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.