#!/bin/bash
#
# Synopsis:
#
#   #!/bin/bash
#   . export-json.bash
#   export_JSON <variables>
#
# Example:
#
#   $ (
#           source export-json.bash
#           export_JSON SSH_TTY \
#                       SSH_CONNECTION \
#                       SSH_CLIENT \
#                       SSH_AUTH_SOCK
#     )
#
#   {
#     "SSH_TTY": "/dev/pts/0",
#     "SSH_CONNECTION": "192.0.2.140 41564 198.51.100.25 22",
#     "SSH_CLIENT": "192.0.2.140 41564 22",
#     "SSH_AUTH_SOCK": "/tmp/ssh-XXXXcePKJl/agent.36326"
#   }
#
# This bash function exports shell variables as
# JSON ("javascript object notation") strings.
#
# The string is printed to stdout.
#
# JSON is a format that represents data as
# Javascript source code so modern programmers
# can read it unlike bash source code.
#
# Variable names are given to the function as its
# argument list:
#
#       $ (source export-json.bash; export_JSON PATH)
#
# The output is a single JSON object containing
# key-value mappings between JSON strings:
#
#       {
#         "PATH": "/usr/local/sbin:[...]"
#       }
#
# It uses the external tool "jq" to parse string
# values placed in jq's argument list by bash and
# then encode them as JSON string values. This
# is no accidental dependency. The jq program is
# the foundation of the trustworthiness of this
# code. If we were encoding JSON strings in bash
# we would have to be a lot more careful.

arg1_to_env0()
{
	case "$1" in
		*[^a-zA-Z0-9_=]* | *=*=* )
			echo "Error: invalid variable: '$1'" >&2
			return 10
			;;
		[a-zA-Z_]* )
			set -- "${1#*=}" "${1%%=*}"
			;;
		* )
			echo "Error: invalid variable: '$1'" >&2
			return 20
			;;
	esac
	if [ -v "$2" ]
	then
		printf '%s=%s\0' "$1" "${!2}"
	else
		echo "Warning: ignoring unset variable: '$2'" >&2
	fi
}

vars_to_env0()
{
	while [ $# -gt 0 ]
	do
		arg1_to_env0 "$1" || return
		shift
	done
}

env0_to_JSON()
{
	set --
	while read -d ''
	do
		set -- "$@" --arg "${REPLY%%=*}" "${REPLY#*=}"
	done
	jq -n -r '$ARGS.named' "$@"
}

export_JSON_unsafe()
{
	(
		set -o pipefail
		vars_to_env0 "$@" | env0_to_JSON
	)
}

safe_stdout()
{
	set -- "$(mktemp)" "$@"
	if (shift; "$@") > "$1"
	then
		r=0
		cat < "$1"
	else
		r=$?
	fi
	rm "$1"
	return $r
}

export_JSON()
{
	safe_stdout export_JSON_unsafe "$@"
}

try()
{
	"$@"
	printf '(Exit %s) <- [%s]\n' "$?" "${*@Q}" >&2
}

runtest()
{
	set -- SSH_CLIENT SSH_TTY SSH_AUTH_SOCK SSH_CONNECTION
	try export_JSON "$@"
	unset unsetvar
	try export_JSON SSH_TTY unsetvar
	try export_JSON unsetvar SSH_TTY
	try export_JSON
	try export_JSON ''
	try export_JSON '' SSH_TTY
}