1catch(n) Tcl Built-In Commands catch(n)
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8 catch - Evaluate script and trap exceptional returns
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11 catch script ?resultVarName? ?optionsVarName?
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16 The catch command may be used to prevent errors from aborting command
17 interpretation. The catch command calls the Tcl interpreter recur‐
18 sively to execute script, and always returns without raising an error,
19 regardless of any errors that might occur while executing script.
20 If script raises an error, catch will return a non-zero integer value
22 corresponding to the exceptional return code returned by evaluation of
23 script. Tcl defines the normal return code from script evaluation to
24 be zero (0), or TCL_OK. Tcl also defines four exceptional return
25 codes: 1 (TCL_ERROR), 2 (TCL_RETURN), 3 (TCL_BREAK), and 4 (TCL_CON‐
26 TINUE). Errors during evaluation of a script are indicated by a return
27 code of TCL_ERROR. The other exceptional return codes are returned by
28 the return, break, and continue commands and in other special situa‐
29 tions as documented. Tcl packages can define new commands that return
30 other integer values as return codes as well, and scripts that make use
31 of the return -code command can also have return codes other than the
32 five defined by Tcl.
33 If the resultVarName argument is given, then the variable it names is
35 set to the result of the script evaluation. When the return code from
36 the script is 1 (TCL_ERROR), the value stored in resultVarName is an
37 error message. When the return code from the script is 0 (TCL_OK), the
38 value stored in resultVarName is the value returned from script.
39 If the optionsVarName argument is given, then the variable it names is │
41 set to a dictionary of return options returned by evaluation of script. │
42 Tcl specifies two entries that are always defined in the dictionary: │
43 -code and -level. When the return code from evaluation of script is │
44 not TCL_RETURN, the value of the -level entry will be 0, and the value │
45 of the -code entry will be the same as the return code. Only when the │
46 return code is TCL_RETURN will the values of the -level and -code │
47 entries be something else, as further described in the documentation │
48 for the return command. │
49 When the return code from evaluation of script is TCL_ERROR, three │
51 additional entries are defined in the dictionary of return options │
52 stored in optionsVarName: -errorinfo, -errorcode, and -errorline. The │
53 value of the -errorinfo entry is a formatted stack trace containing │
54 more information about the context in which the error happened. The │
55 formatted stack trace is meant to be read by a person. The value of │
56 the -errorcode entry is additional information about the error stored │
57 as a list. The -errorcode value is meant to be further processed by │
58 programs, and may not be particularly readable by people. The value of │
59 the -errorline entry is an integer indicating which line of script was │
60 being evaluated when the error occurred. The values of the -errorinfo │
61 and -errorcode entries of the most recent error are also available as │
62 values of the global variables ::errorInfo and ::errorCode respec‐ │
63 tively. │
64 Tcl packages may provide commands that set other entries in the dictio‐ │
66 nary of return options, and the return command may be used by scripts │
67 to set return options in addition to those defined above.
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70 The catch command may be used in an if to branch based on the success
71 of a script.
72 if { [catch {open $someFile w} fid] } {
73 puts stderr "Could not open $someFile for writing\n$fid"
74 exit 1
75 }
76 There are more complex examples of catch usage in the documentation for
78 the return command.
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82 break(n), continue(n), dict(n), error(n), return(n), tclvars(n)
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86 catch, error
87Tcl 8.5 catch(n)