The observed condition is input getting split between the shell and the
background process. I couldn’t figure out what the general case is to
trigger this, but I can observe it when an ssh process with an open unix
socket is placed in the background.
Below is my recipe to reproduce. First step is to create an SSH “master”
connection, which creates a socket and runs in the background. This can be
done in a separate shell.
The step that triggers the issue is to start an ssh process that connects
to that socket, and then put that in the background.
# set up control master
mkdir -p ~/.ssh/sockets
chmod 0600 ~/.ssh/sockets
ssh -Nf -o 'ControlMaster=auto' \
-o 'ControlPath=~/.ssh/sockets/%r@%h:%p' \
-o 'ControlPersist=600' \
localhost
# start an ssh session that uses the above socket and put it in the background
ssh -o 'ControlMaster=auto' \
-o 'ControlPath=~/.ssh/sockets/%r@%h:%p' \
localhost 'read -d0; echo; echo "read: $REPLY"' &
# type in 1234567890000
Produces output like:
bash-4.4$ ssh -o 'ControlMaster=auto' -o
'ControlPath=~/.ssh/sockets/%r@%h:%p' localhost 'read -d0; echo; echo
$REPLY' &
[1] 26868*bash-4.4$ 1346800
read: 2579*
^C
[1]+ Done ssh -o 'ControlMaster=auto' -o
'ControlPath=~/.ssh/sockets/%r@%h:%p' localhost 'read -d0; echo; echo
$REPLY'
bash-4.4$
