1tclvars(n) Tcl Built-In Commands tclvars(n)
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8 argc, argv, argv0, auto_path, env, errorCode, errorInfo, tcl_interac‐
9 tive, tcl_library, tcl_nonwordchars, tcl_patchLevel, tcl_pkgPath,
10 tcl_platform, tcl_precision, tcl_rcFileName, tcl_traceCompile,
11 tcl_traceExec, tcl_wordchars, tcl_version - Variables used by Tcl
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15 The following global variables are created and managed automatically by
16 the Tcl library. Except where noted below, these variables should nor‐
17 mally be treated as read-only by application-specific code and by
18 users.
19 auto_path
21 If set, then it must contain a valid Tcl list giving directories
22 to search during auto-load operations (including for package
23 index files when using the default package unknown handler).
24 This variable is initialized during startup to contain, in
25 order: the directories listed in the TCLLIBPATH environment
26 variable, the directory named by the tcl_library global vari‐
27 able, the parent directory of tcl_library, the directories
28 listed in the tcl_pkgPath variable. Additional locations to
29 look for files and package indices should normally be added to
30 this variable using lappend.
31 Additional variables relating to package management exist. More
33 details are listed in the VARIABLES section of the library man‐
34 ual page.
35 env This variable is maintained by Tcl as an array whose elements
37 are the environment variables for the process. Reading an ele‐
38 ment will return the value of the corresponding environment
39 variable. Setting an element of the array will modify the cor‐
40 responding environment variable or create a new one if it does
41 not already exist. Unsetting an element of env will remove the
42 corresponding environment variable. Changes to the env array
43 will affect the environment passed to children by commands like
44 exec. If the entire env array is unset then Tcl will stop moni‐
45 toring env accesses and will not update environment variables.
46 Under Windows, the environment variables PATH and COMSPEC in any
48 capitalization are converted automatically to upper case. For
49 instance, the PATH variable could be exported by the operating
50 system as “path”, “Path”, “PaTh”, etc., causing otherwise simple
51 Tcl code to have to support many special cases. All other envi‐
52 ronment variables inherited by Tcl are left unmodified. Setting
53 an env array variable to blank is the same as unsetting it as
54 this is the behavior of the underlying Windows OS. It should be
55 noted that relying on an existing and empty environment variable
56 will not work on Windows and is discouraged for cross-platform
57 usage.
58 The following elements of env are special to Tcl:
60 env(HOME)
62 This environment variable, if set, gives the location of
63 the directory considered to be the current user's home
64 directory, and to which a call of cd without arguments or
65 with just “~” as an argument will change into. Most plat‐
66 forms set this correctly by default; it does not normally
67 need to be set by user code.
68 env(TCL_LIBRARY)
70 If set, then it specifies the location of the directory
71 containing library scripts (the value of this variable
72 will be assigned to the tcl_library variable and there‐
73 fore returned by the command info library). If this
74 variable is not set then a default value is used.
75 Note that this environment variable should not normally
77 be set.
78 env(TCLLIBPATH)
80 If set, then it must contain a valid Tcl list giving
81 directories to search during auto-load operations.
82 Directories must be specified in Tcl format, using “/” as
83 the path separator, regardless of platform. This vari‐
84 able is only used when initializing the auto_path vari‐
85 able.
86 env(TCL_TZ), env(TZ)
88 These specify the default timezone used for parsing and
89 formatting times and dates in the clock command. On many
90 platforms, the TZ environment variable is set up by the
91 operating system.
92 env(LC_ALL), env(LC_MESSAGES), env(LANG)
94 These environment variables are used by the msgcat pack‐
95 age to determine what locale to format messages using.
96 env(TCL_INTERP_DEBUG_FRAME)
98 If existing, it has the same effect as running interp
99 debug {} -frame 1 as the very first command of each new
100 Tcl interpreter.
101 errorCode
103 This variable holds the value of the -errorcode return option
104 set by the most recent error that occurred in this interpreter.
105 This list value represents additional information about the
106 error in a form that is easy to process with programs. The
107 first element of the list identifies a general class of errors,
108 and determines the format of the rest of the list. The follow‐
109 ing formats for -errorcode return options are used by the Tcl
110 core; individual applications may define additional formats.
111 ARITH code msg
113 This format is used when an arithmetic error occurs (e.g.
114 an attempt to divide zero by zero in the expr command).
115 Code identifies the precise error and msg provides a
116 human-readable description of the error. Code will be
117 either DIVZERO (for an attempt to divide by zero), DOMAIN
118 (if an argument is outside the domain of a function, such
119 as acos(-3)), IOVERFLOW (for integer overflow), OVERFLOW
120 (for a floating-point overflow), or UNKNOWN (if the cause
121 of the error cannot be determined).
122 Detection of these errors depends in part on the underly‐
124 ing hardware and system libraries.
125 CHILDKILLED pid sigName msg
127 This format is used when a child process has been killed
128 because of a signal. The pid element will be the
129 process's identifier (in decimal). The sigName element
130 will be the symbolic name of the signal that caused the
131 process to terminate; it will be one of the names from
132 the include file signal.h, such as SIGPIPE. The msg ele‐
133 ment will be a short human-readable message describing
134 the signal, such as “write on pipe with no readers” for
135 SIGPIPE.
136 CHILDSTATUS pid code
138 This format is used when a child process has exited with
139 a non-zero exit status. The pid element will be the
140 process's identifier (in decimal) and the code element
141 will be the exit code returned by the process (also in
142 decimal).
143 CHILDSUSP pid sigName msg
145 This format is used when a child process has been sus‐
146 pended because of a signal. The pid element will be the
147 process's identifier, in decimal. The sigName element
148 will be the symbolic name of the signal that caused the
149 process to suspend; this will be one of the names from
150 the include file signal.h, such as SIGTTIN. The msg ele‐
151 ment will be a short human-readable message describing
152 the signal, such as “background tty read” for SIGTTIN.
153 NONE This format is used for errors where no additional infor‐
155 mation is available for an error besides the message
156 returned with the error. In these cases the -errorcode
157 return option will consist of a list containing a single
158 element whose contents are NONE.
159 POSIX errName msg
161 If the first element is POSIX, then the error occurred
162 during a POSIX kernel call. The errName element will
163 contain the symbolic name of the error that occurred,
164 such as ENOENT; this will be one of the values defined in
165 the include file errno.h. The msg element will be a
166 human-readable message corresponding to errName, such as
167 “no such file or directory” for the ENOENT case.
168 TCL ...
170 Indicates some sort of problem generated in relation to
171 Tcl itself, e.g. a failure to look up a channel or vari‐
172 able.
173 To set the -errorcode return option, applications should use
175 library procedures such as Tcl_SetObjErrorCode, Tcl_SetReturnOp‐
176 tions, and Tcl_PosixError, or they may invoke the -errorcode
177 option of the return command. If none of these methods for set‐
178 ting the error code has been used, the Tcl interpreter will
179 reset the variable to NONE after the next error.
180 errorInfo
182 This variable holds the value of the -errorinfo return option
183 set by the most recent error that occurred in this interpreter.
184 This string value will contain one or more lines identifying the
185 Tcl commands and procedures that were being executed when the
186 most recent error occurred. Its contents take the form of a
187 stack trace showing the various nested Tcl commands that had
188 been invoked at the time of the error.
189 tcl_library
191 This variable holds the name of a directory containing the sys‐
192 tem library of Tcl scripts, such as those used for auto-loading.
193 The value of this variable is returned by the info library com‐
194 mand. See the library manual entry for details of the facili‐
195 ties provided by the Tcl script library. Normally each applica‐
196 tion or package will have its own application-specific script
197 library in addition to the Tcl script library; each application
198 should set a global variable with a name like $app_library
199 (where app is the application's name) to hold the network file
200 name for that application's library directory. The initial
201 value of tcl_library is set when an interpreter is created by
202 searching several different directories until one is found that
203 contains an appropriate Tcl startup script. If the TCL_LIBRARY
204 environment variable exists, then the directory it names is
205 checked first. If TCL_LIBRARY is not set or doesn't refer to an
206 appropriate directory, then Tcl checks several other directories
207 based on a compiled-in default location, the location of the
208 binary containing the application, and the current working
209 directory.
210 tcl_patchLevel
212 When an interpreter is created Tcl initializes this variable to
213 hold a string giving the current patch level for Tcl, such as
214 8.4.16 for Tcl 8.4 with the first sixteen official patches, or
215 8.5b3 for the third beta release of Tcl 8.5. The value of this
216 variable is returned by the info patchlevel command.
217 tcl_pkgPath
219 This variable holds a list of directories indicating where pack‐
220 ages are normally installed. It is not used on Windows. It
221 typically contains either one or two entries; if it contains two
222 entries, the first is normally a directory for platform-depen‐
223 dent packages (e.g., shared library binaries) and the second is
224 normally a directory for platform-independent packages (e.g.,
225 script files). Typically a package is installed as a subdirec‐
226 tory of one of the entries in the tcl_pkgPath variable. The
227 directories in the tcl_pkgPath variable are included by default
228 in the auto_path variable, so they and their immediate subdirec‐
229 tories are automatically searched for packages during package
230 require commands. Note: tcl_pkgPath is not intended to be modi‐
231 fied by the application. Its value is added to auto_path at
232 startup; changes to tcl_pkgPath are not reflected in auto_path.
233 If you want Tcl to search additional directories for packages
234 you should add the names of those directories to auto_path, not
235 tcl_pkgPath.
236 tcl_platform
238 This is an associative array whose elements contain information
239 about the platform on which the application is running, such as
240 the name of the operating system, its current release number,
241 and the machine's instruction set. The elements listed below
242 will always be defined, but they may have empty strings as val‐
243 ues if Tcl could not retrieve any relevant information. In
244 addition, extensions and applications may add additional values
245 to the array. The predefined elements are:
246 byteOrder
248 The native byte order of this machine: either littleEn‐
249 dian or bigEndian.
250 debug If this variable exists, then the interpreter was com‐
252 piled with and linked to a debug-enabled C run-time.
253 This variable will only exist on Windows, so extension
254 writers can specify which package to load depending on
255 the C run-time library that is in use. This is not an
256 indication that this core contains symbols.
257 engine The name of the Tcl language implementation. When the
259 interpreter is first created, this is always set to the
260 string Tcl.
261 machine
263 The instruction set executed by this machine, such as
264 intel, PPC, 68k, or sun4m. On UNIX machines, this is the
265 value returned by uname -m.
266 os The name of the operating system running on this machine,
268 such as Windows NT or SunOS. On UNIX machines, this is
269 the value returned by uname -s.
270 osVersion
272 The version number for the operating system running on
273 this machine. On UNIX machines, this is the value
274 returned by uname -r.
275 pathSeparator
277 The character that should be used to split PATH-like │
278 environment variables into their corresponding list of │
279 directory names.
280 platform
282 Either windows, or unix. This identifies the general
283 operating environment of the machine.
284 pointerSize
286 This gives the size of the native-machine pointer in
287 bytes (strictly, it is same as the result of evaluating
288 sizeof(void*) in C.)
289 threaded
291 If this variable exists, then the interpreter was com‐
292 piled with threads enabled.
293 user This identifies the current user based on the login
295 information available on the platform. This value comes
296 from the getuid() and getpwuid() system calls on Unix,
297 and the value from the GetUserName() system call on Win‐
298 dows.
299 wordSize
301 This gives the size of the native-machine word in bytes
302 (strictly, it is same as the result of evaluating
303 sizeof(long) in C.)
304 tcl_precision
306 This variable controls the number of digits to generate when
307 converting floating-point values to strings. It defaults to 0.
308 Applications should not change this value; it is provided for
309 compatibility with legacy code.
310 The default value of 0 is special, meaning that Tcl should con‐
312 vert numbers using as few digits as possible while still distin‐
313 guishing any floating point number from its nearest neighbours.
314 It differs from using an arbitrarily high value for tcl_preci‐
315 sion in that an inexact number like 1.4 will convert as 1.4
316 rather than 1.3999999999999999 even though the latter is nearer
317 to the exact value of the binary number.
318 If tcl_precision is not zero, then when Tcl converts a floating
320 point number, it creates a decimal representation of at most
321 tcl_precision significant digits; the result may be shorter if
322 the shorter result represents the original number exactly. If no
323 result of at most tcl_precision digits is an exact representa‐
324 tion of the original number, the one that is closest to the
325 original number is chosen. If the original number lies pre‐
326 cisely between two equally accurate decimal representations,
327 then the one with an even value for the least significant digit
328 is chosen; for instance, if tcl_precision is 3, then 0.3125 will
329 convert to 0.312, not 0.313, while 0.6875 will convert to 0.688,
330 not 0.687. Any string of trailing zeroes that remains is
331 trimmed.
332 a tcl_precision value of 17 digits is “perfect” for IEEE float‐
334 ing-point in that it allows double-precision values to be con‐
335 verted to strings and back to binary with no loss of informa‐
336 tion. For this reason, you will often see it as a value in
337 legacy code that must run on Tcl versions before 8.5. It is no
338 longer recommended; as noted above, a zero value is the pre‐
339 ferred method.
340 All interpreters in a thread share a single tcl_precision value:
342 changing it in one interpreter will affect all other inter‐
343 preters as well. Safe interpreters are not allowed to modify
344 the variable.
345 Valid values for tcl_precision range from 0 to 17.
347 tcl_rcFileName
349 This variable is used during initialization to indicate the name
350 of a user-specific startup file. If it is set by application-
351 specific initialization, then the Tcl startup code will check
352 for the existence of this file and source it if it exists. For
353 example, for wish the variable is set to ~/.wishrc for Unix and
354 ~/wishrc.tcl for Windows.
355 tcl_traceCompile
357 The value of this variable can be set to control how much trac‐
358 ing information is displayed during bytecode compilation. By
359 default, tcl_traceCompile is zero and no information is dis‐
360 played. Setting tcl_traceCompile to 1 generates a one-line sum‐
361 mary in stdout whenever a procedure or top-level command is com‐
362 piled. Setting it to 2 generates a detailed listing in stdout
363 of the bytecode instructions emitted during every compilation.
364 This variable is useful in tracking down suspected problems with
365 the Tcl compiler.
366 This variable and functionality only exist if TCL_COMPILE_DEBUG
368 was defined during Tcl's compilation.
369 tcl_traceExec
371 The value of this variable can be set to control how much trac‐
372 ing information is displayed during bytecode execution. By
373 default, tcl_traceExec is zero and no information is displayed.
374 Setting tcl_traceExec to 1 generates a one-line trace in stdout
375 on each call to a Tcl procedure. Setting it to 2 generates a
376 line of output whenever any Tcl command is invoked that contains
377 the name of the command and its arguments. Setting it to 3 pro‐
378 duces a detailed trace showing the result of executing each
379 bytecode instruction. Note that when tcl_traceExec is 2 or 3,
380 commands such as set and incr that have been entirely replaced
381 by a sequence of bytecode instructions are not shown. Setting
382 this variable is useful in tracking down suspected problems with
383 the bytecode compiler and interpreter.
384 This variable and functionality only exist if TCL_COMPILE_DEBUG
386 was defined during Tcl's compilation.
387 tcl_wordchars
389 The value of this variable is a regular expression that can be
390 set to control what are considered “word” characters, for
391 instances like selecting a word by double-clicking in text in
392 Tk. It is platform dependent. On Windows, it defaults to \S,
393 meaning anything but a Unicode space character. Otherwise it
394 defaults to \w, which is any Unicode word character (number,
395 letter, or underscore).
396 tcl_nonwordchars
398 The value of this variable is a regular expression that can be
399 set to control what are considered “non-word” characters, for
400 instances like selecting a word by double-clicking in text in
401 Tk. It is platform dependent. On Windows, it defaults to \s,
402 meaning any Unicode space character. Otherwise it defaults to
403 \W, which is anything but a Unicode word character (number, let‐
404 ter, or underscore).
405 tcl_version
407 When an interpreter is created Tcl initializes this variable to
408 hold the version number for this version of Tcl in the form x.y.
409 Changes to x represent major changes with probable incompatibil‐
410 ities and changes to y represent small enhancements and bug
411 fixes that retain backward compatibility. The value of this
412 variable is returned by the info tclversion command.
413
415 The following variables are only guaranteed to exist in tclsh and wish
416 executables; the Tcl library does not define them itself but many Tcl
417 environments do.
418 argc The number of arguments to tclsh or wish.
420 argv Tcl list of arguments to tclsh or wish.
422 argv0 The script that tclsh or wish started executing (if it was speci‐
424 fied) or otherwise the name by which tclsh or wish was invoked.
425 tcl_interactive
427 Contains 1 if tclsh or wish is running interactively (no script
428 was specified and standard input is a terminal-like device), 0
429 otherwise.
430
432 To add a directory to the collection of locations searched by package
433 require, e.g., because of some application-specific packages that are
434 used, the auto_path variable needs to be updated:
435 lappend ::auto_path [file join [pwd] "theLibDir"]
437 A simple though not very robust way to handle command line arguments of
439 the form “-foo 1 -bar 2” is to load them into an array having first
440 loaded in the default settings:
441 array set arguments {-foo 0 -bar 0 -grill 0}
442 array set arguments $::argv
443 puts "foo is $arguments(-foo)"
444 puts "bar is $arguments(-bar)"
445 puts "grill is $arguments(-grill)"
446 The argv0 global variable can be used (in conjunction with the info
448 script command) to determine whether the current script is being exe‐
449 cuted as the main script or loaded as a library. This is useful
450 because it allows a single script to be used as both a library and a
451 demonstration of that library:
452 if {$::argv0 eq [info script]} {
454 # running as: tclsh example.tcl
455 } else {
456 package provide Example 1.0
457 }
458
460 eval(n), library(n), tclsh(1), tkvars(n), wish(1)
461
463 arithmetic, bytecode, compiler, error, environment, POSIX, precision,
464 subprocess, user, variables
465Tcl 8.0 tclvars(n)