Каков был метод сжатия SquashFS?

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 to 5 as of SquashFS 4.3). 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 ║ ╚═══╩════════════════════╩════════════════════╝ 

^_(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.

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/'. Example:

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

Also looking for a better answer. I'm assuming you want to know because your OS doesn't seem to support the compression used in providing you with the squashedfs in question. Or perhaps like the unsquashfs on CentOS6, yours doesn't report the Compression type.

So I strings | head on the compressed file. The first line will be hsqs and the second line should indicate the compression method used. I believe I used this method to determine that 7zXZ indicating that 7zip was in fact the method. However, with gzip, no such luck. I have to use od -b on the file to get the octal dump and to try to match the file signature with those associated with the file program. Again, however, no such luck (the string I expected to find is 037 213 but I see only 037 221.