Tcl source Command

last modified April 3, 2025

The Tcl source command reads and executes a script from a file. It is used to load external Tcl scripts into the current interpreter. This allows for modular programming and code reuse.

Basic Definition

The source command evaluates the contents of a file as a Tcl script. It takes a single argument - the path to the file to be sourced.

Syntax: source fileName. The command returns the result of the last command executed from the sourced file. File paths can be absolute or relative.

Basic File Sourcing

This example shows the simplest usage of source to load a script.

basic_source.tcl

# Contents of greetings.tcl:
# puts "Hello from external script!"
# set greeting "Welcome"

source greetings.tcl
puts $greeting

This sources a file called greetings.tcl that contains Tcl commands. After sourcing, the variable greeting is available in the main script.

Relative Path Sourcing

The source command can load files using relative paths from the current directory.

relative_source.tcl

# File structure:
# main.tcl
# lib/utils.tcl

source lib/utils.tcl
puts "Utility functions loaded"

This demonstrates sourcing a file from a subdirectory. The path is relative to the location of the script containing the source command.

Absolute Path Sourcing

For more control, absolute paths can be used with the source command.

absolute_source.tcl

source /home/user/tclscripts/config.tcl
puts "Configuration loaded from absolute path"

This example shows sourcing a file using its full absolute path. This method is useful when the script location is fixed and known in advance.

Sourcing with Variables

The file path for source can be constructed using variables and command substitution.

variable_source.tcl

set script_dir "lib"
set script_name "helpers.tcl"
source [file join $script_dir $script_name]
puts "Helpers sourced successfully"

This demonstrates building the source path dynamically using variables and the file join command for proper path construction across platforms.

Error Handling

When sourcing files, it's important to handle potential errors gracefully.

error_handling.tcl

if {[file exists "important.tcl"]} {
    source important.tcl
} else {
    puts "Warning: important.tcl not found"
}

This shows how to check for a file's existence before attempting to source it. The file exists command helps prevent runtime errors.

Return Value

The source command returns the result of the last command executed from the sourced file.

return_value.tcl

# Contents of calc.tcl:
# expr {5 * 5}

set result [source calc.tcl]
puts "The calculation result is $result"

This example demonstrates capturing the return value from a sourced file. The calculation in calc.tcl is executed and its result is stored in a variable.

Best Practices

Organization: Keep sourced files logically organized.
Paths: Use relative paths for portability when possible.
Error Checking: Verify file existence before sourcing.
Scope: Be aware of variable scope in sourced files.
Dependencies: Document file dependencies clearly.

This tutorial covered the Tcl source command with practical examples showing its usage in different 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.

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