catch(1T) Tcl Built-In Commands catch(1T)
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NAME
catch - Evaluate script and trap exceptional returns
SYNOPSIS
catch script ?varName?
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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 recur-
sively 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 situa-
tions 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 varName 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 varName 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 script does not raise an error, catch will return 0 (TCL_OK) and set the variable to the value returned from script.
Note that catch catches all exceptions, including those generated by break and continue as well as errors. The only errors that are not
caught are syntax errors found when the script is compiled. This is because the catch command only catches errors during runtime. When
the catch statement is compiled, the script is compiled as well and any syntax errors will generate a Tcl error.
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
$fid"
exit 1
}
The catch command will not catch compiled syntax errors. The first time proc foo is called, the body will be compiled and a Tcl error will
be generated.
proc foo {} {
catch {expr {1 +- }}
}
SEE ALSO
break(1T), continue(1T), error(1T), return(1T), tclvars(1T)
KEYWORDS
catch, error
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
+--------------------+-----------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-----------------+
|Availability | SUNWTcl |
+--------------------+-----------------+
|Interface Stability | Uncommitted |
+--------------------+-----------------+
NOTES
Source for Tcl is available on http://opensolaris.org.
Tcl 8.0 catch(1T)