Add PATH searching

* geesh/shell.scm (slashless?) New function.
(split-search-path): New function.
(find-utility): New function.
(exec-let): Use them to search PATH for a utility when appropriate.
(exec): Update doc string.
* tests/shell.scm: Test it.

geesh/shell.scm

@@ -22,12 +22,38 @@ will be @var{args}."
       (pid (match-let (((pid . status) (waitpid pid)))
              (set-var! env "?" (number->string (status:exit-val status))))))))
 
+(define (slashless? s)
+  "Test if the string @var{s} does not contain any slashes ('/')."
+  (not (string-index s #\/)))
+
+(define (split-search-path s)
+  "Split the search path string @var{s}."
+  (if (string-null? s) '() (string-split s #\:)))
+
+(define (find-utility env name)
+  "Search for the path of the utility @var{name} using @var{env}.  If
+it cannot be found, return @code{#f}."
+  (let loop ((prefixes (split-search-path (var-ref* env "PATH"))))
+    (and (pair? prefixes)
+         (let* ((prefix (car prefixes))
+                (path (if (string-suffix? "/" prefix)
+                          (string-append prefix name)
+                          (string-append prefix "/" name))))
+           (if (access? path X_OK)
+               path
+               (loop (cdr prefixes)))))))
+
 (define (sh:exec-let env bindings name . args)
-  "Execute @var{name} with arguments @var{args}, environment
+  "Find and execute @var{name} with arguments @var{args}, environment
 @var{env}, and extra environment variable bindings @var{bindings}."
-  (exec-utility env bindings name name args))
+  (if (slashless? name)
+      (or (and=> (find-utility env name)
+                 (lambda (path)
+                   (exec-utility env bindings path name args)))
+          (error "Command not found."))
+      (exec-utility env bindings name name args)))
 
 (define (sh:exec env name . args)
-  "Execute @var{name} with arguments @var{args} and environment
-@var{env}."
+  "Find and execute @var{name} with arguments @var{args} and
+environment @var{env}."
   (apply sh:exec-let env '() name args))

tests/shell.scm

@@ -94,4 +94,30 @@
        (sh:exec env utility)
        (file-exists? sentinal)))))
 
+(test-assert "Executes a utility by searching PATH"
+  (call-with-temporary-directory
+   (lambda (directory)
+     (let ((utility (string-append directory "/utility"))
+           (sentinal (string-append directory "/sentinal.txt"))
+           (env (make-environment `(("PATH" . ,directory)))))
+       (make-script utility
+         (with-output-to-file ,sentinal
+           (lambda ()
+             (display "x"))))
+       (sh:exec env "utility")
+       (file-exists? sentinal)))))
+
+(test-assert "Throws error if a utility cannot be found"
+  (call-with-temporary-directory
+   (lambda (directory)
+     (let ((env (make-environment `(("PATH" . ,directory)))))
+       (catch #t
+         (lambda ()
+           (sh:exec env "utility")
+           #f)
+         (lambda args
+           (match args
+             (('misc-error _ _ ("Command not found.") _) #t)
+             (_ #f))))))))
+
 (test-end)