bash-completion for alias of full command line (with subcommands)

bash-completion-alias.bash

#!/bin/bash

# ==============================================================================
#  |#
#  |# Example aliases with bash-completion support
#  |#
#
#complete-alias sc systemctl           # sc <tab>  => systemctl <tab>
#complete-alias scs systemctl status   # scs <tab> => systemctl status <tab>
#
# ==============================================================================

# Wrapper for `_completion_alias` that automatically determines the completion
# function to use via `_completion_handler`.
complete-alias() {
	if [[ ${#} -lt 2 ]]; then
		printf -- 'usage:\n\tcomplete-alias alias command...\n' >&2
		return 1
	fi
	_completion_alias "${1}" $( _completion_handler "${2}" ) "${@:2}"
}

# _completion_alias installs an existing bash-completion handler for a seperate
# executable or alias. 
#
# For example, "complete-alias scs _systemctl systemctl status" will use the
# existing bash-completion function "_systemctl" to auto-complete arguments
# for "systemctl status" when typing the command "scs <tab>".
#
# The bash-completion function for a given command can be found using the
# function `_completion_handler` below.
_completion_alias() {
	local name func
	case ${#} in
	0|1)	return 1 ;;
	2)	_completion_alias "${1}" $( _completion_handler "${2}" ) "${2}"
		return $? ;;
	*)	name=${1}
		func=${2} ;;
	esac
	# if the completer is dynamic and not yet loaded, try to load it
	# automatically using the given command
	if [[ $( type -t "${func}" ) != function ]]; then
		type -p _completion_loader &> /dev/null &&
			_completion_loader "${@:3}"
	fi
	[[ $( type -t "${func}" ) == function ]] || return 2
	comp=$( tr -d -c '[A-Za-z_]' <<< "${name}" )
	eval "
		function _${comp} {
			(( COMP_CWORD += $(( ${#} - 3 )) ))
			COMP_LINE=\${COMP_LINE//${name}/${*:3}}
			_comp_split COMP_WORDS \"\${COMP_LINE}\"
			local -a args=( \"\${@}\" )
			for i in \${!args[@]}; do
				[[ \"\${args[\${i}]}\" != "${name}" ]] ||
					args[\${i}]=${3}
			done
			"${func}" \"\${args[@]}\"
			return 0
		}
	"
	complete -F "_${comp}" "${name}"
	alias ${name}="${*:3}"
}

# Print the bash-completion function used to generate possible completions for
# a given command. Any completion options defined with the original spec are
# also carried over to the resulting spec.
# 
# If no completion spec is defined for the given command, uses the default
# function "_minimal" provided by the bash-completion package.
#
# The primary use-case is for installing aliases that use the bash-completion
# results of other command lines, as with `_completion_alias` defined above.
_completion_handler() {
	local cmd=${1} compcmd=${1}
	local cspec=$( complete -p "${cmd}" 2>/dev/null )
	if [[ -z ${cspec} && "${cmd}" == */* ]]; then
		cspec=$( complete -p ${cmd##*/} 2>/dev/null )
		[[ -n ${cspec} ]] && compcmd=${cmd##*/}
	fi
	if [[ -z ${cspec} ]]; then
		compcmd=${cmd##*/}
		_completion_loader ${compcmd}
		cspec=$( complete -p ${compcmd} 2>/dev/null )
	fi
	if [[ -n ${cspec} ]]; then
		# apply and remove all option arguments
		while [[ ${cspec} == *" -o "* ]]; do
			cspec=${cspec#*-o }
			compopt -o "${cspec%% *}" "${compcmd}"
			cspec=${cspec#${cspec%% *} }
		done
		if [[ ${cspec#* -F } != "${cspec}" ]]; then
			# easy case, we have a "<function>" arg of flag -F
			local func=${cspec#*-F }
			echo ${func%% *}
		elif [[ ${cspec#* -C } != "${cspec}" ]]; then
			local call=${cspec#*-C }
			call=${call%% *}
			# declare a named function that calls "<command>" arg of flag -C
			eval "function _${call##*/} { ${call}; }"
			#export -f _${call##*/}
			echo "_${call##*/}"
		else
			# all other cases, we remove what we don't want.
			cspec=${cspec#complete}    # leading "complete" command
			cspec=${cspec%%${compcmd}} # trailing "<compcmd>" (target) command
			# whatever remains is our function
			echo "${cspec}"
		fi
		# =============================================
		#  TBD: handle builtin and action completions?
		# =============================================
	else
		# default completion handler
		echo '_minimal'
	fi
}