[flang] Document the requirements for the `flang` wrapper script

[awarzynski]

Summary

This was discussed in more detail in
https://github.com/flang-compiler/f18-llvm-project/issues/1344. Many
thanks to Anthony Cabrera for the suggestion.

[awarzynski]

Add a function to check the Bash version. It will exit immediately if an unsupported version of Bash is used.

[kiranchandramohan]

LGTM. Tested by changing the expectation to a higher version.

flang/tools/f18/flang

 #! /usr/bin/env bash
 #===-- tools/f18/flang.sh -----------------------------------------*- sh -*-===#
 #
 # Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 # See https://llvm.org/LICENSE.txt for license information.
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 #
 #===------------------------------------------------------------------------===#
 # A wrapper script for Flang's compiler driver that was developed for testing and
 # experimenting. You should be able to use it as a regular compiler driver. It
 # will:
 #   * run Flang's compiler driver to unparse the input source files
 #   * use the external compiler (defined via FLANG_FC environment variable) to
 #   compile the unparsed source files
+#
+# Tested with Bash 4.4.23. This script will exit immediately if you use an
+# older version of Bash.
 #===------------------------------------------------------------------------===#
 set -euo pipefail
 
 # Global variables to make the parsing of input arguments a bit easier
 INPUT_FILES=()
 OPTIONS=()
 OUTPUT_FILE=""
 MODULE_DIR=""
 INTRINSICS_MOD_DIR=""
 COMPILE_ONLY="False"
 PREPROCESS_ONLY="False"
 PRINT_VERSION="False"
 
+# === check_bash_version ======================================================
+#
+# Checks the Bash version that's used to run this script. Exits immediately if
+# it's lower than 4.4.23
+# =============================================================================
+check_bash_version() {
+  message="Error: Your Bash is too old. Please use Bash >= 4.4.23"
+  # Major version
+  [[ "${BASH_VERSINFO[0]:-0}" -lt 4 ]] && echo $message && exit 1
+
+  # Minor version
+  if [[ "${BASH_VERSINFO[0]}" == 4 ]]; then
+    [[ "${BASH_VERSINFO[1]:-0}" -lt 4 ]] && echo $message && exit 1
+  fi
+
+  # Patch version
+  if [[ "${BASH_VERSINFO[0]}" == 4 ]] && [[ "${BASH_VERSINFO[1]}" == 4 ]] ; then
+    [[ "${BASH_VERSINFO[2]:-0}" -lt 23 ]] && echo $message && exit 1
+  fi
+}
+
 # === parse_args ==============================================================
 #
 # Parse the input arguments passed to this script. Sets the global variables
 # declared at the top.
 #
 # INPUTS:
 #   $1 - all input arguments
 # OUTPUTS:
get_relocatable_name() {
 
   echo "$out_file"
 }
 
 # === main ====================================================================
 # Main entry point for this script
 # =============================================================================
 main() {
+  check_bash_version
   parse_args "$@"
 
   if [[ $PRINT_VERSION == "True" ]]; then
     echo "flang version @FLANG_VERSION@"
     exit 0
   fi
 
   # Source, object and library files provided by the user