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 date 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 $$ PID TTY TIME CMD 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 USER TTY FROM LOGIN@ IDLE JCPU PCPU WHAT root pts/1 100.127.65.241 00:39 2.00s 0.33s 0.33s -bash root pts/2 100.127.65.241 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 console: # ./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 ^C Neat! USAGE: # ./ttysnoop.py -h usage: ttysnoop.py [-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 -s, --datasize size of the transmitting buffer (default 256) -c, --datacount number of times ttysnop checks for data (default 16) examples: ./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 ./ttysnoop /dev/pts/2 -s 1024 # snoop output from /dev/pts/2 with data size 1024 ./ttysnoop /dev/pts/2 -c 2 # snoop output from /dev/pts/2 with 2 checks for 256 bytes of data in buffer (potentionaly retrieving 512 bytes)