Fix erroneous function call on NetBSD

Some non-Linux systems do not have a `/sys/` directory. Therefore,
check it only if we are in a Linux system.

Fixes #19

eshell-info-banner.el

@@ -2,7 +2,7 @@
 
 ;; Author: Lucien Cartier-Tilet <lucien@phundrak.com>
 ;; Maintainer: Lucien Cartier-Tilet <lucien@phundrak.com>
-;; Version: 0.8.2
+;; Version: 0.8.4
 ;; Package-Requires: ((emacs "25.1") (f "0.20") (s "1"))
 ;; Homepage: https://github.com/Phundrak/eshell-info-banner.el
 
@@ -237,7 +237,9 @@ Ensures the command is ran with LANG=C."
   "Display a progress bar without its prefix.
 Display a progress bar of BAR-LENGTH length, followed by an
 indication of how full the memory is with a human readable USED
-and TOTAL size."
+and TOTAL size.
+Optional argument NEWLINE: Whether to output a newline at the end
+of the progress bar."
   (let ((percentage (if (= used 0)
                         0
                       (/ (* 100 used) total))))
@@ -468,6 +470,8 @@ For TEXT-PADDING and BAR-LENGTH, see the documentation of
                         t)))
 
 (defun eshell-info-banner--get-memory-unix-command-to-mem (command)
+  "Get the output of COMMAND corresponding to memory information.
+This function is to be only used on platforms which support sysctl."
   (string-to-number
    (s-trim
     (car (last
@@ -478,7 +482,7 @@ For TEXT-PADDING and BAR-LENGTH, see the documentation of
 (defun eshell-info-banner--get-memory-netbsd ()
   "Get memory usage for NetBSD systems.
 See `eshell-info-banner--get-memory'."
-  (let* ((total (eshell-info-banner--get-memory-unix-command-to-mem `("sysctl hw.physmem64")))
+  (let* ((total (eshell-info-banner--get-memory-unix-command-to-mem "sysctl hw.physmem64"))
          (used  (- total
                    (* 1024 (string-to-number
                             (s-trim
@@ -655,7 +659,9 @@ The usage of `eshell-info-banner-warning-percentage' and
 thought of as the “percentage of discharge” of the computer.
 Thus, setting the warning at 75% will be translated as showing
 the warning face with a battery level of 25% or less."
-  (let ((battery-level (when (file-readable-p "/sys/") (battery))))
+  (let ((battery-level (unless (and (equal system-type 'gnu/linux)
+                                    (not (file-readable-p "/sys/")))
+                         (battery))))
     (if (or (null battery-level)
             (string= battery-level "Battery status not available")
             (string-match-p (regexp-quote "N/A") battery-level))