How many users does Linux support being logged in at the same time via SSH?

Question

I know Linux supports multiple users being logged in at the same time.

But what's the maximum number of users that can be logged into Linux at the same time?

I see there are there are 69 tty files (ttyn or ttysn, where n is an integer, such as tty0, tty1, tty2... ) in my /dev directory. I assume that these files are the shells. So I am thinking that this Linux system will support only 69 user logged in simultaneously.

Is my thinking correct? If my assumption is wrong, please explain the users limit of Linux, including how it's implemented.

Also, how do I access the details of already logged in users? I know commands w, who, but I am looking for sophisticated tools.

Solution

When logging in using SSH, you use a pseudo-terminal (a pty) allocated to the SSH daemon, not a real one (a tty). Pseudo-terminals are created and destroyed as needed. You can find the number of ptys allowed to be allocated at one time at /proc/sys/kernel/pty/max, and this value can be modified using the kernel.pty.max sysctl variable. Assuming that no other ptys are in use, that would be your limit.

w, who, and users are the canonical tools for accessing information about logged in users. last and lastlog also contain historical data.

Comments

[neotam]

could you please tell me about the original tty limit , and where the information will be saved that is returned by last command?

[clerksx]

@usernaveen The maximum number of TTYs on modern Unices is largely limited by the number of virtual consoles you have available. The information shown by last is usually stored in /var/log/wtmp.

[Chad Harrison]

Probably the upper limit of an unsigned int.

[clerksx]

@ott-- Not necessarily. You can have multiple interfaces, so you essentially have an infinite number of available ports.

[clerksx]

@usernaveen Many init systems try and cover as many use cases as possible when choosing how many TTYs to have available, as it costs very little (almost nothing).

[Edward]

CentOS is RHEL derived, not Debian.

[Ezra A.Mosomi]

yes you are right, a typo there @Edward