The exptab file is only required when multiple MIDAS experiments are defined for a particular MIDAS installation. It contains a table of each experiment name, directory and username.
Creating the exptab file
The MIDAS system supports multiple experiments running at the same time on a single computer.
Even though it may not be efficient, this capability makes sense when the experiments are simple detector lab setups which share hardware resources for data collection. In order to support this feature, MIDAS requires a uniquely identified set of parameters for each experiment that is used to define the location of the Online Database (ODB).
Every experiment under MIDAS has its own ODB. In order to differentiate them, an experiment name and directory are assigned to each experiment. This allows several experiments to run concurrently on the same host, using a common MIDAS installation.
Where more than one experiment is defined, the default name of the experiment to connect to can be provided using the MIDAS_EXPT_NAME environment variable. or can be specified as a command line argument see Common Parameters to MIDAS Utilities).
A list of all possible running experiments (unique to each MIDAS installation) on a given machine is kept in the file exptab, created by the user. This file exptab is expected by default to be located under /etc/exptab. This default location can be overwritten by the Environment Variable MIDAS_EXPTAB.
Common exptab /etc/exptab
Where there is one common installation of MIDAS, accessed by several users, each defining one or more experiments, it makes sense to keep the common exptab under /etc/exptab. In this case, the exptab file defines each experiment on the machine, with one line per experiment. Each line contains three parameters, i.e: experiment name, experiment directory name and user name, e.g.
# /etc/exptab # Midas experiment list for single MIDAS installation test /home/johnfoo/online johnfoo e232 /home/johnfoo/online johnfoo decay /home/jackfoo/decay_daq jackfoo zexp /home/janefoo/zexp/online janefoo
Local exptab and MIDAS_EXPTAB
However, it is more common for users each to download their own installation of MIDAS, so that they can update their MIDAS version independently. In this case, they will require their own exptab file containing only their experiment(s), and so will define the Environment Variable MIDAS_EXPTAB.
e.g. setenv MIDAS_EXPTAB /home/frefoo/online/exptab
and the exptab file e.g.
# /home/fredfoo/online/exptab # Midas experiment list for user fredfoo test /home/fredfoo/online fredfoo decay /home/fredfoo/decay_daq fredfoo e512 /home/fredfoo/expt_512 fredfoo
Experiments not defined in exptab
Experiments not defined in exptab are not accessible remotely, but can still be accessed locally using the environment variable MIDAS_DIR if defined. This environment variable supersedes the exptab definition.