If the umask is set up to exlude execute, the installer will set up
calibre to be unreadable by any user other than the installing user.
When installing as `root`, a starting umask of `0o077` results in the
installer choosing a new umask of `0o033`, which means the calibre
install directory (and child directories) are installed with permissions
`0o744`, so only the installing user can actually use the resulting
calibre install.
To test, start with a system umask of `0o077` and run the installer
script as root:
```
% sudo sh ./setup/linux-installer.sh
Using python executable: /usr/bin/python3
WARNING: Your current umask disallows reading of files by some users, this can cause system breakage when running the installer because of bugs in common system utilities.
Should the installer (f)ix the umask, (i)gnore it or (a)bort [f/i/a Default is abort]: f
umask changed to: 022
Installing to /opt/calibre
Downloading tarball signature securely...
Using previously downloaded calibre-3.26.1-x86_64.txz
Extracting files to /opt/calibre ...
Extracting application files...
Creating symlinks...
Symlinking /opt/calibre/ebook-edit to /usr/bin/ebook-edit
Symlinking /opt/calibre/calibre-debug to /usr/bin/calibre-debug
Symlinking /opt/calibre/web2disk to /usr/bin/web2disk
Symlinking /opt/calibre/calibre-parallel to /usr/bin/calibre-parallel
Symlinking /opt/calibre/calibredb to /usr/bin/calibredb
Symlinking /opt/calibre/lrs2lrf to /usr/bin/lrs2lrf
Symlinking /opt/calibre/calibre to /usr/bin/calibre
Symlinking /opt/calibre/ebook-convert to /usr/bin/ebook-convert
Symlinking /opt/calibre/calibre-server to /usr/bin/calibre-server
Symlinking /opt/calibre/ebook-viewer to /usr/bin/ebook-viewer
Symlinking /opt/calibre/calibre-smtp to /usr/bin/calibre-smtp
Symlinking /opt/calibre/ebook-meta to /usr/bin/ebook-meta
Symlinking /opt/calibre/ebook-device to /usr/bin/ebook-device
Symlinking /opt/calibre/fetch-ebook-metadata to /usr/bin/fetch-ebook-metadata
Symlinking /opt/calibre/ebook-polish to /usr/bin/ebook-polish
Symlinking /opt/calibre/lrfviewer to /usr/bin/lrfviewer
Symlinking /opt/calibre/calibre-customize to /usr/bin/calibre-customize
Symlinking /opt/calibre/lrf2lrs to /usr/bin/lrf2lrs
Symlinking /opt/calibre/markdown-calibre to /usr/bin/markdown-calibre
Setting up command-line completion...
Installing zsh completion to: /usr/share/zsh/site-functions/_calibre
Installing bash completion to: /usr/share/bash-completion/completions/calibre
Setting up desktop integration...
Creating un-installer: /usr/bin/calibre-uninstall
Run "calibre" to start calibre
```
And the resulting directory:
```
% ls -ld /opt/calibre
drwxr-xr-x. 5 root root 4.0K Jun 22 10:53 /opt/calibre/
```
Also verify the Python version, starting with no `/opt/calibre` and a system umask of `0o077`:
```
% sudo python3 ./setup/linux-installer.py
WARNING: Your current umask disallows reading of files by some users, this can cause system breakage when running the installer because of bugs in common system utilities.
Should the installer (f)ix the umask, (i)gnore it or (a)bort [f/i/a Default is abort]: f
umask changed to: 022
Installing to /opt/calibre
Downloading tarball signature securely...
Using previously downloaded calibre-3.26.1-x86_64.txz
Extracting files to /opt/calibre ...
Extracting application files...
Creating symlinks...
Symlinking /opt/calibre/calibre-customize to /usr/bin/calibre-customize
Symlinking /opt/calibre/calibre-smtp to /usr/bin/calibre-smtp
Symlinking /opt/calibre/calibredb to /usr/bin/calibredb
Symlinking /opt/calibre/calibre to /usr/bin/calibre
Symlinking /opt/calibre/ebook-polish to /usr/bin/ebook-polish
Symlinking /opt/calibre/ebook-meta to /usr/bin/ebook-meta
Symlinking /opt/calibre/calibre-server to /usr/bin/calibre-server
Symlinking /opt/calibre/markdown-calibre to /usr/bin/markdown-calibre
Symlinking /opt/calibre/fetch-ebook-metadata to /usr/bin/fetch-ebook-metadata
Symlinking /opt/calibre/lrf2lrs to /usr/bin/lrf2lrs
Symlinking /opt/calibre/calibre-parallel to /usr/bin/calibre-parallel
Symlinking /opt/calibre/ebook-convert to /usr/bin/ebook-convert
Symlinking /opt/calibre/ebook-viewer to /usr/bin/ebook-viewer
Symlinking /opt/calibre/web2disk to /usr/bin/web2disk
Symlinking /opt/calibre/calibre-debug to /usr/bin/calibre-debug
Symlinking /opt/calibre/ebook-device to /usr/bin/ebook-device
Symlinking /opt/calibre/lrfviewer to /usr/bin/lrfviewer
Symlinking /opt/calibre/ebook-edit to /usr/bin/ebook-edit
Symlinking /opt/calibre/lrs2lrf to /usr/bin/lrs2lrf
Setting up command-line completion...
Installing zsh completion to: /usr/share/zsh/site-functions/_calibre
Installing bash completion to: /usr/share/bash-completion/completions/calibre
Setting up desktop integration...
Creating un-installer: /usr/bin/calibre-uninstall
Run "calibre" to start calibre
% ls -ld /opt/calibre
drwxr-xr-x. 5 root root 4.0K Jun 22 10:55 /opt/calibre/
```
After each, verify calibre starts as a non-root user.
Needed because Ubuntu 18.04 has decided, in its infinite wisdom to
have no default python executable, which makes it impossible to run
polyglot python scripts.
