**fScan Reference Manual, Chapter 1 (Topics): status**
===== Status queries -- Command operators to query data status =====
fScan command lines and interactive command strings can contain special query operators that test the status of data elements. The queries return either a 1 or 0 value, depending on whether the query result was yes or no.

==== The data definition query -- ? ====
**usage: ?data**

The '?' query operator can be used in place of the '$' data name identifier in any fScan command string. 
The value of a query is:\\
-1 if the DATA name is not defined\\
0 if the DATA name is defined but empty (string data only)\\
1 if the DATA name is defined and not an empty string.

This is different than the '$' operator, which is replaced by the actual value of the named DATA.

==== The file found query -- ?exists ====
**usage: ?exists(filename)**\\
** or: $exists(filename)**

The EXISTS query operator test whether its argument string is the name of an existing data file. It returns 1 if the file was found, 0 if the file was not found.

**See Also:**\\
[[jvs:fscan:manual:chapter2:if|IF]] 

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[[jvs:fscan:manual|fScan Manual]], 
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