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tclvars(n)		     Tcl Built-In Commands		    tclvars(n)

______________________________________________________________________________

NAME
       argc,  argv,  argv0, auto_path, env, errorCode, errorInfo, tcl_interac‐
       tive,  tcl_library,  tcl_nonwordchars,	tcl_patchLevel,	  tcl_pkgPath,
       tcl_platform,	tcl_precision,	  tcl_rcFileName,    tcl_traceCompile,
       tcl_traceEval, tcl_wordchars, tcl_version - Variables used by Tcl
_________________________________________________________________

DESCRIPTION
       The following global variables are created and managed automatically by
       the Tcl library.	 Except where noted below, these variables should nor‐
       mally be treated as  read-only  by  application-specific	 code  and  by
       users.

       auto_path
	      If set, then it must contain a valid Tcl list giving directories
	      to search during auto-load  operations  (including  for  package
	      index  files  when  using	 the default package unknown handler).
	      This variable is	initialized  during  startup  to  contain,  in
	      order:  the  directories	listed	in  the TCLLIBPATH environment
	      variable, the directory named by the  tcl_library	 global	 vari‐
	      able,  the  parent  directory  of	 tcl_library,  the directories
	      listed in the tcl_pkgPath	 variable.   Additional	 locations  to
	      look  for	 files and package indices should normally be added to
	      this variable using lappend.

	      Additional variables relating to package management exist.  More
	      details  are listed in the VARIABLES section of the library man‐
	      ual page.

       env    This variable is maintained by Tcl as an	array  whose  elements
	      are  the environment variables for the process.  Reading an ele‐
	      ment will return the  value  of  the  corresponding  environment
	      variable.	  Setting an element of the array will modify the cor‐
	      responding environment variable or create a new one if  it  does
	      not  already exist.  Unsetting an element of env will remove the
	      corresponding environment variable.  Changes to  the  env	 array
	      will  affect the environment passed to children by commands like
	      exec.  If the entire env array is unset then Tcl will stop moni‐
	      toring env accesses and will not update environment variables.

	      Under Windows, the environment variables PATH and COMSPEC in any
	      capitalization are converted automatically to upper  case.   For
	      instance,	 the  PATH variable could be exported by the operating
	      system as “path”, “Path”, “PaTh”, etc., causing otherwise simple
	      Tcl code to have to support many special cases.  All other envi‐
	      ronment variables inherited by Tcl are left unmodified.  Setting
	      an  env  array  variable to blank is the same as unsetting it as
	      this is the behavior of the underlying Windows OS.  It should be
	      noted that relying on an existing and empty environment variable
	      will not work on Windows and is discouraged  for	cross-platform
	      usage.

	      The following elements of env are special to Tcl:

	      env(HOME)
		     This  environment variable, if set, gives the location of
		     the directory considered to be the	 current  user's  home
		     directory, and to which a call of cd without arguments or
		     with just “~” as an argument will change into. Most plat‐
		     forms set this correctly by default; it does not normally
		     need to be set by user code.

	      env(TCL_LIBRARY)
		     If set, then it specifies the location of	the  directory
		     containing	 library  scripts  (the value of this variable
		     will be assigned to the tcl_library variable  and	there‐
		     fore  returned  by	 the  command  info library).  If this
		     variable is not set then a default value is used.

		     Note that this environment variable should	 not  normally
		     be set.

	      env(TCLLIBPATH)
		     If	 set,  then  it	 must  contain a valid Tcl list giving
		     directories  to  search  during   auto-load   operations.
		     Directories must be specified in Tcl format, using “/” as
		     the path separator, regardless of platform.   This	 vari‐
		     able  is  only used when initializing the auto_path vari‐
		     able.

	      env(TCL_TZ), env(TZ)
		     These specify the default timezone used for  parsing  and
		     formatting	 times and dates in the clock command. On many
		     platforms, the TZ environment variable is set up  by  the
		     operating system.

	      env(LC_ALL), env(LC_MESSAGES), env(LANG)
		     These  environment variables are used by the msgcat pack‐
		     age to determine what locale to format messages using.

	      env(TCL_INTERP_DEBUG_FRAME)
		     If existing, it has the same  effect  as  running	interp
		     debug  {}	-frame 1 as the very first command of each new
		     Tcl interpreter.

       errorCode
	      This variable holds the value of the  -errorcode	return	option
	      set  by the most recent error that occurred in this interpreter.
	      This list value  represents  additional  information  about  the
	      error  in	 a  form  that	is easy to process with programs.  The
	      first element of the list identifies a general class of  errors,
	      and  determines the format of the rest of the list.  The follow‐
	      ing formats for -errorcode return options are used  by  the  Tcl
	      core; individual applications may define additional formats.

	      ARITH code msg
		     This format is used when an arithmetic error occurs (e.g.
		     an attempt to divide zero by zero in the  expr  command).
		     Code  identifies  the  precise  error  and msg provides a
		     human-readable description of the error.	Code  will  be
		     either DIVZERO (for an attempt to divide by zero), DOMAIN
		     (if an argument is outside the domain of a function, such
		     as	 acos(-3)), IOVERFLOW (for integer overflow), OVERFLOW
		     (for a floating-point overflow), or UNKNOWN (if the cause
		     of the error cannot be determined).

		     Detection of these errors depends in part on the underly‐
		     ing hardware and system libraries.

	      CHILDKILLED pid sigName msg
		     This format is used when a child process has been	killed
		     because  of  a  signal.   The  pid	 element  will	be the
		     process's identifier (in decimal).	 The  sigName  element
		     will  be  the symbolic name of the signal that caused the
		     process to terminate; it will be one of  the  names  from
		     the include file signal.h, such as SIGPIPE.  The msg ele‐
		     ment will be a short  human-readable  message  describing
		     the  signal,  such as “write on pipe with no readers” for
		     SIGPIPE.

	      CHILDSTATUS pid code
		     This format is used when a child process has exited  with
		     a	non-zero  exit	status.	  The  pid element will be the
		     process's identifier (in decimal) and  the	 code  element
		     will  be  the  exit code returned by the process (also in
		     decimal).

	      CHILDSUSP pid sigName msg
		     This format is used when a child process  has  been  sus‐
		     pended  because of a signal.  The pid element will be the
		     process's identifier, in decimal.	 The  sigName  element
		     will  be  the symbolic name of the signal that caused the
		     process to suspend; this will be one of  the  names  from
		     the include file signal.h, such as SIGTTIN.  The msg ele‐
		     ment will be a short  human-readable  message  describing
		     the signal, such as “background tty read” for SIGTTIN.

	      NONE   This format is used for errors where no additional infor‐
		     mation is available for  an  error	 besides  the  message
		     returned  with  the error.	 In these cases the -errorcode
		     return option will consist of a list containing a	single
		     element whose contents are NONE.

	      POSIX errName msg
		     If	 the  first  element is POSIX, then the error occurred
		     during a POSIX kernel call.   The	errName	 element  will
		     contain  the  symbolic  name  of the error that occurred,
		     such as ENOENT; this will be one of the values defined in
		     the  include  file	 errno.h.   The	 msg element will be a
		     human-readable message corresponding to errName, such  as
		     “no such file or directory” for the ENOENT case.

	      TCL ...
		     Indicates	some  sort of problem generated in relation to
		     Tcl itself, e.g. a failure to look up a channel or	 vari‐
		     able.

	      To  set  the  -errorcode	return option, applications should use
	      library procedures such as Tcl_SetObjErrorCode, Tcl_SetReturnOp‐
	      tions,  and  Tcl_PosixError,  or	they may invoke the -errorcode
	      option of the return command.  If none of these methods for set‐
	      ting  the	 error	code  has  been used, the Tcl interpreter will
	      reset the variable to NONE after the next error.

       errorInfo
	      This variable holds the value of the  -errorinfo	return	option
	      set  by the most recent error that occurred in this interpreter.
	      This string value will contain one or more lines identifying the
	      Tcl  commands  and  procedures that were being executed when the
	      most recent error occurred.  Its contents take  the  form	 of  a
	      stack  trace  showing  the  various nested Tcl commands that had
	      been invoked at the time of the error.

       tcl_library
	      This variable holds the name of a directory containing the  sys‐
	      tem library of Tcl scripts, such as those used for auto-loading.
	      The value of this variable is returned by the info library  com‐
	      mand.   See  the library manual entry for details of the facili‐
	      ties provided by the Tcl script library.	Normally each applica‐
	      tion  or	package	 will have its own application-specific script
	      library in addition to the Tcl script library; each  application
	      should  set  a  global  variable	with  a name like $app_library
	      (where app is the application's name) to hold the	 network  file
	      name  for	 that  application's  library  directory.  The initial
	      value of tcl_library is set when an interpreter  is  created  by
	      searching	 several different directories until one is found that
	      contains an appropriate Tcl startup script.  If the  TCL_LIBRARY
	      environment  variable  exists,  then  the	 directory it names is
	      checked first.  If TCL_LIBRARY is not set or doesn't refer to an
	      appropriate directory, then Tcl checks several other directories
	      based on a compiled-in default location,	the  location  of  the
	      binary  containing  the  application,  and  the  current working
	      directory.

       tcl_patchLevel
	      When an interpreter is created Tcl initializes this variable  to
	      hold  a  string  giving the current patch level for Tcl, such as
	      8.4.16 for Tcl 8.4 with the first sixteen official  patches,  or
	      8.5b3  for the third beta release of Tcl 8.5.  The value of this
	      variable is returned by the info patchlevel command.

       tcl_pkgPath
	      This variable holds a list of directories indicating where pack‐
	      ages  are	 normally  installed.	It is not used on Windows.  It
	      typically contains either one or two entries; if it contains two
	      entries,	the  first is normally a directory for platform-depen‐
	      dent packages (e.g., shared library binaries) and the second  is
	      normally	a  directory  for platform-independent packages (e.g.,
	      script files). Typically a package is installed as  a  subdirec‐
	      tory  of	one  of	 the  entries in the tcl_pkgPath variable. The
	      directories in the tcl_pkgPath variable are included by  default
	      in the auto_path variable, so they and their immediate subdirec‐
	      tories are automatically searched for  packages  during  package
	      require commands.	 Note: tcl_pkgPath is not intended to be modi‐
	      fied by the application.	Its value is  added  to	 auto_path  at
	      startup;	changes to tcl_pkgPath are not reflected in auto_path.
	      If you want Tcl to search additional  directories	 for  packages
	      you  should add the names of those directories to auto_path, not
	      tcl_pkgPath.

       tcl_platform
	      This is an associative array whose elements contain  information
	      about  the platform on which the application is running, such as
	      the name of the operating system, its  current  release  number,
	      and  the	machine's  instruction set.  The elements listed below
	      will always be defined, but they may have empty strings as  val‐
	      ues  if  Tcl  could  not	retrieve any relevant information.  In
	      addition, extensions and applications may add additional	values
	      to the array.  The predefined elements are:

	      byteOrder
		     The  native  byte order of this machine: either littleEn‐
		     dian or bigEndian.

	      debug  If this variable exists, then the	interpreter  was  com‐
		     piled  with  and  linked  to  a debug-enabled C run-time.
		     This variable will only exist on  Windows,	 so  extension
		     writers  can  specify  which package to load depending on
		     the C run-time library that is in use.  This  is  not  an
		     indication that this core contains symbols.

	      machine
		     The  instruction  set  executed  by this machine, such as
		     intel, PPC, 68k, or sun4m.	 On UNIX machines, this is the
		     value returned by uname -m.

	      os     The name of the operating system running on this machine,
		     such as Windows  95,  Windows  NT,	 or  SunOS.   On  UNIX
		     machines,	this  is  the  value returned by uname -s.  On
		     Windows 95 and Windows 98, the  value  returned  will  be
		     Windows  95  to provide better backwards compatibility to
		     Windows 95; to distinguish between	 the  two,  check  the
		     osVersion.

	      osVersion
		     The  version  number  for the operating system running on
		     this machine.   On	 UNIX  machines,  this	is  the	 value
		     returned by uname -r.  On Windows 95, the version will be
		     4.0; on Windows 98, the version will be 4.10.

	      pathSeparator
		     The character that should	be  used  to  split  PATH-like │
		     environment  variables  into  their corresponding list of │
		     directory names.

	      platform
		     Either windows, or unix.	This  identifies  the  general
		     operating environment of the machine.

	      pointerSize
		     This  gives  the  size  of	 the native-machine pointer in
		     bytes (strictly, it is same as the result	of  evaluating
		     sizeof(void*) in C.)

	      threaded
		     If	 this  variable	 exists, then the interpreter was com‐
		     piled with threads enabled.

	      user   This identifies the  current  user	 based	on  the	 login
		     information  available  on the platform.  This comes from
		     the USER or LOGNAME environment variable on Unix, and the
		     value from GetUserName on Windows.

	      wordSize
		     This  gives  the size of the native-machine word in bytes
		     (strictly,	 it  is	 same  as  the	result	of  evaluating
		     sizeof(long) in C.)

       tcl_precision
	      This  variable  controls	the  number of digits to generate when
	      converting floating-point values to strings.  It defaults to  0.
	      Applications  should  not	 change this value; it is provided for
	      compatibility with legacy code.

	      The default value of 0 is special, meaning that Tcl should  con‐
	      vert numbers using as few digits as possible while still distin‐
	      guishing any floating point number from its nearest  neighbours.
	      It  differs  from using an arbitrarily high value for tcl_preci‐
	      sion in that an inexact number like  1.4	will  convert  as  1.4
	      rather  than 1.3999999999999999 even though the latter is nearer
	      to the exact value of the binary number.

	      If tcl_precision is not zero, then when Tcl converts a  floating
	      point  number,  it  creates  a decimal representation of at most
	      tcl_precision significant digits; the result may be  shorter  if
	      the shorter result represents the original number exactly. If no
	      result of at most tcl_precision digits is an  exact  representa‐
	      tion  of	the  original  number,	the one that is closest to the
	      original number is chosen.  If the  original  number  lies  pre‐
	      cisely  between  two  equally  accurate decimal representations,
	      then the one with an even value for the least significant	 digit
	      is chosen; for instance, if tcl_precision is 3, then 0.3125 will
	      convert to 0.312, not 0.313, while 0.6875 will convert to 0.688,
	      not  0.687.  Any	string	of  trailing  zeroes  that  remains is
	      trimmed.

	      a tcl_precision value of 17 digits is “perfect” for IEEE	float‐
	      ing-point	 in  that it allows double-precision values to be con‐
	      verted to strings and back to binary with no  loss  of  informa‐
	      tion.  For  this	reason,	 you  will  often see it as a value in
	      legacy code that must run on Tcl versions before 8.5. It	is  no
	      longer  recommended;  as	noted  above, a zero value is the pre‐
	      ferred method.

	      All interpreters in a thread share a single tcl_precision value:
	      changing	it  in	one  interpreter  will affect all other inter‐
	      preters as well.	Safe interpreters are not  allowed  to	modify
	      the variable.

	      Valid values for tcl_precision range from 0 to 17.

       tcl_rcFileName
	      This variable is used during initialization to indicate the name
	      of a user-specific startup file.	If it is set  by  application-
	      specific	initialization,	 then  the Tcl startup code will check
	      for the existence of this file and source it if it exists.   For
	      example,	for wish the variable is set to ~/.wishrc for Unix and
	      ~/wishrc.tcl for Windows.

       tcl_traceCompile
	      The value of this variable can be set to control how much	 trac‐
	      ing  information	is  displayed during bytecode compilation.  By
	      default, tcl_traceCompile is zero and  no	 information  is  dis‐
	      played.  Setting tcl_traceCompile to 1 generates a one-line sum‐
	      mary in stdout whenever a procedure or top-level command is com‐
	      piled.   Setting	it to 2 generates a detailed listing in stdout
	      of the bytecode instructions emitted during  every  compilation.
	      This variable is useful in tracking down suspected problems with
	      the Tcl compiler.

	      This variable and functionality only exist if  TCL_COMPILE_DEBUG
	      was defined during Tcl's compilation.

       tcl_traceExec
	      The  value of this variable can be set to control how much trac‐
	      ing information is  displayed  during  bytecode  execution.   By
	      default,	tcl_traceExec is zero and no information is displayed.
	      Setting tcl_traceExec to 1 generates a one-line trace in	stdout
	      on  each	call  to a Tcl procedure.  Setting it to 2 generates a
	      line of output whenever any Tcl command is invoked that contains
	      the name of the command and its arguments.  Setting it to 3 pro‐
	      duces a detailed trace showing  the  result  of  executing  each
	      bytecode	instruction.   Note that when tcl_traceExec is 2 or 3,
	      commands such as set and incr that have been  entirely  replaced
	      by  a  sequence of bytecode instructions are not shown.  Setting
	      this variable is useful in tracking down suspected problems with
	      the bytecode compiler and interpreter.

	      This  variable and functionality only exist if TCL_COMPILE_DEBUG
	      was defined during Tcl's compilation.

       tcl_wordchars
	      The value of this variable is a regular expression that  can  be
	      set  to  control	what  are  considered  “word”  characters, for
	      instances like selecting a word by double-clicking  in  text  in
	      Tk.   It	is platform dependent.	On Windows, it defaults to \S,
	      meaning anything but a Unicode space  character.	 Otherwise  it
	      defaults	to  \w,	 which	is any Unicode word character (number,
	      letter, or underscore).

       tcl_nonwordchars
	      The value of this variable is a regular expression that  can  be
	      set  to  control	what are considered “non-word” characters, for
	      instances like selecting a word by double-clicking  in  text  in
	      Tk.   It	is platform dependent.	On Windows, it defaults to \s,
	      meaning any Unicode space character.  Otherwise it  defaults  to
	      \W, which is anything but a Unicode word character (number, let‐
	      ter, or underscore).

       tcl_version
	      When an interpreter is created Tcl initializes this variable  to
	      hold the version number for this version of Tcl in the form x.y.
	      Changes to x represent major changes with probable incompatibil‐
	      ities  and  changes  to  y  represent small enhancements and bug
	      fixes that retain backward compatibility.	  The  value  of  this
	      variable is returned by the info tclversion command.

OTHER GLOBAL VARIABLES
       The  following variables are only guaranteed to exist in tclsh and wish
       executables; the Tcl library does not define them itself but  many  Tcl
       environments do.

       argc  The number of arguments to tclsh or wish.

       argv  Tcl list of arguments to tclsh or wish.

       argv0 The script that tclsh or wish started executing (if it was speci‐
	     fied) or otherwise the name by which tclsh or wish was invoked.

       tcl_interactive
	     Contains 1 if tclsh or wish is running interactively  (no	script
	     was  specified  and  standard input is a terminal-like device), 0
	     otherwise.

EXAMPLES
       To add a directory to the collection of locations searched  by  package
       require,	 e.g.,	because of some application-specific packages that are
       used, the auto_path variable needs to be updated:

	      lappend ::auto_path [file join [pwd] "theLibDir"]

       A simple though not very robust way to handle command line arguments of
       the  form  “-foo	 1  -bar 2” is to load them into an array having first
       loaded in the default settings:
	      array set arguments {-foo 0 -bar 0 -grill 0}
	      array set arguments $::argv
	      puts "foo is $arguments(-foo)"
	      puts "bar is $arguments(-bar)"
	      puts "grill is $arguments(-grill)"

       The argv0 global variable can be used (in  conjunction  with  the  info
       script  command)	 to determine whether the current script is being exe‐
       cuted as the main script or  loaded  as	a  library.   This  is	useful
       because	it  allows  a single script to be used as both a library and a
       demonstration of that library:

	      if {$::argv0 eq [info script]} {
		  # running as: tclsh example.tcl
	      } else {
		  package provide Example 1.0
	      }

SEE ALSO
       eval(n), library(n), tclsh(1), tkvars(n), wish(1)

KEYWORDS
       arithmetic, bytecode, compiler, error, environment,  POSIX,  precision,
       subprocess, user, variables

Tcl				      8.0			    tclvars(n)
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