env

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



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NAME
       argc, argv, argv0, auto_path, env, errorCode, errorInfo, tcl_interactive,
       tcl_library, tcl_nonwordchars, tcl_patchLevel, tcl_pkgPath, tcl_platform,
       tcl_precision, tcl_rcFileName, tcl_traceCompile, tcl_traceExec,
       tcl_wordchars, tcl_version - Variables used by Tcl
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DESCRIPTION
       The following global variables are created and managed automatically by
       the Tcl library.  Except where noted below, these variables should
       normally 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 variable, 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 manual
              page.

       env    This variable is maintained by Tcl as an array whose elements are
              the environment variables for the process.  Reading an element
              will return the value of the corresponding environment variable.
              Setting an element of the array will modify the corresponding
              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 monitoring 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
              environment 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
                     platforms 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 therefore
                     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 variable
                     is only used when initializing the auto_path variable.

              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 package
                     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 following 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 underlying
                     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 element 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 suspended
                     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 element 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
                     information 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 variable.

              To set the -errorcode return option, applications should use
              library procedures such as Tcl_SetObjErrorCode,
              Tcl_SetReturnOptions, and Tcl_PosixError, or they may invoke the
              -errorcode option of the return command.  If none of these methods
              for setting 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 system
              library of Tcl scripts, such as those used for auto-loading.  The
              value of this variable is returned by the info library command.
              See the library manual entry for details of the facilities
              provided by the Tcl script library.  Normally each application 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
              packages 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-dependent
              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 subdirectory
              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 subdirectories are
              automatically searched for packages during package require
              commands.  Note: tcl_pkgPath is not intended to be modified 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 values 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 littleEndian
                     or bigEndian.

              debug  If this variable exists, then the interpreter was compiled
                     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.

              engine The name of the Tcl language implementation.  When the
                     interpreter is first created, this is always set to the
                     string Tcl.

              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 NT or SunOS.  On UNIX machines, this is the
                     value returned by uname -s.

              osVersion
                     The version number for the operating system running on this
                     machine.  On UNIX machines, this is the value returned by
                     uname -r.

              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 compiled
                     with threads enabled.

              user   This identifies the current user based on the login
                     information available on the platform.  This value comes
                     from the getuid() and getpwuid() system calls on Unix, and
                     the value from the GetUserName() system call 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 convert
              numbers using as few digits as possible while still distinguishing
              any floating point number from its nearest neighbours.  It differs
              from using an arbitrarily high value for tcl_precision 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 representation
              of the original number, the one that is closest to the original
              number is chosen.  If the original number lies precisely 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 floating-
              point in that it allows double-precision values to be converted to
              strings and back to binary with no loss of information. 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 preferred method.

              All interpreters in a thread share a single tcl_precision value:
              changing it in one interpreter will affect all other interpreters
              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 tracing
              information is displayed during bytecode compilation.  By default,
              tcl_traceCompile is zero and no information is displayed.  Setting
              tcl_traceCompile to 1 generates a one-line summary in stdout
              whenever a procedure or top-level command is compiled.  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 tracing
              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 produces 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, letter, 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
              incompatibilities 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
             specified) 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 executed
       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)