POSIX Shell collapse_string function.

collapse_string.sh

#!/usr/bin/env sh

# Disable this error because ShellCheck doesn't understand the conditional
# usage of positional arguments.
# shellcheck disable=SC2120
collapse_string() {
	{
		if [ $# -gt 0 ]; then
			# Use positional arguments.
			echo "$@"
		elif [ ! -t 0 ]; then
			# Use piped input.
			cat < /dev/stdin
		else
			# No input.
			echo ''
		fi
	} | tr -d '\a\b\f\r\n\t\v'
}

# positional example: collapse_string "$(printf '\thello\n \nworld\t')" => 'hello world'
# pipe example: printf '\thello\n \nworld\t' | collapse_string => 'hello world'