source, a built-in command, evaluates a file or resource as a Tcl script.


source filename

source -encoding encodingName fileName


official reference

See Also

source with args
pre-processing for source
Overloaded source for C-style comments
\u001a is an end-of-file character in scripts
about the use of ^Z
sourcecache , by David Welton
acts as source does, but with caching. This is useful, as he notes, "for applications such as web servers running dynamic pages ..."
how to change a running system
RS: source is the weakest way of . Relying on auto_index or package mechanisms is more robust, and easier once set up - do nothing (maybe extend auto_path), or package require.


source evaluates the contents of the specified file as a script at the level of its caller. As with uplevel, the result is returned to source itself, not to the caller of source. This is in contrast to eval, where the result is returned to the caller of eval. If needed, the -code and -level options of return provide finer control.

The first occurrence of ^Z (ASCII character 26) is interpreted as a marker indicating the end of the script. This makes it possible to store additional data in the same file. If necessary, use read or gets, to work around this. . To use the character ^Z in the script, encode it as \032 or \u001a.

filename is normally read using the system encoding. Override this behaviour with -encoding.

Ensure that a file is only sourced once

Joe Mistachkin 2003-08-25: The following routine sources a file only once:

proc sourceOnce file {
    upvar 1 sources sources
    if {![info exists sources([file normalize $file])]} then {
        # don't catch errors, since that may indicate we failed to load it...?
        uplevel "1" [list source $file]
        # mark it as loaded since it was source'd with no error...
        set sources([file normalize $file]) "1"

DGP: I don't think that quite works. Different callers might well refer to different sources arrays. Better to encapsulate all in the same namespace.

namespace eval my {
    namespace export sourceOnce
    variable sources
    array set sources {}
    proc sourceOnce file {
        # Remaining exercise for the next reader.  Adapt argument
        # processing to support the -rsrc and -encoding options
        # that [::source] provides (or will in Tcl 8.5)
        variable sources
        if {![info exists sources([file normalize $file])]} then {
            # don't catch errors, since that may indicate we failed to load it...?
            #     Extra challenge:  Use the techniques outlined in TIP 90
            #     to catch errors, then re-raise them so the [uplevel] does
            #     not appear on the stack trace.
            # We don't know what command is [source] in the caller's context,
            # so fully qualify to get the [::source] we want.
            uplevel 1 [list ::source $file]
            # mark it as loaded since it was source'd with no error...
            set sources([file normalize $file]) 1
namespace import my::sourceOnce

elfring 2003-08-26: The discussion about sourceOnce is a continuation of feature request #301: Include source files only once .

I do not see that the variable "sources" is documented on the manual page tclvars [L1 ] or anywhere else. Will this happen with Tcl 8.5 or earlier? I get the feeling that the requested addition of a function "include_once" or "source -once" need more safety checks and conditions than I have expected. Who would like to show the standard argument processing for it?

DKF 2003-08-26: It's not documented anywhere in the core because it is not in the core. It's defined right here, just a few lines above. :^)

elfring 2003-08-27: Well, thank you for this fact. - The current implementation of the function "source" does not record into a variable which files were read.

Making Use of Environment PATH

RS: /bin/sh-like source: When playing Bourne shell, I found out that their source command "." searches the environment PATH, a feature that has occasionally been missed in Tcl's source. Well, here's an emulation that caters for Unix/Win differences:

proc path'separator {} {
    switch -- $::tcl_platform(platform) {
        unix    {return ":"}
        windows {return ";"}
        default {error "unknown platform"}
if {[llength [info global tk*]]==0} {
    # The name . is taboo in a wish, would exit immediately
    proc . filename {
        foreach path [split $::env(PATH) [path'separator]] {
            set try [file join $path $filename]
            if [file readable $try] {return [source $try]}
        error ".: no such file or directory: $filename"
} ;# RS

Evaluate Channel Contents

proc sourceOpen {channel} {
    while 1 {
        append line [gets $channel line]
        if {[info complete $line]} {
            eval $line
        if {[eof $channel]} break

PYK: This behaves differently from source: ^Z is not, lines are evaluated within the scope of sourceOpen instead of its caler, and return is treated differently.

Evaluate Channel Contents in Chunks

hans 2008-02-25: source reads the whole file into memory first before evaluating it. I'm saving updates to a large Tcl source file whenever any variables change, so the "tape" file can become quite large. Of course the memory is freed after sourcing, but still there is a huge spike.

Here's my solution. After every update to my data.tcl file, I'm inserting a line with: "#~". Of course, I have to escape that special char everywhere. Now it's easy to read and eval the huge file in chunks, timing is very close to that of source.

proc load_data {fname} {
    set f  [open $fname r]
    fconfigure $f -translation binary \
    -buffersize 1000000 -encoding "iso8859-1"
    while {![eof $f]} {
        set r [read $f 1000000]
        set l [string length $r]
        set c [string last "~" $r]
        while {$c eq -1} {
            if {[eof $f]} { error "Missing ~" }
            set r2 [read $f 1000000]
            append r $r2
            set l [string length $r]
            set c [string last "~" $r]
        set e [string range $r 0 $c]
        uplevel #0 eval $e 
        seek $f [expr $c-$l-1] current
        if {$l<1000000} { break }
   close $f

Perhaps someone will come along and help out with error handling and plug logic holes here...

Determining whether the current script was sourced

Arjen Markus: Sometimes it is useful to distinguish between the Tcl/Tk shell sourcing the file (as in tclsh aa.tcl) and the script being evaluated to do this (via the command "source aa.tcl"). This can be used to:

  • Provide a library of useful procedures and
  • Provide a self-contained "main" proc in the same file

One example of this, a library of procs that together compare two files. If called directly:

tclsh numcomp.tcl a.inp b.inp

the main proc gets executed and the comparison is started. If sourced inside another script, however, this is not done and the sourcing script has full control. (In this case: it is a larger script controlling a number of programs whose results must be compared against reference files).

The trick is the code below:

if { [info script] eq $::argv0 } {
    puts {In main}
} else {
    puts {Sourcing [info script]}

pyk 2019-08-07: See also main script.

return in a sourced file

Arjen Markus: Another trick I recently learned: return works in source as in any other script - it allows you to return from the source command, so that the rest of the file is not handled. Quite useful for recovering from errors or making the processing depend on external influences.


loading data

You can also source (almost) pure data files like this:

array set data [source datafile]

where datafile contains:

return {
1 {some data}
2 {other data}
. ...
foo {note that the element name does not have to be numeric}

results in:

% parray data
data(.)   = ...
data(1)   = some data
data(2)   = other data
data(foo) = note that the element name does not have to be numeric

Multiple files

Just glob the files you want, and source them all.

foreach x [glob -dir $myDirectory *.tcl] {
    source $x

Exclude myself when sourcing everything in my directory

If you want to have it load all scripts in its own directory, instead of that "source $x" do an "if $x not itself.."

Encoding and TCL prior to 8.5.0

Notice that sourcing is always done in the system encoding. For sources in Unicode or other goodies you'd have to open a file, fconfigure -encoding, read, eval, close - see Unicode file reader or source with encoding. source $file does something like:

set fp [open $file]
set data [read $fp]
close $fp
eval $data

HaO 2015-07-07: Since TCL 8.5.0, the optional parameter '-encoding' may be used to specify the encoding.

I hope, TCL 9.0 will use utf-8 encoding for sources instead of the system encoding...

utf-8 BOM

HaO 2015-07-07: If a file encoded in utf-8 with a BOM at the beginning is sourced, the following error is shown for me (system encoding: cp1252 (Windows western Europe), TCL 8.6.4):

% source bom.tcl
invalid command name "puts"

Nevertheless, the BOM is accepted if utf-8 encoding is specified:

% source -encoding utf-8 bom.tcl

File contents of bom.tcl (you don't see the leading BOM in the editor):

puts "hello"

The binary contents is:

% set h [open bom.tcl rb]
% set d [read $h]
puts "hello"

% close $h
% binary scan $d H* dh
% regexp -all -inline .. $dh
ef bb bf 70 75 74 73 20 22 68 65 6c 6c 6f 22 0d 0a


CMcC 2006-04-12:

Following discussions on tcler's chat in which dgp suggested (on, I presume, aesthetic grounds) that pkgIndex.tcl ought not to contain more than one package definition:

One implementation problem with advice is that a package may contain many source files, but there is no way to discover the directory within which those source files reside.

One possible solution would be to have the pkgIndex.tcl file entry for the multi-source package source all of the components of the package.

The most elegant way to achieve this goal would be to allow source to take multiple file arguments as follows: source ?-dir directory? fileName ?fileName?, and this is what I propose for general consideration.


Special treatment of ^Z (ASCII 26) was introduced in Tcl version 8.4.