snapcraft felt it was important I share this with you :-) Running snapcraft on Ubuntu 19.04. $ snapcraft --version snapcraft, version 2.43.1+18.10 The log: $ snapcraft clean Sorry, an error occurred in Snapcraft: Traceback (most recent call last): File "/usr/bin/snapcraft", line 11, in <module> load_entry_point('snapcraft==2.43.1+18.10', 'console_scripts', 'snapcraft')() ... File "/usr/lib/python3/dist-packages/yaml/composer.py", line 127, in compose_mapping_node while not self.check_event(MappingEndEvent): File "/usr/lib/python3/dist-packages/yaml/parser.py", line 98, in check_event self.current_event = self.state() File "/usr/lib/python3/dist-packages/yaml/parser.py", line 439, in parse_block_mapping_key "expected <block end>, but found %r" % token.id, token.start_mark) yaml.parser.ParserError: while parsing a block mapping in "snapcraft.yaml", line 19, column 3 expected <block end>, but found '<block mapping start>' in "snapcraft.yaml", line 36, column 4 We would appreciate it if you created a bug report at https://launchpad.net/snapcraft/+filebug with the above text included. The snapcraft.yaml provoking the crash (note the inconsistent indent level between the two parts): name: fpm version: 1.1 summary: Effing Package Management description: | Build packages for multiple platforms (deb, rpm, etc) with great ease and sanity. http://fpm.readthedocs.io/en/latest/ grade: devel confinement: devmode apps: fpm: environment: RUBYLIB: $SNAP/usr/lib/ruby/2.3.0:$SNAP/usr/lib/x86_64-linux-gnu/ruby/2.3.0 GEM_HOME: $SNAP/gems GEM_PATH: $SNAP command: ruby $SNAP/bin/fpm parts: fpm: source: . plugin: nil build-packages: - build-essential - ruby-dev - rubygems-integration - libreadline-dev - libssl-dev - libtinfo-dev stage-packages: - ruby - rubygems-integration override-build: | snapcraftctl build gem install fpm -i $SNAPCRAFT_PART_INSTALL mangle-shebang: plugin: nil after: [fpm] override-build: | snapcraftctl build sed -i 's/#!\/usr\/bin\/ruby/#!\/usr\/bin\/env ruby/' $SNAPCRAFT_STAGE/usr/bin/{gem2.3,rdoc2.3,fpm,rake} Removing the extra space at the start of the 'mangle-shebang:' line worked around the problem