POSIX sh stdin watchdog for a remote helper: copy stdin through a fifo with cat, and kill the child process tree when cat sees EOF

To make sure a remote process started over ssh (no pty) dies when the session ends, even if its main thread is blocked, wrap it like this. Save the real stdin with exec 9<&0. Start the child in the background reading from a mkfifo. Start a watchdog subshell that runs cat <&9 > fifo. When cat returns (stdin EOF, or cat killed by a group signal), wait N seconds; if the child is still a child of the top shell (compare ps -o ppid= with the top shell pid), STOP it, then recursively STOP and KILL its descendants (pgrep -P), then KILL it. Stopping a parent before listing its children keeps their pids from being reused, because a stopped parent cannot reap. Gotchas: (1) trap : HUP TERM in the top shell so it survives group signals. Subshells reset the trap to the default, so the child keeps normal handlers. A trapped signal interrupts wait, so loop on wait while the child is still your child, or you report 128+sig instead of the real exit status. (2) Only cat may hold the fifo write end, otherwise the child never sees EOF. (3) rm the fifo dir as soon as the child has opened it. (4) When testing with a local fifo as ssh stdin, a background holder like ( exec 3>fifo; sleep 900 ) & keeps the fifo open after you kill the subshell, because sleep inherited fd 3. Use sleep 900 > fifo & so killing that pid really closes it. Likewise sleep 900 | ssh ... makes bash wait for sleep after ssh exits.