XMM-Newton Science Analysis System: User Guide

2.7 General settings affecting all SAS tasks

On a given SAS session, the SAS environment variables define the way in which SAS commands are executed. Several of these environment variables are part of the SAS initialization (e.g. SAS_DIR and SAS_PATH), others are optional.

The following is a description of some of the most useful environment variables in SAS:

• SAS_DIR is the top-level directory where SAS is installed.

• SAS_CCFPATH points to a list of colon separated directories where the CCF constituents are to be sought.

• SAS_ODF points to a SAS summary file (*SUM.SAS, see § 2.3.4). Alternatively, it can point to a directory containing an ODF. If a name is specified via -o and SAS_ODF is defined, the former takes precedence over the latter. This environment variable is only relevant to tasks making use of the OAL.

• SAS_CCF points to the calibration index file (see § 2.3.3). If a value is provided via the command line option -i from any SAS task (see § 2.6) this takes precedence over any environment specification.

• SAS_MEMORY_MODEL determines how the internal memory is used by the data access (through the Data Access Layer (DAL)). There are basically two options for the memory model: High Memory (high) and HighLow Memory (highlow or low) model. In the high model every time a dataset is opened it is kept entirely in memory and all subsequent operations are performed on the memory-loaded dataset. In the low model the data is loaded into memory only when their access is needed (when opening a dataset only their attributes are kept in memory). While the high model should bring higher performance, its use implies normally a high memory consumption, which can lead to swapping, producing a poorer performance. Therefore for machines with less than about 1GB RAM it is recommended to use the low model. The default value for SAS_MEMORY_MODEL is high.

• SAS_VERBOSITY determines the debug level of the task. The value ranges from 0 to 10 with increasing verbosity level. The default value is 1, a value of 4 is recommended for getting a very communicative SAS, larger values should only be used for debugging purposes.

• SAS_SUPPRESS_WARNING determines whether warning messages produced during the execution of any given task are displayed or not. It is a boolean which means any value not equal to 0 sets it to true. By default is set to true.

• SAS_BROWSER specifies the browser to be launched when any SAS task is invoked with the -m command line option, to display the HTML documentation on such task. It could be specified either as the absolute path to the browser binary (e.g. /usr/bin/firefox) or just simply the browser name. For that latter, the browser binary must be accessible through the $PATH variable.

The SAS tasks produce error messages at levels called message, warning, error and fatal.

• At message level, a message is reported to the user and processing continues.
• At warning level, a warning message is reported to the user and processing continues.
• At error level, an error message is reported and the current operation is aborted. Control may return to the calling programme which can take an appropriate action.
• At fatal level, a fatal error is reported to the user and all processing is aborted. A fatal error is generated if the internal state of the programme is disrupted, for example if an invalid value is found in a variable.

Error messages are characterized by a layer and a verbosity level. The layer indicates the layer in the system where the message is coming from ( e.g. application, user interface, application library or system library). The verbosity level determines whether or not the message will be reported. The user can specify the verbosity level. If the verbosity of a message is large enough compared to the system verbosity level, the message will be reported. The verbosity level differs for the different layers.