scripts: environment: Use system* instead of system.

This allows for direct program invokation without needing a shell to act
as a command interpreter.

* guix/scripts/environment.scm (%default-shell): New variable.
  (show-help): Adjust description.  Remove '--exec' reference.
  (%default-options): Use '%default-shell'.
  (%options): Adjust '--exec' to run command via the default shell.
  (parse-args): New procedure.
  (guix-environment): Use 'parse-args'.  Use 'system*' instead of
  'system'.
* tests/guix-environment.sh: Add test for '--' command invokation.
* doc/guix.texi ("Invoking guix environment"): Use new syntax.  Remove
  '--exec' documentation.

doc/guix.texi

@@ -4538,11 +4538,12 @@ and Emacs are available:
 guix environment guile emacs
 @end example
 
-Sometimes an interactive shell session is not desired.  The
-@code{--exec} option can be used to specify the command to run instead.
+Sometimes an interactive shell session is not desired.  An arbitrary
+command may be invoked by placing the @code{--} token to separate the
+command from the rest of the arguments:
 
 @example
-guix environment guile --exec=make
+guix environment guile -- make -j4
 @end example
 
 In other situations, it is more convenient to specify the list of
@@ -4551,7 +4552,7 @@ runs @command{python} from an environment containing Python@tie{}2.7 and
 NumPy:
 
 @example
-guix environment --ad-hoc python2-numpy python-2.7 -E python
+guix environment --ad-hoc python2-numpy python-2.7 -- python
 @end example
 
 The available options are summarized below.
@@ -4582,11 +4583,6 @@ As an example, @var{file} might contain a definition like this
 @verbatiminclude environment-gdb.scm
 @end example
 
-
-@item --exec=@var{command}
-@item -E @var{command}
-Execute @var{command} in the new environment.
-
 @item --ad-hoc
 Include all specified packages in the resulting environment, as if an
 @i{ad hoc} package were defined with them as inputs.  This option is
@@ -4596,7 +4592,7 @@ package expression to contain the desired inputs.
 For instance, the command:
 
 @example
-guix environment --ad-hoc guile guile-sdl -E guile
+guix environment --ad-hoc guile guile-sdl -- guile
 @end example
 
 runs @command{guile} in an environment where Guile and Guile-SDL are

guix/scripts/environment.scm

@@ -57,6 +57,9 @@ (define (evaluate-input-search-paths inputs search-paths)
 (define %precious-variables
   '("HOME" "USER" "LOGNAME" "DISPLAY" "TERM" "TZ" "PAGER"))
 
+(define %default-shell
+  (or (getenv "SHELL") "/bin/sh"))
+
 (define (purify-environment)
   "Unset almost all environment variables.  A small number of variables such
 as 'HOME' and 'USER' are left untouched."
@@ -103,17 +106,15 @@ (define (package+propagated-inputs package output)
     ,@(package-transitive-propagated-inputs package)))
 
 (define (show-help)
-  (display (_ "Usage: guix environment [OPTION]... PACKAGE...
-Build an environment that includes the dependencies of PACKAGE and execute a
-shell command in that environment.\n"))
+  (display (_ "Usage: guix environment [OPTION]... PACKAGE... [-- COMMAND...]
+Build an environment that includes the dependencies of PACKAGE and execute
+COMMAND or an interactive shell in that environment.\n"))
   (display (_ "
   -e, --expression=EXPR  create environment for the package that EXPR
                          evaluates to"))
   (display (_ "
   -l, --load=FILE        create environment for the package that the code within
                          FILE evaluates to"))
-  (display (_ "
-  -E, --exec=COMMAND     execute COMMAND in new environment"))
   (display (_ "
       --ad-hoc           include all specified packages in the environment instead
                          of only their inputs"))
@@ -135,7 +136,7 @@ (define (show-help)
 
 (define %default-options
   ;; Default to opening a new shell.
-  `((exec . ,(or (getenv "SHELL") "/bin/sh"))
+  `((exec . (,%default-shell))
     (system . ,(%current-system))
     (substitutes? . #t)
     (max-silent-time . 3600)
@@ -153,9 +154,9 @@ (define %options
          (option '("pure") #f #f
                  (lambda (opt name arg result)
                    (alist-cons 'pure #t result)))
-         (option '(#\E "exec") #t #f
+         (option '(#\E "exec") #t #f ; deprecated
                  (lambda (opt name arg result)
-                   (alist-cons 'exec arg result)))
+                   (alist-cons 'exec (list %default-shell "-c" arg) result)))
          (option '("search-paths") #f #f
                  (lambda (opt name arg result)
                    (alist-cons 'search-paths #t result)))
@@ -230,14 +231,24 @@ (define (build-inputs inputs opts)
                (built-derivations derivations)
                (return derivations))))))))
 
-;; Entry point.
-(define (guix-environment . args)
+(define (parse-args args)
+  "Parse the list of command line arguments ARGS."
   (define (handle-argument arg result)
     (alist-cons 'package arg result))
 
+  ;; The '--' token is used to separate the command to run from the rest of
+  ;; the operands.
+  (let-values (((args command) (split args "--")))
+    (let ((opts (parse-command-line args %options (list %default-options)
+                                    #:argument-handler handle-argument)))
+      (if (null? command)
+          opts
+          (alist-cons 'exec command opts)))))
+
+;; Entry point.
+(define (guix-environment . args)
   (with-error-handling
-    (let* ((opts     (parse-command-line args %options (list %default-options)
-                                         #:argument-handler handle-argument))
+    (let* ((opts     (parse-args args))
            (pure?    (assoc-ref opts 'pure))
            (ad-hoc?  (assoc-ref opts 'ad-hoc?))
            (command  (assoc-ref opts 'exec))
@@ -282,4 +293,7 @@ (define (handle-argument arg result)
                      (return #t))
                     (else
                      (create-environment inputs paths pure?)
-                     (return (exit (status:exit-val (system command)))))))))))))
+                     (return
+                      (exit
+                       (status:exit-val
+                        (apply system* command)))))))))))))

tests/guix-environment.sh

@@ -40,7 +40,15 @@ test "`wc -l < "$tmpdir/a"`" = 1
 cmp "$tmpdir/a" "$tmpdir/b"
 
 # Make sure the exit value is preserved.
-if guix environment --ad-hoc guile-bootstrap --pure -E 'guile -c "(exit 42)"'
+if guix environment --ad-hoc guile-bootstrap --pure -- guile -c '(exit 42)'
+then
+    false
+else
+    test $? = 42
+fi
+
+# Same as above, but with deprecated -E flag.
+if guix environment --ad-hoc guile-bootstrap --pure -E "guile -c '(exit 42)'"
 then
     false
 else
@@ -66,7 +74,7 @@ then
     # as returned by '--search-paths'.
     guix environment -e '(@@ (gnu packages commencement) gnu-make-boot0)'	\
 	 --no-substitutes --pure						\
-         --exec='echo $PATH $CPATH $LIBRARY_PATH' > "$tmpdir/b"
+         -- /bin/sh -c 'echo $PATH $CPATH $LIBRARY_PATH' > "$tmpdir/b"
     ( . "$tmpdir/a" ; echo $PATH $CPATH $LIBRARY_PATH ) > "$tmpdir/c"
     cmp "$tmpdir/b" "$tmpdir/c"