The test harness provides the commands it wishes to run in Bourne
syntax. This fails if the user uses a different shell. For example,
with fish:
machine.wait_for_unit("graphical-session.target", "alice")
machine # fish: Unsupported use of '='. To run '-u`' with a modified environment, please use 'env XDG_RUNTIME_DIR=/run/user/`id -u`…'
machine # XDG_RUNTIME_DIR=/run/user/`id -u` systemctl --user --no-pager show "graphical-session.target"
machine # ^
machine # [ 16.329957] su[1077]: pam_unix(su:session): session closed for user alice
error: retrieving systemctl info for unit "graphical-session.target" under user "alice" failed with exit code 127
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