Every well known operating system has a concept of standardized streams. These consist of standard input, output, and error. As their name purports, they are the standard places for proccesses to read input, and send output. They’re a crucial concept that allows various running processes to easily communicate.
Standardized streams enable you to string together multiple commands with a pipe (|) in your shell like this:
$~ cat /dev/urandom | less
In this example, bash executes the commands on both sides of the pipe. It writes the stdout of cat to stdin of less. For both, stderr is written to the terminal. Most programs, like less, are made to write to stdout/stderr and read from stdin for this exact reason.
When a new process is created, file descriptors are opened for each of these standardized streams. You can view each processes open file descriptors in /proc like this:
$~ ls -l /proc/$PPID/fd
lrwx------ 1 root root 64 May 20 16:58 0 -> /dev/pts/0
lrwx------ 1 root root 64 May 20 16:58 1 -> /dev/pts/0
lrwx------ 1 root root 64 May 20 16:58 2 -> /dev/pts/0
In the above output you can see that the process has file descriptors 0, 1, and 2. By convention these correspond to stdin, stdout, and stderr respectively. These all point to the pseudo-terminal /dev/pts/0.
Your terminal application runs a shell (such as bash or zsh) and provides for it a GUI to display input and output. When you open a terminal it has its own set of standard streams. As mentioned above, it also creates a special device file like /dev/pts/0 which other processes can read from and write to. In addition to running the commands that you enter, it listens for their input and output from the pseudo-terminal device file. In the case of input, it passes it to the running command. In the case of output, it displays it to the terminal GUI.
You can actually access the standard streams of other processes. Try taking the following steps:
Open up two terminal sessions.
From the first terminal run the command ls -l /proc/$PPID/fd. This will tell you what pseudo-terminal your shell is using to read from and write to.
In the second terminal write to the first pseudo-terminal device file with echo "testing" > /dev/pts/<pts_number>.
Take a look at the first terminal for a special surprise!