What was the SquashFS compression method?

unsquashfs -s did not have the capability of displaying the compression type used until this commit on 07 August 2009. This means that if you are running squashfs-tools 4.0 or older, you wouldn't be able to see the compression method used.

From this information, I derived a way to read the SquashFS 4.0 superblock to determine the compression method used (where $SQUASHFS is the path to your SquashFS file):

dd if=$SQUASHFS bs=1 count=2 skip=20 2>/dev/zero | od -An -tdI | xargs

Alternatively, here's a function for those who would like to type in the filename at the end of the line:

sqsh_comp_method(){ dd if="$1" bs=1 count=2 skip=20 2>/dev/zero|od -An -tdI | xargs;};sqsh_comp_method

You will get a number (between 1 and 6 as of SquashFS 4.4). You can match that number to the following table to see what compression method was used:

╔═══╦════════════════════╦════════════════════╗
║ # ║ Compression Method ║ Compatible Version ║
╠═══╬════════════════════╬════════════════════╣
║ 1 ║ gzip               ║ 1.0 and newer      ║
║ 2 ║ lzma               ║ 4.1 and newer      ║
║ 3 ║ lzo                ║ 4.1 and newer      ║
║ 4 ║ xz                 ║ 4.2 and newer      ║
║ 5 ║ lz4                ║ 4.3 and newer      ║
║ 6 ║ zstd               ║ 4.4 and newer      ║
╚═══╩════════════════════╩════════════════════╝

^_(Source)

Note that the above dd command will only provide a reliable output if the file you specified had a SquashFS 4.0 superblock. The following command will output "Not SquashFS 4.0" if the file $SQUASHFS does not have the SquashFS 4.0 magic number:

if [[ "$(dd if="$SQUASHFS" bs=1 count=4 skip=28 2>/dev/zero | xxd -p)" != "04000000" ]] ; then echo -n "Not " ; fi ; echo "SquashFS 4.0"

Explanation

In SquashFS 4.0 filesystems, the compression method is stored on the 21^st and 22^nd bytes of the superblock as a data type short. dd bs=1 count=2 skip=20 will retrieve the short, od -An -tdI will turn the short into a human-readable number, and xargs is just to get rid of the leading spaces.

Before SquashFS 4.0, there was only the gzip method.

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Old answer

unsquashfs has the -s flag for displaying SquashFS filesystem information.

Example usage:

deltik@node51 [/tmp]# unsquashfs -s template.squashfs
Found a valid SQUASHFS 4:0 superblock on template.squashfs.
Creation or last append time Thu Apr 30 23:07:23 2015
Filesystem size 47225242.44 Kbytes (46118.40 Mbytes)
Compression gzip
Block size 131072
Filesystem is exportable via NFS
Inodes are compressed
Data is compressed
Fragments are compressed
Always_use_fragments option is not specified
Xattrs are compressed
Duplicates are removed
Number of fragments 23629
Number of inodes 437076
Number of ids 1

If you just want the compression type identified, you could pipe the output through awk '/^Compression/{print $2}'. Example:

deltik@node51 [/tmp]# unsquashfs -s template.squashfs | awk '/^Compression/{print $2}'
gzip