Unable to write on /dev/* files - linux

Question

I'm writing a basic char device driver for Linux kernel.
For this, the code flow I have considered is as follows:
alloc_chrdev_region() -> to use dynamic allocation of major number
class_create() -> to create device class in sysfs
device_creat() -> to create device under /dev/
cdv_init() -> to initialize char device structure
cdev_add() -> to add my device structure in kernel
I have added read, write, open, release methods in code.
When I try to read device file under /dev/ my read method is called.
But when I try to write on /dev/ file using echo it gives error
"bash: /dev/scull: Permission denied"
I have checked permissions of file using ls -l, and I have permissions to read or write on this file.
This problem occurs for every device driver module I have written. It works well in on another machine.
I'm working on ubuntu 15.10, custom compiled kernel 4.3.0
the result of ls -l /dev/scull:
crw------- 1 root root 247, 0 Dec 30 18:06 /dev/scull
the exact command I used to open the file
$ sudo echo 54 > /dev/scull
the source code for the open implementation
ssize_t scull_write(struct file *filp, const char __user *buf, size_t count, loff_t *f_pos){
pr_alert("Device Written\n");
return 0;
}
Behavior I'm seeking here is, I should be able to see 'Device Written' in dmesg ouput?

Answer 1

I assume that you are normally not root on your bash shell. Then this command line
sudo echo 54 > /dev/scull
does not what you think. The command is executed in two steps:
The bash setups the output redirection, i.e., it tries to open /dev/scull with the current user privileges.
The command sudo echo 54 is executed whereas stdout is connected to the file.
As you have no write-permissions as non-root user, the first step fails and the bash reports
"bash: /dev/scull: Permission denied"
You must already be root to setup the output redirection. Thus execute
sudo -i
which gives you an interactive shell with root privileges. The you can execute
echo 54 > /dev/scull
within that root shell.

Answer 2

I know the thread is too old to answer but just in case if someone wants to know alternative method without switching to root user, here is the solution:
sudo bash -c 'echo "54" > /dev/my_dev'

Answer 3

I wanted to note that on your system only root (file owner) has read / write permissions. Your (normal) user account has not! So another (fast) solution would be to give all users read / write permissions.
Probably this is not the safest solution! Only do this in your test environment!
sudo chmod a+rw /dev/scull
But now you test your module with your user account (without sudo)
echo "hello, world!" > /dev/scull
cat < /dev/scull

Answer 4

You can do so while going root with the command
sudo su
and then going into the /dev folder and enter your command (to save data into /dev/scull).
cd /dev
echo 54 > scull