SHELL env var assumed to exist during install

[maxrake]

Overview

When the install script is run in an environment where the SHELL environment variable is not set, the output includes a warning. This is not a failure of the script because it will still work. It is more just a blemish on the output, which may cause confusion with users who may think that an error has occurred.

Further, the instructions that get printed after install include a repeated word (file) when the SHELL env var is an unrecognized value.

How To Reproduce

Steps to reproduce this behavior:

• Install the Phylum CLI in an environment where the SHELL environment variable is not set
  • This is the case in some/most CI systems (e.g., GitHub, GitLab, etc.)
  • unset the env var
• See the error in the output:

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Expected Behavior

A sane default value is provided when there is no SHELL environment variable set.

Additional Context

N/A