2.4 XAS procedures

XAS is not Yet Another Scripting Language and does not explicitly supports the writing of procedure scripts, but defers this to the facilities provided by the various operating systems (e.g. shell scripts or DCL command files), with which the user is perhaps more familiar.

We give here only a brief tutorial on the possibilities of assembling sequences of XAS commands to perform repetitive tasks (with particular emphasis on the Unix side).

Simple sequences

If one needs to execute often a standard sequence of XAS commands to perform a standard reduction, using always the same standard file names, without need to pass any argument, and without the need of "branching" (no IF statements depending on previous results), it is sufficient

to write into a file the sequence of XAS commands
to invoke the file in the current process, without the need to make it an executable script, e.g. in Unix C shell using the source command.
The file header keywords and the XAS environment will take care of exchanging information between one program and the next.

Parametric sequences

By "parametric" sequence we mean a standard sequence of XAS commands used to perform a standard reduction, without the need of "branching" (no IF statements depending on previous results), but which requires some arguments (e.g. file names or numeric parameters).

This can be arranged as an executable shell script with arguments, or even as a "source" file by setting the arguments in specific shell environment variables. The same concepts given above for simple sequences are valid here too, provided one does not need to access the XAS environment at shell level.

Note also that, if one wishes to supply some arguments interactively, one can use a simple sequence containing XAS commands (and invocation of command files ) in which some runstring or command file arguments are left "blank" and will therefore be prompted interactively.

Procedure scripts

One will need to write a proper script under two conditions :

one needs programming facilities like branching (IF statements) or manipulation of shell variables
one need to interact with XAS environment variables

Programming is left to the facilities of the user's favourite shell and is not of our concern here.
Coming to the interaction of the XAS and system variables one shall instead note what follows.

XAS environment variables are usually used to contain setup information arranged by the user. There are however cases in which they are set by XAS programs. In particular there is a result variable intended to pass back program results.
The XAS and system environment shall be regarded as two disjoinct subsets of a larger environment : each "half" can be accessed as "read-write" from either XAS or the shell, and as "read-only" from the other.
use only xasset to set XAS variables.
use only shell commands (e.g. set, setenv) to set system environment variables
In particular if a XAS environment variable is not set, but a system variable of the same name exists, XAS program are able to use its value.
Conversely in principle all XAS environment variable are accessible to the system shells as system environment variables with uppercase names prefixed by XAS_. E.g. the XAS environment variable ECHO will be known to the system as XAS_ECHO
- This is immediately true in the case of VMS, where XAS variables are global symbols.
- This is not straightforward in the case of Unix, where the system environment of a process (e.g. a XAS program, like xasset itself) is not inherited back to the parent process (e.g. the shell). The XAS environment is stored in a file linked to the current session, and must be re-imported in the shell.
- one way to import back all XAS variables (deleting and resetting the file) is, under csh, to issue the following command (which can be aliased)
```
 eval `importback`
```
- another way to copy back a single XAS variable into another is to intercept the result of a xasset query. Compare e.g. the following csh example (note the use of uppercase) :
```
set temp = `xasset fotdir \? | grep FOTDIR`
set fotdirname = $temp[3]
```

Other useful Unix tricks

One can, if desired, access directly the content of the file where the environment is stored. This file is in the user login directory, and has a name linked to the current terminal name (e.g. ttyp1) and to the hostname (e.g. poseidon), like ttyp1_poseidon.environment.

One may then use also a command of the following form to copy back a XAS environment variable into a shell variable :

set fotdirname = `grep FOTDIR ~/ttyp1_poseidon.environment | cut -f2 -d'='`

Note also that the environment files remain there after you log off, but are in general cleared when you log in again and issue your first XAS command in the new session. If you wish to continue preserving the last session XAS environment, just touch the environment file before issuing ANY XAS command.
A similar effect is obtained if you copy or rename the environment file of a session into another.

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sax.iasf-milano.inaf.it/Xashelp/xasprog.4.html :: original creation 2002 Sep 04 17:38:28 CEST :: last edit 2002 Sep 04 17:38:28 CEST