[terrelln]

Both PEXs and XARs package a python script and its dependencies in single hermetic file.

PEX is a self-extracting zip file which has to be fully extracted before being run. The extracted files could potentially be modified.

XAR is a self-mounting compressed SquashFS filesystem image. SquashFS will decompress pages lazily and cache the result in the page cache, so the startup time is much faster. Since SquashFS is read-only, the files can't be modified.