Update page '6-8-2023 Cohort 1 Sync Notes'

6-8-2023 Cohort 1 Sync Notes.md

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+1425h Started late
+
+* Karen can't execute the python file in the `pricer` repository; attempting to do `./fp2_14.py` results in syntax errors
+* Explained how the Linux execution model works
+  * Executable permission, octal permission bits, what octal is, world|group|user, not guaranteed on Windows
+  * 1) binfmt subsystem checks for valid *binary* formats at https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/exec.c?h=v6.3
+  * 2) if none of the registered binfmt handlers think it's one of theirs, then
+  * 3) fall back to #! processing (pronounced "sh-bang")
+  * 4) give it to /bin/sh
+  * 5) if `/bin/sh` can't figure out what's in the file, then the kernel won't execute it, and will return an error
+  * https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/
+  * What shell means, tootsie roll pops, family tree: https://i.pinimg.com/736x/39/29/41/392941169141711c54c7de093f6c8938--software-libre-software-development.jpg
+  * What polyglot means
+  * Polyglot interpretation of the `#` character
+  * \#\!
+  * bourne shell reserved words; ` ! case  coproc  do done elif else esac fi for function if in select then until while { } time [[ ]]`
+  * `command not found` is a strong hint that the executing interpreter is some kind of command language, usually bourne shell
+  * `exec.c`
+  * `/usr/bin/env` is used to do path lookup on an executable in cases where the shell does not have a PATH, or other environment variables need to be specified
+  * Kernel does not enforce namespace policies, or filesystem layout policies
+  * Binary formats: ELF: unix/linux, Mach-O: macos, PE/COFF: windows
+  * default shell: bourne, decoupling of the `/bin/sh` *name* and the actual binary
+  * Best practices: 
+    * begin every program with a shebang (`#!`)
+    * have a company standard for what to put after the shebang, whether to use /usr/bin/env or not
+    * make sure scripts have the executable permission when checked in to the source repository
+
+Questions:
+
+ * What happens if you put random garbage into a file and try to execute it?
+ * What scripting languages do not use `#` as a comment character, and cannot use the `#!` mechanism to be directly executed
+
+1525h Karen used the suggested troubleshooting procedure:
+```
+the troubleshooting process should be 1) I know I checked out the repository 2) running the file from the default current directory ('.' is the current directory) doesn't work 3) logically, I can conclude that the file is not in my default current directory, so 4) it's somewhere else 5) I can find where it is by using `find . -name git` as shown around 1450h in the notes yesterday https://gogs.gtsplab.com:5432/SVIG/python-bootcamp/wiki/6-7-2023+Cohort+1+Sync+Notes
+```
+
+She assumed that she didn't set up the python environment, then logged into the container and ran the program, got an error when using `python`, tried `python3` instead, and now the program executes.
+
+Make sure that executable files do not touch any unfiltered data, especially from the Internet, because the bourne shell has zero security and falling back to it for execution is a security hazard.
+
+Enrichment reading: shellshock https://en.wikipedia.org/wiki/Shellshock_(software_bug)
+
+Karen points out that selenium does not require explicit installation, because it's included in `requirements.txt`
+
+The next major hurdle will be getting `chromium-browser` to run headless.  Search terms: X-windows, Weyland, Xvfb, framebuffer
+