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Demonstrations of ttysnoop, the Linux eBPF/bcc version.
ttysnoop watches a tty or pts device, and prints the same output that is
appearing on that device. It can be used to mirror the output from a shell
session, or the system console.
Let's snoop /dev/pts/2:
# ./ttysnoop 2
<screen clears>
Sun Oct 16 01:28:47 UTC 2016
# uname -a
Linux bgregg-xenial-bpf-i-xxx 4.8.0-rc4-virtual #1 SMP Wed Aug 31 22:54:37 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux
# df -h
Filesystem Size Used Avail Use% Mounted on
udev 7.4G 0 7.4G 0% /dev
tmpfs 1.5G 89M 1.4G 6% /run
/dev/xvda1 7.8G 4.5G 3.3G 59% /
tmpfs 7.4G 0 7.4G 0% /dev/shm
tmpfs 5.0M 0 5.0M 0% /run/lock
tmpfs 7.4G 0 7.4G 0% /sys/fs/cgroup
tmpfs 250M 0 250M 0% /run/shm
/dev/md0 160G 20G 141G 13% /mnt
tmpfs 1.5G 0 1.5G 0% /run/user/0
# ^C
What we're seeing is another shell session. The first line was "date" without
the shell prompt ("#") because we began tracing after the prompt was printed.
The other commands appeared, keystroke by keystroke, as the user was typing
them. Spooky!
Remember to Ctrl-C to exit ttysnoop.
To figure out which pts device number to use, you can check your own with "ps"
and other's with "w". For example:
# ps -p $$
9605 pts/1 00:00:00 bash
# w
01:26:37 up 9 days, 35 min, 2 users, load average: 0.22, 0.22, 0.15
root pts/1 00:39 2.00s 0.33s 0.33s -bash
root pts/2 00:40 16.00s 1.06s 1.06s -bash
So I'm pts/1, and there's another session that's pts/2.
This can also snoop tty devices using their full path. Eg, snooping the system
# ./ttysnoop /dev/console
Oct 16 01:32:06 bgregg-xenial-bpf-i-xxx kernel: [780087.407428] bash (9888): drop_caches: 1
Oct 16 01:32:38 bgregg-xenial-bpf-i-xxx snmpd[2708]: Cannot statfs /sys/kernel/debug/tracing: Permission denied
Oct 16 01:33:32 bgregg-xenial-bpf-i-xxx snmpd[2708]: Cannot statfs /sys/kernel/debug/tracing: Permission denied
Oct 16 01:34:26 bgregg-xenial-bpf-i-xxx snmpd[2708]: Cannot statfs /sys/kernel/debug/tracing: Permission denied
# ./ -h
usage: [-h] [-C] device
Snoop output from a pts or tty device, eg, a shell
positional arguments:
device path to a tty device (eg, /dev/tty0) or pts number
optional arguments:
-h, --help show this help message and exit
-C, --noclear don't clear the screen
./ttysnoop /dev/pts/2 # snoop output from /dev/pts/2
./ttysnoop 2 # snoop output from /dev/pts/2 (shortcut)
./ttysnoop /dev/console # snoop output from the system console
./ttysnoop /dev/tty0 # snoop output from /dev/tty0