Describe the bug
The documentation at https://help.github.com/en/actions/reference/workflow-syntax-for-github-actions#jobsjob_idstepsrun implies that when a shell is not specified on a non-Windows platform, bash (if found) will be invoked as bash --noprofile --norc -eo pipefail {0}.

Fail-fast behavior using set -e o pipefail: Default for bash and built-in shell. It is also the default when you don't provide an option on non-Windows platforms.

However, this is not exactly the case.

To Reproduce
Steps to reproduce the behavior:

1. Create an action without specifying a shell value:

    - name: No shell specified (default bash)
      run: echo Hello, world!

2. Observe that it is executed as shell: /bin/bash -e {0} as per this example at https://github.com/johnstevenson/runner-actions-test/actions/runs/48283923 which also shows the documented behaviour when the shell is specified.

Expected behavior
For the command to be executed as shell: /bin/bash --noprofile --norc -e -o pipefail {0}

Runner Version and Platform

Current runner version: 2.165.2
Tested on ubuntu-latest and macos-latest

What's not working?

Regardless of whether this is a documentation problem (or my incorrect interpretation), there is no consistency when invoking bash with and without a shell value.

The ScriptHandler:RunAsync code (also duplicated in ScriptHandler:PrintActionDetails) sets shellCommand as sh even if bash is found, resulting in the wrong format string being used:

So any fix would be:

#if OS_WINDOWS
                shellCommand = "pwsh";
                commandPath = WhichUtil.Which(shellCommand, require: false, Trace, prependPath);
                if (string.IsNullOrEmpty(commandPath))
                {
                    shellCommand = "powershell";
                    Trace.Info($"Defaulting to {shellCommand}");
                    commandPath = WhichUtil.Which(shellCommand, require: true, Trace, prependPath);
                }
                ArgUtil.NotNullOrEmpty(commandPath, "Default Shell");
#else
                shellCommand = "bash";
                commandPath = WhichUtil.Which(shellCommand, require: false, Trace, prependPath)
                if (string.IsNullOrEmpty(commandPath))
                {
                    shellCommand = "sh";
                    Trace.Info($"Defaulting to {shellCommand}");
                    commandPath = WhichUtil.Which(shellCommand, require: true, Trace, prependPath);
                }
                ArgUtil.NotNullOrEmpty(commandPath, "Default Shell");
#endif
                argFormat = ScriptHandlerHelpers.GetScriptArgumentsFormat(shellCommand);

I'd happily PR this if needed and if I could find out how you debug an action in Visual Studio (are there any docs about this?)