cqfd: add exec command

TL;DR; The command run takes arguments as if they are given to system()
(i.e. the command_string is intepreted by sh -c). The command shell
takes arguments as if they are given to execlp($SHELL, ...), and the new
command exec takes arguments as they are given to execlp() (i.e. for the
two last, the command is not interpreted by sh; except for the arguments
that are not simple-quoted and that are subject to get interpreted by
the current shell). In short:
 - cqfd run -> sh -c
 - cqfd shell -> sh
 - cqfd exec -> ssh

This adds the exec command to run a command from within the container.

The command consists of the exec-program and its arguments (if any), as
it is given to the C function execlp(). The exec-program could be either
a binary or a script.

	$ cqfd exec COMMAND [ARGUMENTS...]

It is a distinct CLI to the commands run and shell that both takes a
command_string as it is given to the C function system() (i.e. given to
sh -c):
 - cqfd shell has three different forms:
   cqfd shell [command_file [argument...]]
   cqfd shell -c command_string [command_name [argument...]]
   cqfd shell -s [argument...] (requires CQFD_EXTRA_RUN_ARGS=-i for now)
 - cqfd run [-c] [command_string] takes a command_string as specified by
   sh(1) or the command= attribute from the file .cqfdrc.
   Important: the option -c of run is for concatenate and it is
   different to the option -c of sh(1).

Note: The command exec (as the command shell) does not run the command=
set in the file .cqfdrc.

According to sh(1):

	argument

	The positional parameters ($1, $2, and so on) shall be set to
	arguments, if any.

	command_file

	The pathname of a file containing commands. If the pathname
	contains one or more <slash> characters, the implementation
	attempts to read that file; the file need not be executable. If
	the pathname does not contain a <slash> character:
	* The implementation shall attempt to read that file from the
	  current working directory; the file need not be executable.
	* If the file is not in the current working directory, the
	  implementation may perform a search for an executable file
	  using the value of PATH, as described in Section 2.9.1.1,
	  Command Search and Execution.

	Special parameter 0 (see Section 2.5.2, Special Parameters)
	shall be set to the value of command_file. If sh is called using
	a synopsis form that omits command_file, special parameter 0
	shall be set to the value of the first argument passed to sh
	from its parent (for example, argv[0] for a C program), which is
	normally a pathname used to execute the sh utility.

	command_name

	A string assigned to special parameter 0 when executing the
	commands in command_string. If command_name is not specified,
	special parameter 0 shall be set to the value of the first
	argument passed to sh from its parent (for example, argv[0] for
	a C program), which is normally a pathname used to execute the
	sh utility.

	command_string

	A string that shall be interpreted by the shell as one or more
	commands, as if the string were the argument to the system()
	function defined in the System Interfaces volume of
	POSIX.1‐2017. If the command_string operand is an empty string,
	sh shall exit with a zero exit status.

Fixes: savoirfairelinux#99

bash-completion

@@ -48,9 +48,9 @@ _cqfd() {
 
 	local arg=
 	_get_first_arg
-	if [[ "$arg" == run ]]; then
+	if [[ "$arg" =~ ^(exec|run)$ ]]; then
 		for (( i=1; i <= cword; i++ )); do
-			if [[ ${words[i]} == run ]]; then
+			if [[ ${words[i]} =~ ^(exec|run)$ ]]; then
 				if [[ $((i+1)) -eq $cword ]]; then
 					break
 				elif [[ ${words[i+1]} == -c ]]; then
@@ -61,7 +61,11 @@ _cqfd() {
 			fi
 		done
 
-		COMPREPLY=( $(compgen -c -W "-c" -- "$cur") )
+		if [[ "$arg" == exec ]]; then
+			COMPREPLY=( $(compgen -c -- "$cur") )
+		else
+			COMPREPLY=( $(compgen -c -W "-c" -- "$cur") )
+		fi
 		return
 	elif [[ "$arg" == shell ]]; then
 		for (( i=1; i <= cword; i++ )); do
@@ -84,7 +88,7 @@ _cqfd() {
 		return
 	fi
 
-	local cmds="init flavors run release shell version help"
+	local cmds="init flavors exec run release shell version help"
 	COMPREPLY=( $(compgen -W "$cmds $opts" -- "$cur") )
 } &&
 complete -F _cqfd cqfd

cqfd

@@ -44,6 +44,7 @@ Options:
 Commands:
     init                 Initialize project build container.
     flavors              List flavors from config file to stdout.
+    exec cmd [args]      Run argument(s) inside build container.
     run [cmdstring]      Run argument(s) inside build container.
     release [cmdstring]  Run argument(s) and release software.
     shell [shargs]       Run shell command inside build container.
@@ -493,6 +494,16 @@ while [ $# -gt 0 ]; do
 	-q)
 		quiet=true
 		;;
+	exec)
+		if [ "$#" -lt 2 ]; then
+			die "command exec requires arguments"
+		fi
+		shift
+		config_load $flavor
+		command_string="${@@Q}"
+		docker_run "$command_string"
+		exit
+		;;
 	run|release)
 		if [ "$1" = "release" ]; then
 			has_to_release=true