1unknown(n) Tcl Built-In Commands unknown(n)
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8 unknown - Handle attempts to use non-existent commands
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11 unknown cmdName ?arg arg ...?
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15 This command is invoked by the Tcl interpreter whenever a script tries
16 to invoke a command that does not exist. The default implementation of
17 unknown is a library procedure defined when Tcl initializes an inter‐
18 preter. You can override the default unknown to change its functional‐
19 ity, or you can register a new handler for individual namespaces using
20 the namespace unknown command. Note that there is no default implemen‐
21 tation of unknown in a safe interpreter.
22 If the Tcl interpreter encounters a command name for which there is not
24 a defined command (in either the current namespace, or the global
25 namespace), then Tcl checks for the existence of an unknown handler for
26 the current namespace. By default, this handler is a command named
27 ::unknown. If there is no such command, then the interpreter returns
28 an error. If the unknown command exists (or a new handler has been
29 registered for the current namespace), then it is invoked with argu‐
30 ments consisting of the fully-substituted name and arguments for the
31 original non-existent command. The unknown command typically does
32 things like searching through library directories for a command proce‐
33 dure with the name cmdName, or expanding abbreviated command names to
34 full-length, or automatically executing unknown commands as sub-pro‐
35 cesses. In some cases (such as expanding abbreviations) unknown will
36 change the original command slightly and then (re-)execute it. The
37 result of the unknown command is used as the result for the original
38 non-existent command.
39 The default implementation of unknown behaves as follows. It first
41 calls the auto_load library procedure to load the command. If this
42 succeeds, then it executes the original command with its original argu‐
43 ments. If the auto-load fails then unknown calls auto_execok to see if
44 there is an executable file by the name cmd. If so, it invokes the Tcl
45 exec command with cmd and all the args as arguments. If cmd cannot be
46 auto-executed, unknown checks to see if the command was invoked at top-
47 level and outside of any script. If so, then unknown takes two addi‐
48 tional steps. First, it sees if cmd has one of the following three
49 forms: !!, !event, or ^old^new?^?. If so, then unknown carries out
50 history substitution in the same way that csh would for these con‐
51 structs. Finally, unknown checks to see if cmd is a unique abbrevia‐
52 tion for an existing Tcl command. If so, it expands the command name
53 and executes the command with the original arguments. If none of the
54 above efforts has been able to execute the command, unknown generates
55 an error return. If the global variable auto_noload is defined, then
56 the auto-load step is skipped. If the global variable auto_noexec is
57 defined then the auto-exec step is skipped. Under normal circumstances
58 the return value from unknown is the return value from the command that
59 was eventually executed.
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62 Arrange for the unknown command to have its standard behavior except
63 for first logging the fact that a command was not found:
64 # Save the original one so we can chain to it
66 rename unknown _original_unknown
67 # Provide our own implementation
69 proc unknown args {
70 puts stderr "WARNING: unknown command: $args"
71 uplevel 1 [list _original_unknown {*}$args]
72 }
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75 info(n), proc(n), interp(n), library(n), namespace(n)
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78 error, non-existent command
79Tcl unknown(n)