename c mode uid gid major minor"
    create a character device.

-p "filename f mode uid gid command"
    create file from stdout of command.

-p "filename s mode uid gid symlink"
    create a symbolic link.

-p "filename i mode uid gid [s|f]"
    create a socket (s) or FIFO (f).

-p "filename x name=val"
    create an extended attribute.

-p "filename h linkname"
    create a hard-link to linkname, follows symlinks.

-p "filename l linkname"
    create a hard-link to linkname.

-p "filename L pseudo_filename"
    same, but link to pseudo file.

-p "filename D time mode uid gid"
    create a directory with timestamp time.

-p "filename M time mode uid gid"
    modify a file with timestamp time.

-p "filename B time mode uid gid major minor"
    create block device with timestamp time.

-p "filename C time mode uid gid major minor"
    create char device with timestamp time.

-p "filename F time mode uid gid command"
    create file with timestamp time.

-p "filename S time mode uid gid symlink"
    create symlink with timestamp time.

-p "filename I time mode uid gid [s|f]"
    create socket/fifo with timestamp time.

SYMBOLIC MODE SPECIFICATION
The symbolic mode is of the format [ugoa]*[[+-=]PERMS]+. PERMS = [rwxXst]+ or [ugo], and the sequence can be repeated separated with commas.

A combination of the letters ugoa specify which permission bits will be affected, u means user, g means group, o means other, and a means all or ugo.

The next letter is +, - or =. The letter + means add to the existing permission bits, - means remove the bits from the existing permission bits, and = means set the permission bits.

The permission bits (PERMS) are a combination of [rwxXst] which sets/adds/removes those bits for the specified ugoa combination, r means read, w means write and x means execute for files or search for directories. X has a special meaning, if the file is a directory it is equivalent to x or search, but if it is a non-directory, it only takes effect if execute is already set for user, group or other. The s flag sets user or group ID on execution, and the t flag on a directory sets restricted deletion, or historically made the file sticky if a non-directory.

The permission bits can also be u, g or o, which takes the permission bits from the user, group or other of the file respectively.

EXIT STATUS

0   Mksquashfs successfully generated a filesystem.

1   Fatal errors occurred, Mksquashfs aborted and did not generate a filesystem (or update if appending).

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, arm64, powerpc, sparc, ia64, riscv.

-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 -131072 .. -1 or 1 .. 22 (default 15). Negative compression levels correspond to the zstd --fast option.

  lzma:

    (no options) (deprecated - no kernel support)

ENVIRONMENT

SQFS_CMDLINE
    If set, this is used as the directory to write the file sqfs_cmdline which contains the command line arguments given to Mksquashfs. Each command line argument is wrapped in quotes to ensure there is no ambiguity when arguments contain spaces. If the file already exists then the command line is appended to the file.

SOURCE_DATE_EPOCH
    If set, this is used as the filesystem creation timestamp. Also any file timestamps which are after SOURCE_DATE_EPOCH will be clamped to SOURCE_DATE_EPOCH. See https://reproducible-builds.org/docs/source-date-epoch/ for more information.

PAGER
    If set, this is used as the name of the program used to display the help text. The value can be a simple command or a pathname. The default is /usr/bin/pager.

EXAMPLES

mksquashfs DIRECTORY IMAGE.SQFS
    Create a Squashfs filesystem from the contents of DIRECTORY, writing the output to IMAGE.SQFS. Mksquashfs will use the default compressor (normally gzip), and a block size of 128 Kbytes.

mksquashfs DIRECTORY FILE1 FILE2 IMAGE.SQFS
    Create a Squashfs filesystem containing DIRECTORY and FILE1 and FILE2. If multiple sources are specified on the command line they will be combined into a single directory.

mksquashfs DIRECTORY IMAGE.SQFS -b 1M -comp zstd
    Use a block size of 1 Mbyte and Zstandard compression to create the filesystem.

mksquashfs DIRECTORY IMAGE.SQFS -all-time now
    Set all file and directory timestamps to "now" (current time) in the filesystem.

mksquashfs DIRECTORY IMAGE.SQFS -force-uid phillip -force-gid phillip
    Make all files and directories in the filesystem owned by "phillip".

mksquashfs DIRECTORY IMAGE.SQFS -force-file-mode ugo+r,go-w
    Make all files readable by everyone, but only writable by user, all other permission bits are unaffected.

mksquashfs DIRECTORY IMAGE.SQFS -e file1 file2
    Exclude file1 and file2 from DIRECTORY when creating filesystem. No wildcard matching of files.

mksquashfs DIRECTORY IMAGE.SQFS -wildcards -e "*.gz"
    Exclude anything in DIRECTORY which matches the wildcard pattern "*.gz".

mksquashfs DIRECTORY IMAGE.SQFS -wildcards -e "... *.gz"
    Exclude files which match the wildcard pattern "*.gz" anywhere within DIRECTORY and its sub-directories. The initial "..." indicates the wildcard pattern is "non-anchored" and will match anywhere.

Note: when passing wildcarded names to Mksquashfs, they should be quoted (as in the above examples), to ensure that they are not processed by the shell.

  Using pseudo file definitions

mksquashfs DIRECTORY IMAGE.SQFS -p "build_dir d 0644 0 0"
    Create a directory called "build_dir" in the output filesystem.

mksquashfs DIRECTORY IMAGE.SQFS -p "version.txt l /tmp/build/version"
    Create a reference called "version.txt" to a file outside DIRECTORY, which acts as if the file "/tmp/build/version" was copied or hard-linked into DIRECTORY before calling Mksquashfs.

mksquashfs DIRECTORY IMAGE.SQFS -p "date.txt f 0644 0 0 date"
    Create a file called "date.txt" which holds the output (stdout) from running the "date" command.

mksquashfs DIRECTORY IMAGE.SQFS -p "\"hello world\" f 0644 0 0 date"
    As above, but, showing that filenames can have spaces, if they are quoted. The quotes need to be blackslashed to protect them from the shell.

mksquashfs - IMAGE.SQFS -p "input f 0644 root root dd if=/dev/sda1 bs=1024" -p "/ d 0644 0 0"
    Create a file containing the contents of partition /dev/sda1". Ordinarily Mksquashfs 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. Note there are no other sources than the pseudo file, and so the command line source is "-". If there are no other sources than pseudo files, the root (/) directory must be defined too, as seen in this example.

unsquashfs -pf - IMAGE.SQFS | mksquashfs - NEW.SQFS -pf -
    Transcode IMAGE.SQFS to NEW.SQFS by piping the pseudo file output from Unsquashfs to Mksquashfs using stdout and stdin. This can convert from earlier Squashfs filesystems or change compression algorithm, block size etc. without needing to unpack into an intermediate directory or file.

Note: pseudo file definitions should be quoted (as in the above examples), to ensure that they are passed to Mksquashfs as a single argument, and to ensure that they are not processed by the shell.

  Using extended attribute options

mksquashfs DIRECTORY IMAGE.SQFS -no-xattrs
    Do not store any extended attributes in the Squashfs filesystem. Any extended attributes in the source files will be ignored.

mksquashfs DIRECTORY IMAGE.SQFS -xattrs-include "^user."
    Filter the extended attributes in the source files, and only store extended attributes in the user namespace in the Squashfs filesystem.

mksquashfs DIRECTORY IMAGE.SQFS -xattrs-exclude "^user."
    Filter the extended attributes in the source files, and don't store any extended attributes in the user namespace in the Squashfs filesystem.

mksquashfs DIRECTORY IMAGE.SQFS -xattrs-add "user.comment=hello world"
    Add the extended attribute called "user.comment" with the content "hello world" to all files and directories in the Squashfs filesystem.

mksquashfs DIRECTORY IMAGE.SQFS -xattrs-add "user.comment=0thello world\012"
    Add the extended attribute called "user.comment" to all files and directories, but in this case the contents of the extended attribute will be "hello world" with a trailing newline character (012 octal).

mksquashfs DIRECTORY IMAGE.SQFS -xattrs-add "user.comment=0saGVsbG8gd29ybGQ="
    Add the extended attribute called "user.comment" to all files and directories, where the value is given in base64 encoding, representing "hello world".

mksquashfs DIRECTORY IMAGE.SQFS -action "-xattrs-include(^user.) @ type(f)"
    Filter the extended attributes but only in regular files (type f), and only store extended attributes in the user namespace.

mksquashfs DIRECTORY IMAGE.SQFS -p "hello_world x user.comment=0tsalve mundi\012"
    Add the extended attribute called "user.comment" to the file called "hello_world", with the contents of the extended attribute being "salve mundi" with a trailing newline character (012 octal).

  Using Actions to not compress, change attributes etc.

mksquashfs DIRECTORY IMAGE.SQFS -action "uncompressed @ (name(*.jpg) || name(*.mpg) ) || (name(*.img) && filesize(>=1G))"
    Specify that any files matching the wildcards "*.jpg" and "*.mpg" should not be compressed. Additionally, it also specifies any files matching the wildcard "*.img" and are 1 Gigabyte or larger should be uncompressed too. This shows test operators can be combined with logical expressions.

mksquashfs DIRECTORY IMAGE.SQFS -action "chmod(o+r)@! perm(o+r)"
    If any files within DIRECTORY are not readable by "others", then make them readable by others in the Squashfs filesystem.

mksquashfs DIRECTORY IMAGE.SQFS -action "uid(phillip)@! perm(o+r)"
    As previous, match on any files which are not readable by "others", but, in this case change the owner of the file to "phillip" in the Squashfs filesystem.

mksquashfs DIRECTORY IMAGE.SQFS -action "prune @ type(l) && ! exists"
    Delete any symbolic link within DIRECTORY which points outside of DIRECTORY, i.e. will be unresolvable in the Squashfs filesystem.

mksquashfs DIRECTORY IMAGE.SQFS -action "exclude @ depth(3)"
    Create a Squashfs filesystem containing the two top most levels (contents of DIRECTORY and immediate sub-directories), and exclude anything at level 3 or below.

mksquashfs DIRECTORY IMAGE.SQFS -action "-xattrs-include(^user.) @ type(f)"
    Filter the extended attributes but only in regular files (type f), and only store extended attributes in the user namespace.

Note: actions should be quoted (as in the above examples), to ensure that they are passed to Mksquashfs as a single argument, and to ensure that they are not processed by the shell.

AUTHOR
Written by Phillip Lougher <phillip@squashfs.org.uk>

COPYRIGHT
Copyright © 2025 Phillip Lougher <phillip@squashfs.org.uk>

This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2, or (at your option) any later version.

This program 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 General Public License for more details.

SEE ALSO
unsquashfs(1), sqfstar(1), sqfscat(1)

The README for the Squashfs-tools 4.7.4 release, describing the new features can be read here https://github.com/plougher/squashfs-tools/blob/master/Documentation/4.7.4/README.md

The MKSQUASHFS usage guide can be read here https://github.com/plougher/squashfs-tools/blob/master/Documentation/4.7.4/USAGE-MKSQUASHFS.md

The CHANGELOG for Squashfs tools can be read here https://github.com/plougher/squashfs-tools/blob/master/CHANGES.md

The ACTIONS-README for the actions subsystem can be read here https://github.com/plougher/squashfs-tools/blob/master/Documentation/4.7.4/ACTIONS-README.md