user@vps:~$ sudo apt install squashfs-tools
[sudo] password for user:
Reading package lists... Done
Building dependency tree
Reading state information... Done
squashfs-tools is already the newest version (1:4.3-3ubuntu2).
squashfs-tools set to manually installed.
0 upgraded, 0 newly installed, 0 to remove and 2 not upgraded.
user@vps:~$ cd /tmp
user@vps:/tmp$ mkdir foo
user@vps:/tmp$ mkdir baz
user@vps:/tmp$ touch foo/bar
user@vps:/tmp$ mksquashfs foo foo.squash
Parallel mksquashfs: Using 2 processors
Creating 4.0 filesystem on foo.squash, block size 131072.


Exportable Squashfs 4.0 filesystem, gzip compressed, data block size 131072
	compressed data, compressed metadata, compressed fragments, compressed xattrs
	duplicates are removed
Filesystem size 0.19 Kbytes (0.00 Mbytes)
	89.24% of uncompressed filesystem size (0.22 Kbytes)
Inode table size 44 bytes (0.04 Kbytes)
	66.67% of uncompressed inode table size (66 bytes)
Directory table size 21 bytes (0.02 Kbytes)
	84.00% of uncompressed directory table size (25 bytes)
Number of duplicate files found 1
Number of inodes 2
Number of files 1
Number of fragments 0
Number of symbolic links  0
Number of device nodes 0
Number of fifo nodes 0
Number of socket nodes 0
Number of directories 1
Number of ids (unique uids + gids) 1
Number of uids 1
	user (1000)
Number of gids 1
	user (1000)
user@vps:/tmp$ sudo mount foo.squash baz
mount: /tmp/foo.squash: failed to setup loop device: Operation not permitted
user@vps:/tmp$ ls baz/
user@vps:/tmp$