Testing for correct user input from keyboard

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catch

catch(1T)						       Tcl Built-In Commands							 catch(1T)

__________________________________________________________________________________________________________________________________________________

NAME

       catch - Evaluate script and trap exceptional returns

SYNOPSIS

       catch script ?varName?
_________________________________________________________________

DESCRIPTION

       The  catch  command may be used to prevent errors from aborting command interpretation.	The catch command calls the Tcl interpreter recur-
       sively to execute script, and always returns without raising an error, regardless of any errors that might occur while executing script.

       If script raises an error, catch will return a non-zero integer value corresponding to the exceptional return code returned  by	evaluation
       of  script.  Tcl defines the normal return code from script evaluation to be zero(0), or TCL_OK.  Tcl also defines four exceptional return
       codes: 1 (TCL_ERROR), 2 (TCL_RETURN), 3 (TCL_BREAK), and 4 (TCL_CONTINUE).  Errors during evaluation of a script are indicated by a  return
       code of TCL_ERROR.  The other exceptional return codes are returned by the return, break, and continue commands and in other special situa-
       tions as documented.  Tcl packages can define new commands that return other integer values as return codes as well, and scripts that  make
       use of the return -code command can also have return codes other than the five defined by Tcl.

       If  the varName argument is given, then the variable it names is set to the result of the script evaluation.  When the return code from the
       script is 1 (TCL_ERROR), the value stored in varName is an error message.  When the return code from the script is 0  (TCL_OK),	the  value
       stored in resultVarName is the value returned from script.

       If script does not raise an error, catch will return 0 (TCL_OK) and set the variable to the value returned from script.

       Note  that  catch  catches all exceptions, including those generated by break and continue as well as errors.  The only errors that are not
       caught are syntax errors found when the script is compiled.  This is because the catch command only catches errors  during  runtime.   When
       the catch statement is compiled, the script is compiled as well and any syntax errors will generate a Tcl error.

EXAMPLES

       The catch command may be used in an if to branch based on the success of a script.
	      if { [catch {open $someFile w} fid] } {
		  puts stderr "Could not open $someFile for writing
$fid"
		  exit 1
	      }

       The catch command will not catch compiled syntax errors.  The first time proc foo is called, the body will be compiled and a Tcl error will
       be generated.
	      proc foo {} {
		  catch {expr {1 +- }}
	      }

SEE ALSO

       break(1T), continue(1T), error(1T), return(1T), tclvars(1T)

KEYWORDS

       catch, error

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +--------------------+-----------------+
       |  ATTRIBUTE TYPE    | ATTRIBUTE VALUE |
       +--------------------+-----------------+
       |Availability	    | SUNWTcl	      |
       +--------------------+-----------------+
       |Interface Stability | Uncommitted     |
       +--------------------+-----------------+
NOTES

       Source for Tcl is available on http://opensolaris.org.

Tcl									8.0								 catch(1T)

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