3.1 XAS environment variables

XAS environment variables (or "global variables") are used to retain and exchange information between one XAS program and another, or to set global properties which have to be remembered for a whole XAS session (e.g. default values, or program behaviours).

Global variables are assigned a value either implicitly by some programs, or by using special programs, or, in the most general case, via the xasset program.

The same xasset program can also be used to query the current value of a global variable, or to deassign it (note that for XAS purposes a not existing variable, a variable with a null value, or a variable with all-blank value are equivalent : in all such cases most programs assume a suitable default value). Note however that if a non-XAS global variable with same name exists, its value may be used instead of the missing XAS variable.

XAS environment variables are implemented in VMS as global symbols, and in Unix as environment variables. In both cases their "system" name can be built prefixing their XAS name with the prefix "XAS_". See elsewhere for using XAS variables in procedures ( in Unix via importback).

This achieves a separation between the XAS and the system environment, by which system variables are readonly for XAS (they may be used instead of a missing XAS variable, but not set with xasset while XAS variables can be accessed via system facilities using their prefix).

The XAS environment is linked to a session, which in VMS means a process (and related subprocesses), while in Unix means a login session on a particular machine and a particular pseudo-terminal (in particular one must set the environment separately in each window !). To allow back-inheritance from a child process to the parent, in Unix the XAS environment is saved to a file : the date of creation of this file is compared with the date of login to determine whether it has still to be considered applicable.

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