umask: add page (#834)

pages/linux/umask.md

@@ -0,0 +1,19 @@
+# umask
+
+> Manage the read/write/execute permissions that are masked out (i.e. restricted) for newly created files by the user.
+
+- Display the current mask in octal notation:
+
+`umask`
+
+- Display the current mask in symbolic (human-readable) mode:
+
+`umask -S`
+
+- Change the mask symbolically to allow read permission for all users (the rest of the mask bits are unchanged):
+
+`umask {{a+r}}`
+
+- Set the mask (using octal) to restrict no permissions for the file's owner, and restrict all permissions for everyone else:
+
+`umask {{077}}`