SSH – Kill Process Spawned When SSH Dies

One possibility is that the program catches the Ctrl+C sequence. Check the output of stty -a; the intr setting indicates which key combination (if any) sends a SIGINT signal, and the isig indicates whether the signal keys are enabled (-isig means they are disabled).

If the program consists of several processes, pressing Ctrl+C sends SIGINT to all the processes in the process group. You can obtain the same effect by sending the signal to the process group instead of sending it to one of the processes. To send a signal to a process group, first determine its leader: that's the first process that starts all the others; if you run the process group in the background, that's the PID shown by jobs -l. The PID of the process group leader is the PGID (process group id); send the signal to its negative. E.g. if the PGID is 1234, run kill -INT -1234.

If the program consists of a wrapper script and a main application, there are two cases to consider. If there is no cleanup to do, so that the wrapper script terminates as soon as the main aplication terminates, make the wrapper script call exec:

#!/bin/sh
export SOMEVAR=somevalue
…
exec /path/to/application "$@"

This way the application replaces the script, inheriting its PID. Some shells optimize a script that ends by executing another program, but not all. This approach doesn't work when the script needs to perform some cleanup such as removing temporary files.

Consider making the script detect a signal and transmitting the signal to the application. Here's a sketch of how this goes:

/path/to/application "$@" &
app_pid=$!
trap -INT 'kill -INT $app_pid'
wait $!
rm /temp/file

ssh can be invoked in a few different ways, each resulting slightly different treatment of terminal-initiated signals like Ctrl-C.

• ssh remotehost will run an interactive session on remotehost. On the client side, ssh will try to set the tty used by stdin to "raw" mode, and sshd on the remote host will allocate a pseudo-tty and run your shell as a login shell (e.g. -bash).

  Setting raw mode means that characters that would normally send signals (such as Ctrl-C and Ctrl-\) are instead just inserted into the input stream. ssh will send such characters as-is to the remote host, where they will likely send SIGINT or SIGQUIT and, typically, kill any command and return you to a shell on the remote host. The ssh connection will stay alive, as long as the remote shell is alive.

• ssh -t remotehost command args ... will run an interactive session on remotehost, just like the above, except on the remote side, your_shell -c "command args ..." will be run. As above, if you type Ctrl-C, it will be sent to the remote host, where the command will likely receive SIGINT and immediately exit, and then the remote shell will exit. The remote sshd then closes the connection, and ssh reports Connection to remotehost closed.

• ssh remotehost command args ... will run a non-interactive session on remotehost. On the client side, ssh will not set the tty to raw mode (well, except to read in a password or passphrase). If you type Ctrl-C, ssh will get sent SIGINT and will immediately be terminated, without even issuing a Connection to remotehost closed message.

  The your_shell -c "command args ..." processes will likely remain running on the remote host. Either they will exit on their own, or one process will try to write data to the now-closed ssh socket, which will cause a (typically) fatal SIGPIPE signal to be sent to it.