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3. The SquashFS tools exposed

3.1. Using mksquashfs

mksquashfs is the tool for creating new squashed file systems, and for appending new data to existing squashed file systems. The general command-line format for mksquashfs is:

  bash# mksquashfs source1 source2 ... destination [options]

  • source1, source2, etc.: files and directories to be added to the resulting file system, given with relative and/or absolute paths

  • destination: a regular file (filesystem image file), or a block device (such as /dev/fd0 or /dev/hda3) where you want to have your squashed file system

Notes for default mksquashfs behavior:

  • When the new files are added to the new file system or appended to an existing one, mksquashfs will automatically rename files with duplicate names: if two or more files named text will appear in the same resulting directory, the second file will be renamed to text_1, third one to text_2 and so on.

  • Duplicate files will be removed, so there will be only one physical instance (By the SquashFS 2.x, you can disable the detection/removal of the duplicates with the -no-duplicates option).

  • If destination has a pre-existing SquashFS file system on it, by default, the new source items will be appended to the existing root directory. Examine the options table below to force mksquashfs to overwrite the whole destination and/or change the way new source items are added.

  • If a single source file or directory is given, it becomes the root in a newly created file system. If two or more source files and/or directories are given, they will all become sub-items in the root of the new file system.

  • The resulting filesystem will be padded to a multiple of 4 Kb: this is required for filesystems to be used on block devices. If you are very sure you don't ned this, use the -nopad option to disable this operation.

See the next section for more details about all possible options.

3.2. Command-line options

All possible options for mksquashfs are shown in the table below.

Table 1. Command-line options of the mksquashfs tool

Option Description
-2.0 force mksquashfs version 2.1 to create a version 2.0 filesystem
-all-root or -root-owned make all files in the target file system owned by root (UID=0, GID=0)
-always-use-fragments divide all files greater than block size into fragments (by the version 2.x). It will result in greater compression ratios
-b [block size] use [block size] filesystem block size (32 Kbytes default for 2.x, 128 kbytes for 3.x) - this can be either 4096, 8192, 16384, 32768, 65536 or 131072
-be or -le force a big or little endian file system, respectively
-check-data enable additional file system checks
-e [file1] ( [file2] ... ) specify which files and/or directories to omit from the new file system that is to be created
-ef [file] specify a file which contains the list of files/directories to exclude
-force-gid [GID] set all group IDs in target file system to [GID] (can be specified as a name or a number)
-force-uid [UID] set all user IDs in target file system to [UID] (can be specified as a name or a number)
-info print files, their original size and compression ratio, as they are added to the file system
-keep-as-directory if the source is a single directory, force this directory to be a subdirectory of the root in the created file system
-noappend if the destination file/device already contains a squashed file system, overwrite it, rather than append the new data to an existing file system
-no-duplicates do not detect/remove duplicate file names
-noD or -noDataCompression do not compress the data
-noF or -noFragmentCompression do not compress the fragments (avaliable by 2.x)
-no-fragments do not generate fragment blocks (avaliable by 2.x, this will produce almost the same filesystem as 1.x did)
-noI or -noInodeCompression do not compress the inode table
-nopad do not pad the resulting file system to a multiple of 4 KBytes
-root-becomes [name] can be used while appending to a pre-existing squashed file system: it will make a new root, and [name] directory will contain all pre-existing files/directories
-version print the version, copyright and license message
-recover [name] recover filesystem data using recovery file [name] (3.3)
-no-recovery don't create a recovery file (3.3).
-no-exports don't make avaliable file system to export via NFS (3.x)
-no-sparse don't check for sparse files (3.x)
-processors [number] set the number of CPU to create file system. By default it will be used all avaliable processors (3.x)
--read-queue [size] set input queue to [size] Mb. (Default is 64 Mb)(3.x)
-write-queue [size] set output queue to [size] Mb (3.x)
-sort [sort_file] sort files relating to priorities in [sort_file] (3.x)
-wildcards enable the extended shell wildcards to exclude directories/files (to be used with -e)
-regex enable to use POSIX regular expressions (3.3)

In most cases, you should leave all compression/block options by default, as they allow mksquashfs to achieve the best possible compression ratios.

3.3. Using unsquashfs

unsquashfs is the tool for extracting data from squashed file systems. The general command-line format for unsquashfs is:

unsquashfs [options] target [files/directories to extract]

  • target is the squashed file system to extract.

Notes for unsquashfs behavior:

  • By not specifying any destination path, unsquashfs extracts the compressed file system in the ./squashfs-root directory.

  • The tool does not extract a squashed file system on already exsisting directory unless the -f option is specified.

  • You can specify on the command line, a multiple number of files/directories to extract and the items to be extracted can be also be given in a file with -e [file] option.

All possible options for unsquashfs are shown in the table below.

Table 2. Command-line options of the unsquashfs tool

Option Description
-v[ersion] print the version, licence and copyright message
-i[nfo] print the files as they are extracted from the file system
-l[ist] list the squashed file system without extracting files
-li list files with theyr attributes as they are unsquashed (3.3)
-ll list the squashed file system files with attributes without any extraction (3.3)
-d[estination] path specify a destination path for unsquashed items
-f[orce] if files exist overwrite them
-s[tat] display file system's superblock informations (it can discover the file system version and the options used to compress it) (3.3)
-e[f] [extract file] list of directories or files to extract (entries given one per line) (3.3)
-r[egex] treat extract names as POSIX regular expressions (3.3)

Note that by 3.x release you can extract 1.x and 2.x squashed file system too.