How can I hook on to one terminal's output from another terminal?

Question

I need to hook onto output of currently running terminal (tty1) from virtual terminal and capture it (running X server).

I'm pretty sure such a thing is possible by simpler means than debugging (result of my Google searches).

Can anyone help?

Answer 1

I came across this one tool called ttylog. It's a Perl program available on CPAN here. It has a couple caveats, one being that I could only figure out how to attach to a terminal that was created as part of someone ssh'ing into my box. The other being that you have to run it with elevated privileges (i.e. root or sudo).

But it works!

For example

First ssh into your box in TERM#1:

TERM#1% ssh saml@grinchy

Note this new terminal's tty:

TERM#1% tty
/dev/pts/3

Now in another terminal (TERM#2) run this command:

TERM#2% ttylog pts/3
DEBUG: Scanning for psuedo terminal pts/3
DEBUG: Psuedo terminal [pts/3] found.
DEBUG: Found parent sshd pid [13789] for user [saml]

Now go back to TERM#1 and type stuff, it'll show up in TERM#2.

All the commands I tried, (top, ls, etc.) worked without incident using ttylog.

Answer 2

Indeed it is. The /dev/vcs* and /dev/vcsa* devices corresponds to the /dev/tty* devices (the virtual terminals). F1=tty1=vcs1/vcsa1 and so on. The vcs/vcsa is like tty for the "current" virtual terminal.

As root, you can just cat these devices (e.g. cat /dev/vcs2), and see what's on the corresponding VT (e.g. /dev/tty2 the on on F2) like taking a snapshot. vcsa* differs from vcs* in that they include information about the dimensions of the terminal (the screen). Mind you, it's just a raw snapshot of the characters as they show on the screen - gathered from the memory allocated to the terminal - so don't expect nice, easily parseble output.

The drawback is that if the information flashes past too fast, it may be difficult to capture. Perhaps tail -f /dev/vcs1 will work, if you need to follow several screenfulls (haven't tried myself)? It may be easiest to simply redirect it to a file first. It may also be a good idea to use a VT (F1-F6) to look at it, as the terminals will have the same dimensions. In my experience, it's best to use the vcs* - not the vcsa* - devices.

If that doesn't work, perhaps one of the "big brotherish" packages that allows an admin to keep an eye on the activity on a terminal may work.

PS: I forgot to ask what OS you use. This is for Linux, though similar devices probably exists on other OSes too. Try searching for "virtual console memory" among the man-pages for devices.

Answer 3

look at:

man 1 script

for example:

script -f /dev/tty1

Answer 4

This worked for me:

• Using a keyboard on computer "A" (i.e. the physical computer that's to be controlled), run: screen -q

• Connect with ssh from computer "B" to computer "A".

• In the ssh session, type: screen -ls to get a session id to connect to (4 digits on the row that contains tty).

• Connect to the above session with: screen -x <session id> ... using the session id number received from the screen -ls command above.

Anything typed in either "session" will happen in both "sessions", so for example typing screen -d will quit BOTH sessions.

Answer 5

Another approach is to use the gnu screen utility on your local machine. Invoke it with the -L option, or start without that option and use the ^aH command sequence. Either approach causes all input & output to be logged to a file named screenlog.x where x is the screen number.

This is a handy because nothing extra needs to be installed on the remote machine.

Answer 6

Since I didn't start tty1 with screen this script helped:

I used the answer from Baard Kopperud above. "128" is 1 line of my tty1. Sleep can be set to an appropriate number.

#!/bin/bash
while true
do
    sudo tail -c 128 /dev/vcs1 && echo ""
    sleep 10
done

I used this in terminator and sized the column so the scroll is one line of text.

Answer 7

Use the tty command in each terminal to identify them:

$ tty
/dev/pts/0

$ tty
/dev/pts/1

Assuming these TTYs, to redirect the first's stdout to the second, run this in the first terminal:

exec 1>/dev/pts/1

Note: Now every command output will show on pts/1

To restore default behavior stdout of pts/0:

exec 1>/dev/pts/0

See this video for a demonstration.