In this guide I will be showing you how to remount filesystem read-write using the Android operating system on a smartphone, however, since it is Linux based this guide should work on most variants and distributions of Linux.
The system partition on Android is read-only to prevent an application or user changes which may adversely affect the operation or function of the device, however, these are some instances where it is useful to have access to write information to the system partition, for example removing bloatware, changing the host's file or modifying init.d.
If you do not have access to write to the partition your request to save or change data will result in a "Permission Denied" error. To get around this, you need to remount filesystem read-write permission.
Remount filesystem as read-write can only be done with root access. You can do this by installing terminal on the device if it isn't already installed and running the following commands.
$ su $ mount -o rw,remount /system
/system is the filesystem to remount read-write. Now you can perform any action you wish on the system partition. Be careful what you do though, as it was protected for a reason. You should reboot the device to reset permissions back to their defaults and prevent any accidental changes.