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, four additional entries are defined in the
       dictionary  of  return  options  stored  in  optionsVarName:  -errorinfo,  -errorcode,  -errorline,   and 2
       -errorstack.   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 value of the -errorstack entry is  an  even-sized  list  made  of 2
       token-parameter  pairs  accumulated while unwinding the stack. The token may be “CALL”, in which case the 2
       parameter is a list made of the proc name and arguments at the corresponding level; or it may be “UP”, in 2
       which case the parameter is the relative level  (as  in  uplevel)  of  the  previous  CALL.  The  salient 2
       differences with respect to -errorinfo are that:                                                          2

       [1]                                                                                                       2
              it is a machine-readable form that is amenable to processing with [foreach {tok prm} ...],         2

       [2]                                                                                                       2
              it  contains  the true (substituted) values passed to the functions, instead of the static text of 2
              the calling sites, and                                                                             2

       [3]                                                                                                       2
              it is coarser-grained, with only one element per stack frame (like procs; no separate elements for 2
              foreach constructs for example).

       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.  The value  of  the  -errorstack  entry 2
       surfaces as info errorstack.

       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),  errorCode(3tcl),  errorInfo(3tcl),  info(3tcl),
       return(3tcl)

KEYWORDS

       catch, error, exception

Tcl                                                    8.5                                           catch(3tcl)