Default to /bin/sh if SHELL is not set?

[tsarna]

I spent quite a bit of time trying to understand a weird problem with Hashicorp Vault Agent. The post-template-rendering command execution failed. The error was unhelpful:

failed parsing command: fork/exec : no such file or directory:

"

After much befuddlement, it eventually turned out that this only happened when the command contained backticks, and was due to SHELL not being set in the environment, leading to a failure in shellRun in util_posix.

It seems like it would be reasonable to default to /bin/sh here if the variable is not set.