catch(n)						       Tcl Built-In Commands							  catch(n)

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NAME

       catch - Evaluate script and trap exceptional returns

SYNOPSIS

       catch script ?resultVarName? ?optionsVarName?
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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	resultVarName  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 resultVarName 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  the	optionsVarName	argument  is  given, then the variable it names is set to a dictionary of return options returned by evaluation of |
       script.	Tcl specifies two entries that are always defined in the dictionary: -code and -level.	When the return code  from  evaluation	of |
       script  is  not	TCL_RETURN, the value of the -level entry will be 0, and the value of the -code entry will be the same as the return code. |
       Only when the return code is TCL_RETURN will the values of the -level and -code entries be something else, as further described in the doc- |
       umentation for the return command.													   |

       When  the  return  code	from  evaluation  of script is TCL_ERROR, three additional entries are defined in the dictionary of return options |
       stored in optionsVarName: -errorinfo, -errorcode, and -errorline.  The value of the -errorinfo entry is a formatted stack trace	containing |
       more  information  about the context in which the error happened.  The formatted stack trace is meant to be read by a person.  The value of |
       the -errorcode entry is additional information about the error stored as a list.  The -errorcode value is meant to be further processed	by |
       programs,  and may not be particularly readable by people.  The value of the -errorline entry is an integer indicating which line of script |
       was being evaluated when the error occurred.  The values of the -errorinfo and -errorcode entries of the most recent error are also  avail- |
       able as values of the global variables ::errorInfo and ::errorCode respectively. 							   |

       Tcl packages may provide commands that set other entries in the dictionary of return options, and the return command may be used by scripts |
       to set return options in addition to those defined above.

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
	      }

       There are more complex examples of catch usage in the documentation for the return command.

SEE ALSO

       break(n), continue(n), dict(n), error(n), return(n), tclvars(n)

KEYWORDS

       catch, error

Tcl									8.5								  catch(n)