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

______________________________________________________________________________

NAME
       catch - Evaluate script and trap exceptional returns

SYNOPSIS
       catch script ?resultVarName? ?optionsVarName?
_________________________________________________________________

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_CON‐
       TINUE).	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	 documentation │
       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	 available  as │
       values  of  the	global	variables  ::errorInfo and ::errorCode respec‐ │
       tively.								       │

       Tcl packages may provide commands that set other entries in the dictio‐ │
       nary  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 $someFile for writing\n$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)
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