Hold both bash and PowerShell code in a single script file and/or make a PowerShell.ps1 script executable on all platforms. For Windows, *nix, linux and macOS. See Comments below

Bash-or-PowerShell-with-Param.ps1

` # \
# PowerShell Param statement : every line must end in #\ except the last line must with <#\
# And, you can't use backticks in this section        #\
param( [ValidateSet('A','B')]$tabCompletionWorksHere, #\
       [switch]$andHere                               #\
     )                                               <#\
#^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `
#vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv
# Bash Start ------------------------------------------------------------

scriptdir="`dirname "${BASH_SOURCE[0]}"`";
echo BASH. Script is running from $scriptdir

# Bash End --------------------------------------------------------------
# ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
echo > /dev/null <<"out-null" ###
'@ | out-null
#vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv
# Powershell Start ----------------------------------------------------#>

$scriptdir=$PSScriptRoot
Write-host "Powershell. This script is running from $scriptdir."

# Powershell End -------------------------------------------------------
# ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
out-null
#vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv
# Both Bash and Powershell run the rest but with limited capabilities

echo "Some lines work in both bash and powershell."
echo "Powershell picked up the named parameter '$'tabCompletionWorksHere=$tabCompletionWorksHere '$'andHere=$andHere"
echo "Whilst bash got positional parameters \$1=$1 and \$2=$2"

Bash-or-Powershell.ps1

# This file has a bash section followed by a powershell section,
# as well as shared sections.
echo @'
' > /dev/null
#vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv
# Bash Start -----------------------------------------------------------

scriptdir="`dirname "${BASH_SOURCE[0]}"`";
echo BASH. Script is running from $scriptdir

# Bash End -------------------------------------------------------------
# ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
echo > /dev/null <<"out-null" ###
'@ | out-null
#vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv
# Powershell Start -----------------------------------------------------

$scriptdir=$PSScriptRoot
"powershell. Script is running from $scriptdir"

# Powershell End -------------------------------------------------------
# ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
out-null

echo "Some lines work in both bash and powershell. Calculating scriptdir=$scriptdir, requires separate sections."

Powershell-executable-on-win-and-nix.ps1

#! /usr/local/bin/powershell
# A single Powershell file can be recognised as executable on all platforms:  Windows, linux & macOS
@"
This file is PowerShell only, but it can run unmodified on Windows, linux & macOs
Windows recognises it as executable because of the the ps1 suffix. 
linux and macOS recognises the #! header and so will pass the script to powershell to run.
linux and macOS do still require the execute bit set: chmod a+x *.ps1, or to be run like bash *.ps1
"@

[System.Environment]::OSVersion

Powershell-or-bash.ps1

# This file has a powershell section first followed by the bash section,
# as well as shared sections. But it is more complicated. Putting the
# the bash section first, followed by powershell, is simpler.
# ----------------------------------------------------------------------
echo @"
" > /dev/null ; echo > /dev/null <<"out-null" ###
"@ | out-null 
#vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv
# Powershell Start -----------------------------------------------------

$scriptdir=$PSScriptRoot
"powershell. Script is running from $scriptdir"

# Powershell End -------------------------------------------------------
# ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
out-null
echo @'
' > /dev/null
#vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv
# Bash Start -----------------------------------------------------------

scriptdir="`dirname "${BASH_SOURCE[0]}"`";
echo BASH. Script is running from $scriptdir

# Bash End -------------------------------------------------------------
# ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
echo > /dev/null <<out-null
'@ | out-null
out-null
# ------------------------------------------------------

echo "Some lines work in both bash and powershell. Calculating scriptdir=$scriptdir, requires separate sections."