Everything in Linux is a file or a directory. Things in /dev are special files.

The majority of devices are either block or character devices; however other types of devices exist and can be created. In general, 'block devices' are devices that store or hold data, 'character devices' can be thought of as devices that transmit or transfer data. For example, diskette drives, hard drives and CD-ROM drives are all block devices while serial ports, mice and parallel printer ports are all character devices. There is a naming scheme of sorts but in the vast majority of cases these are completely illogical.

Devices are defined by type, such as 'block' or 'character', and 'major' and 'minor' number. The major number is used to categorize a device and the minor number is used to identify a specific device type. For example, all IDE device connected to the primary controller have a major number of 3. Master and slave devices, as well as individual partitions are further defined by the use of minor numbers.

Taken from Linux-Filesystem-Hierarchy

2 options:

1. Try

   
   
   

    cat < /dev/ttyS0
   
   

   
   
   

   and see if that works.




2. You might solve this error by adding your user to the same group (ie. dialout).