Using GNU timeout with SSH -t in a bash script to prevent hanging

Question

I have a script that ssh's to some servers. Sometimes an unexpected problem causes ssh to hang indefinitely. I want to avoid this by killing ssh if it runs too long.

I'm also using a wrapper function for input redirection. I need to force a tty with the -t flag to make a process on the server happy.

function _redirect {
    if [  "$DEBUG" -eq 0 ]; then
        $*   1> /dev/null 2>&1
    else
        $*
    fi
    return $?
exit
}
SSH_CMD="ssh -t  -o BatchMode=yes -l robot"
SERVER="192.168.1.2"
ssh_script=$(cat <<EOF
sudo flock -w 60 -n /path/to/lock -c /path/to/some_golang_binary
EOF
)

_redirect timeout 1m $SSH_CMD $SERVER "($ssh_script)"

The result is a timeout with this message printed:

 tcsetattr: Interrupted system call

The expected result is either the output of the remote shell command, or a timeout and proper exit code.

when I type

  timeout 1m ssh -t -o BatchMode=yes -o  -l robot 192.168.1.2 \
  "(sudo sudo flock -w 60 -n /path/to/lock -c /path/to/some_golang_binary)" \
  1> /dev/null

I get the expected result.

I suspect these two things:

1)The interaction between GNU timeout and ssh is causing the tcsetattr system call to take a very long time (or hang), then timeout sends a SIGTERM to interrupt it and it prints that message. There is no other output because this call is one of the first things done. I wonder if timeout launches ssh in a child process that cannot have a terminal, then uses its main process to count time and kill its child.

I looked here for the reasons this call can fail.

2) _redirect needs a different one of $@, $*, "$@", "$*" etc. Some bad escaping/param munging breaks the arguments to timeout which causes this tcsetattr error. Trying various combinations of this has not yet solved the problem.

Without having _redirect, I had the same problem; therefore I guess your first suspicion is more probably the cause. Anyway, --foreground fixes this, thanks. — emre can
– emre can, Commented Oct 30, 2019 at 9:29

jSp · Accepted Answer · 2017-05-22 17:05:06Z

4

What fixed this was --foreground flag to timeout.

answered May 22, 2017 at 17:05

jSp

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