The main file [source]d by a Tcl interpreter when starting up (during [Tcl_Init]).  See the [library] manpage for more info:

   * http://www.tcl.tk/man/tcl/TclCmd/library.htm

Has in recent versions become ''more'' important for providing the core language environment, as some core commands ([clock], [info], [string]) that used to be implemented in [C] throughout are now [namespace ensemble]s set up in init.tcl.

See also:
   * [Tcl_Init]
   * [Magic names]
   * [interp create]
   * [TCL_LIBRARY] (esp. regarding "cannot find usable init.tcl" errors after compiling Tcl)
-----
`init.tcl` is very well-documented, as usual with Tcl, and worth reading if you're interested. For a quick survey, here are some procs it defines:

======
# $ egrep '^\s*(namespace\ eval|proc)' /usr/share/tcltk/tcl8.6/init.tcl | sed -e 's/{$/{}/'
namespace eval tcl {}
    namespace eval mathfunc {}
        proc min {args} {}
        proc max {args} {}
    namespace eval tcl {}
        proc EnvTraceProc {lo n1 n2 op} {}
        proc InitWinEnv {} {}
    namespace eval ::tcl::clock [list variable TclLibDir $::tcl_library]
    proc clock args {}
        namespace eval ::tcl::clock [list namespace ensemble create -command \
            proc ::tcl::clock::$cmd args {}
    proc tclLog {string} {}
proc unknown args {}
proc auto_load {cmd {namespace {}}} {}
            namespace eval :: $auto_index($name)
            namespace eval :: $auto_index($name)
proc auto_load_index {} {}
proc auto_qualify {cmd namespace} {}
proc auto_import {pattern} {}
                namespace eval :: $auto_index($name)
proc auto_execok name {}
proc auto_execok name {}
proc tcl::CopyDirectory {action src dest} {}
======

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