Systemd socket activation stdin

I have to tranfer a legacy xinetd config to systemd. The requirement is to open a tcp port and listen to incoming transmissions. An application transfers one file per connection simply by netcating it to the port. When a transmission is registered, a shell script is invoked, which saves the incoming data to a file by redirecting the stdin into said file. This construct worked for years with xined. Here is what I have: [root@localhost ~]# cat /etc/systemd/system/foo.socket [Unit] Description=Foo Socket PartOf=foo.service [Socket] ListenStream=127.0.0.1:9999 Accept=yes [Install] WantedBy=sockets.target [root@localhost ~]# cat /etc/systemd/system/foo@.service [Unit] Description=Foo Service After=network.target foo.socket Requires=foo.socket [Service] Type=simple ExecStart=/usr/local/bin/foo.sh TimeoutStopSec=5 [Install] WantedBy=default.target [root@localhost ~]# cat /usr/local/bin/foo.sh #!/bin/bash cat > /tmp/foo.$$ [root@localhost ~]# systemctl start foo.socket [root@localhost ~]# echo "Hello World" > testfile [root@localhost ~]# socat -u FILE:testfile TCP:127.0.0.1:9999 [root@localhost ~]# ls -al /tmp/foo.* -rw-r--r--. 1 root root 0 Nov 7 21:20 /tmp/foo.19820 [root@localhost ~]# The tcp port is open, the service is invoked and the shell script is executed. But the output file size is zero. If I stop the socket and use this command: [root@localhost system]# systemctl stop foo.socket [root@localhost system]# /usr/lib/systemd/systemd-activate -l 127.0.0.1:9999 -a /usr/local/bin/foo.sh & 19833 [root@localhost system]# Listening on 127.0.0.1:9999 as 3. [root@localhost ~]# socat -u FILE:testfile TCP:127.0.0.1:9999 Communication attempt on fd 3. Connection from 127.0.0.1:39924 to 127.0.0.1:9999 Spawned /usr/local/bin/foo.sh (/usr/local/bin/foo.sh) as PID 19840 [root@localhost ~]# Child 19840 died with code 0 [root@localhost ~]# ls -al /tmp/foo* -rw-r--r--. 1 root root 12 Nov 7 21:26 /tmp/foo.19840 [root@localhost ~]# cat /tmp/foo.19840 Hello World [root@localhost ~]# It works like expected. What do I miss?