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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 recursively 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 situations 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 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 $someFile for writing\n$fid"
                  exit 1
              }

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

SEE ALSO

       break(3tcl), continue(3tcl), dict(3tcl), error(3tcl), return(3tcl), tclvars(3tcl)

KEYWORDS

       catch, error