Tcl catch Command

last modified April 3, 2025

The Tcl catch command is used for error handling. It evaluates a script and catches any exceptions that occur. This prevents errors from terminating the program execution.

Basic Definition

The catch command evaluates its argument as a Tcl script. It returns 0 if the script executes successfully, or 1 if an error occurs.

Syntax: catch script ?resultVarName? ?optionsVarName?. The optional arguments capture the error result and options dictionary.

Basic Error Handling

This example shows the simplest usage of catch to handle errors.

basic_catch.tcl

set result [catch {expr {1 / 0}} error]
puts "Result: $result, Error: $error"

This attempts to divide by zero, which would normally cause an error. The catch command prevents termination and stores the error message.

Catching Specific Errors

We can examine the error code to handle specific types of errors differently.

catch_specific.tcl

set script {
    set x "hello"
    expr {$x + 10}
}
set code [catch $script result options]

if {$code == 1} {
    puts "Error occurred: $result"
    puts "Error code: [dict get $options -code]"
}

This catches a type error when trying to add a string to a number. The options dictionary provides additional error information for more precise handling.

Nested Catch Commands

catch commands can be nested to handle errors at different levels.

catch_nested.tcl

proc riskyOperation {x} {
    if {$x == 0} {
        error "Division by zero attempted"
    }
    return [expr {100 / $x}]
}

set result [catch {
    catch {riskyOperation 0} innerResult
} outerResult]

puts "Outer result: $outerResult"
puts "Inner result: $innerResult"

This shows nested error handling where an inner catch handles a function's error, and an outer catch provides additional safety.

Using Finally-like Behavior

Tcl doesn't have a finally clause, but we can simulate it with catch.

catch_finally.tcl

set file [open "nonexistent.txt" r]
set code [catch {
    set content [read $file]
} result]

close $file
if {$code} {
    puts "Error reading file: $result"
} else {
    puts "File content: $content"
}

This demonstrates ensuring a file is closed whether an error occurs or not. The cleanup code executes after the catch block regardless of outcome.

Error Codes and Messages

The catch command can distinguish between different error types.

catch_codes.tcl

set code [catch {error "Custom error" "" 42} result options]

puts "Error code: $code"
puts "Error message: $result"
puts "Custom error code: [dict get $options -errorcode]"
puts "Error info: [dict get $options -errorinfo]"

This shows how to create and catch custom errors with specific codes. The options dictionary contains detailed error information for debugging.

Advanced Error Handling

Combine catch with other commands for robust error handling.

catch_advanced.tcl

proc safeDivide {a b} {
    if {![string is integer $a] || ![string is integer $b]} {
        error "Non-integer arguments" "" 1
    }
    if {$b == 0} {
        error "Division by zero" "" 2
    }
    return [expr {$a / $b}]
}

set code [catch {safeDivide 10 0} result options]

switch [dict get $options -errorcode] {
    1 { puts "Type error: $result" }
    2 { puts "Math error: $result" }
    default { puts "Unknown error: $result" }
}

This demonstrates a comprehensive error handling strategy with custom error codes. Different error types are handled differently based on the error code.

Best Practices

Specificity: Catch only what you can handle meaningfully.
Logging: Always log unexpected errors for debugging.
Recovery: Provide recovery paths where possible.
Cleanup: Ensure resources are released after errors.
Testing: Test error cases as thoroughly as success cases.

This tutorial covered the Tcl catch command with practical examples showing its usage in different error handling scenarios.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

List all Tcl Tutorials.

Ebooks