Comment 2 for bug 1839431

uumas gave me remote and I was able to have a quick look.

The problems start in /usr/share/ldm/rc.d/X01-localapps, at the following lines:
# Get user information from the server
IFS=':' read -r LDM_USERNAME dummy pw_uid pw_gid pw_gecos LDM_HOME pw_shell <<EOF
$(ssh_run '/usr/bin/getent passwd $(/usr/bin/id -u)')
EOF

Fish doesn't use $() but () to execute commands in a subshell, so the line above destroys LDM_USERNAME instead of getting additional info.
$(ssh_run "/usr/bin/getent passwd $LDM_USERNAME")
could be used instead, but this might fail in some cases.

The important thing is to put a test above the `su - ${LDM_USERNAME}` line, to log an error and exit if LDM_USERNAME isn't set.