I was reading a bit about /dev/ and device files.
There are a bunch of different devices and you can ls -lh /dev/ to show some of them. Here are a few on my laptop:
vagrant@ubuntu-xenial:~$ ls -lh /dev/
total 0
crw------- 1 root root 10, 235 May 1 12:18 autofs
drwxr-xr-x 2 root root 260 May 1 12:17 block
drwxr-xr-x 2 root root 80 May 1 12:17 bsg
crw-rw---- 1 root disk 10, 237 May 1 12:18 loop-control
brw-rw---- 1 root disk 7, 0 May 1 12:18 loop0
crw-rw-rw- 1 root root 1, 3 May 1 12:18 null
crw------- 1 root root 10, 1 May 1 12:18 psaux
crw-rw-rw- 1 root tty 5, 2 May 1 13:08 ptmx
drwxr-xr-x 2 root root 0 May 1 12:17 pts
brw-rw---- 1 root disk 8, 0 May 1 12:18 sda
lrwxrwxrwx 1 root root 15 May 1 12:17 stderr -> /proc/self/fd/2
lrwxrwxrwx 1 root root 15 May 1 12:17 stdin -> /proc/self/fd/0
lrwxrwxrwx 1 root root 15 May 1 12:17 stdout -> /proc/self/fd/1
crw-rw-rw- 1 root tty 5, 0 May 1 12:18 tty
crw-rw---- 1 root dialout 4, 65 May 1 12:18 ttyS1
crw-rw-rw- 1 root root 1, 9 May 1 12:18 urandom
crw-rw---- 1 root tty 7, 0 May 1 12:18 vcs
crw-rw-rw- 1 root root 1, 5 May 1 12:18 zero
You might recognize some of them, especially stdin, stdout, urandom, and null. Here is an overview of some of the devices found in /dev/.
Another thing I want to point out is the first character of each line, the file type. You may be familiar with - for regular files, d for directories, and l for symbolic links. In this list you will also see c for character devices and b for block devices. Everything is a file in UNIX, even devices. There are other file types, as well, like p for named pipes and s for sockets.
I’m going to focus on the c character devices. In particular, you’ll notice that pts, tty, and ttyS1 (all types of terminals) are character devices.
Cool! So, if each terminal is just a file, if I want to send a message to another terminal can I just write to it?
vagrant@ubuntu-xenial:~$ tty
/dev/pts/0
vagrant@ubuntu-xenial:~$ echo "Hi /dev/pts/1!" > /dev/pts/1
vagrant@ubuntu-xenial:~$ tty
/dev/pts/1
vagrant@ubuntu-xenial:~$ Hi /dev/pts/1!
Awesome. I can even send the bell character \a to make the other terminal ring.
vagrant@ubuntu-xenial:~$ tty
/dev/pts/1
vagrant@ubuntu-xenial:~$ echo -e 'Hey /dev/pts/0! \a' > /dev/pts/0
vagrant@ubuntu-xenial:~$ tty
/dev/pts/0
vagrant@ubuntu-xenial:~$ Hey /dev/pts/0! <ding!>
Fun. But I’m just sending myself messages (since both terminals are the vagrant user).
I added another user tnargav which is using pts/1:
vagrant@ubuntu-xenial:~$ who
vagrant pts/0 May 2 02:11 (10.0.2.2)
tnargav pts/1 May 2 02:14 (10.0.2.2)
Let’s try it out:
vagrant@ubuntu-xenial:~$ echo "Hi /dev/pts/1!" > /dev/pts/1
-bash: /dev/pts/1: Permission denied
Boo. Okay, that makes sense though, I shouldn’t be able to just send another user messages.
What if I update the file permissions?
tnargav@ubuntu-xenial:~$ chmod a+w /dev/pts/1
vagrant@ubuntu-xenial:~$ echo "Hi tnargav!" > /dev/pts/1
tnargav@ubuntu-xenial:~$ Hi tnargav!
Neat, no magic here, just standard UNIX file permissions.
I think most systems have write(1) which allows you to send messages to another user. A quick peek at the source code and it seems like it checks group permissions.