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 2
       options  returned  by  evaluation  of  script.   Tcl specifies two entries that are always defined in the 2
       dictionary: -code and -level.  When the return code from evaluation of  script  is  not  TCL_RETURN,  the 2
       value  of  the  -level  entry  will be 0, and the value of the -code entry will be the same as the return 2
       code.  Only when the return code is TCL_RETURN will the  values  of  the  -level  and  -code  entries  be 2
       something else, as further described in the documentation for the return command.                         2

       When  the return code from evaluation of script is TCL_ERROR, three additional entries are defined in the 2
       dictionary of return options stored in optionsVarName: -errorinfo, -errorcode, and -errorline.  The value 2
       of the -errorinfo entry is a formatted stack trace containing more information about the context in which 2
       the error happened.  The formatted stack trace is meant to be  read  by  a  person.   The  value  of  the 2
       -errorcode  entry  is  additional  information about the error stored as a list.  The -errorcode value is 2
       meant to be further processed by programs, and may not be particularly readable by people.  The value  of 2
       the  -errorline  entry  is  an integer indicating which line of script was being evaluated when the error 2
       occurred.  The values of the -errorinfo and  -errorcode  entries  of  the  most  recent  error  are  also 2
       available as values of the global variables ::errorInfo and ::errorCode respectively.                     2

       Tcl  packages  may  provide  commands that set other entries in the dictionary of return options, and the 2
       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

Tcl                                                    8.5                                           catch(3tcl)