1exec(n) Tcl Built-In Commands exec(n)
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8 exec - Invoke subprocesses
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11 exec ?switches? arg ?arg ...?
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16 This command treats its arguments as the specification of one or more
17 subprocesses to execute. The arguments take the form of a standard
18 shell pipeline where each arg becomes one word of a command, and each
19 distinct command becomes a subprocess.
20 If the initial arguments to exec start with - then they are treated as
22 command-line switches and are not part of the pipeline specification.
23 The following switches are currently supported:
24 -ignorestderr
26 Stops the exec command from treating the output of mes‐ │
27 sages to the pipeline's standard error channel as an error │
28 case.
29 -keepnewline Retains a trailing newline in the pipeline's output. Nor‐
31 mally a trailing newline will be deleted.
32 -- Marks the end of switches. The argument following this
34 one will be treated as the first arg even if it starts
35 with a -.
36 If an arg (or pair of args) has one of the forms described below then
38 it is used by exec to control the flow of input and output among the
39 subprocess(es). Such arguments will not be passed to the subpro‐
40 cess(es). In forms such as “< fileName”, fileName may either be in a
41 separate argument from “<” or in the same argument with no intervening
42 space (i.e. “<fileName”).
43 | Separates distinct commands in the pipeline. The stan‐
45 dard output of the preceding command will be piped into
46 the standard input of the next command.
47 |& Separates distinct commands in the pipeline. Both stan‐
49 dard output and standard error of the preceding command
50 will be piped into the standard input of the next com‐
51 mand. This form of redirection overrides forms such as
52 2> and >&.
53 < fileName The file named by fileName is opened and used as the
55 standard input for the first command in the pipeline.
56 <@ fileId FileId must be the identifier for an open file, such as
58 the return value from a previous call to open. It is
59 used as the standard input for the first command in the
60 pipeline. FileId must have been opened for reading.
61 << value Value is passed to the first command as its standard
63 input.
64 > fileName Standard output from the last command is redirected to
66 the file named fileName, overwriting its previous con‐
67 tents.
68 2> fileName Standard error from all commands in the pipeline is
70 redirected to the file named fileName, overwriting its
71 previous contents.
72 >& fileName Both standard output from the last command and standard
74 error from all commands are redirected to the file named
75 fileName, overwriting its previous contents.
76 >> fileName Standard output from the last command is redirected to
78 the file named fileName, appending to it rather than
79 overwriting it.
80 2>> fileName Standard error from all commands in the pipeline is
82 redirected to the file named fileName, appending to it
83 rather than overwriting it.
84 >>& fileName Both standard output from the last command and standard
86 error from all commands are redirected to the file named
87 fileName, appending to it rather than overwriting it.
88 >@ fileId FileId must be the identifier for an open file, such as
90 the return value from a previous call to open. Standard
91 output from the last command is redirected to fileId's
92 file, which must have been opened for writing.
93 2>@ fileId FileId must be the identifier for an open file, such as
95 the return value from a previous call to open. Standard
96 error from all commands in the pipeline is redirected to
97 fileId's file. The file must have been opened for writ‐
98 ing.
99 2>@1 Standard error from all commands in the pipeline is
101 redirected to the command result. This operator is only
102 valid at the end of the command pipeline.
103 >&@ fileId FileId must be the identifier for an open file, such as
105 the return value from a previous call to open. Both
106 standard output from the last command and standard error
107 from all commands are redirected to fileId's file. The
108 file must have been opened for writing.
109 If standard output has not been redirected then the exec command
111 returns the standard output from the last command in the pipeline, │
112 unless “2>@1” was specified, in which case standard error is included │
113 as well. If any of the commands in the pipeline exit abnormally or are
114 killed or suspended, then exec will return an error and the error mes‐
115 sage will include the pipeline's output followed by error messages
116 describing the abnormal terminations; the -errorcode return option will
117 contain additional information about the last abnormal termination
118 encountered. If any of the commands writes to its standard error file
119 and that standard error is not redirected and -ignorestderr is not │
120 specified, then exec will return an error; the error message will
121 include the pipeline's standard output, followed by messages about
122 abnormal terminations (if any), followed by the standard error output.
123 If the last character of the result or error message is a newline then
125 that character is normally deleted from the result or error message.
126 This is consistent with other Tcl return values, which do not normally
127 end with newlines. However, if -keepnewline is specified then the
128 trailing newline is retained.
129 If standard input is not redirected with “<”, “<<” or “<@” then the
131 standard input for the first command in the pipeline is taken from the
132 application's current standard input.
133 If the last arg is “&” then the pipeline will be executed in back‐
135 ground. In this case the exec command will return a list whose ele‐
136 ments are the process identifiers for all of the subprocesses in the
137 pipeline. The standard output from the last command in the pipeline
138 will go to the application's standard output if it has not been redi‐
139 rected, and error output from all of the commands in the pipeline will
140 go to the application's standard error file unless redirected.
141 The first word in each command is taken as the command name; tilde-sub‐
143 stitution is performed on it, and if the result contains no slashes
144 then the directories in the PATH environment variable are searched for
145 an executable by the given name. If the name contains a slash then it
146 must refer to an executable reachable from the current directory. No
147 “glob” expansion or other shell-like substitutions are performed on the
148 arguments to commands.
149
151 Windows (all versions)
152 Reading from or writing to a socket, using the “@ fileId” nota‐
153 tion, does not work. When reading from a socket, a 16-bit DOS
154 application will hang and a 32-bit application will return imme‐
155 diately with end-of-file. When either type of application
156 writes to a socket, the information is instead sent to the con‐
157 sole, if one is present, or is discarded.
158 The Tk console text widget does not provide real standard IO
160 capabilities. Under Tk, when redirecting from standard input,
161 all applications will see an immediate end-of-file; information
162 redirected to standard output or standard error will be dis‐
163 carded.
164 Either forward or backward slashes are accepted as path separa‐
166 tors for arguments to Tcl commands. When executing an applica‐
167 tion, the path name specified for the application may also con‐
168 tain forward or backward slashes as path separators. Bear in
169 mind, however, that most Windows applications accept arguments
170 with forward slashes only as option delimiters and backslashes
171 only in paths. Any arguments to an application that specify a
172 path name with forward slashes will not automatically be con‐
173 verted to use the backslash character. If an argument contains
174 forward slashes as the path separator, it may or may not be rec‐
175 ognized as a path name, depending on the program.
176 Additionally, when calling a 16-bit DOS or Windows 3.X applica‐
178 tion, all path names must use the short, cryptic, path format
179 (e.g., using “applba~1.def” instead of “applbakery.default”),
180 which can be obtained with the “file attributes fileName -short‐
181 name” command.
182 Two or more forward or backward slashes in a row in a path refer
184 to a network path. For example, a simple concatenation of the
185 root directory c:/ with a subdirectory /windows/system will
186 yield c://windows/system (two slashes together), which refers to
187 the mount point called system on the machine called windows (and
188 the c:/ is ignored), and is not equivalent to c:/windows/system,
189 which describes a directory on the current computer. The file
190 join command should be used to concatenate path components.
191 Note that there are two general types of Win32 console applica‐
193 tions:
194 [1] CLI — CommandLine Interface, simple stdio
196 exchange. netstat.exe for example.
197 [2] TUI — Textmode User Interface, any application
199 that accesses the console API for doing such
200 things as cursor movement, setting text color,
201 detecting key presses and mouse movement, etc. An
202 example would be telnet.exe from Windows 2000.
203 These types of applications are not common in a
204 windows environment, but do exist.
205 exec will not work well with TUI applications when a console is
207 not present, as is done when launching applications under wish.
208 It is desirable to have console applications hidden and
209 detached. This is a designed-in limitation as exec wants to
210 communicate over pipes. The Expect extension addresses this
211 issue when communicating with a TUI application.
212 Windows NT
214 When attempting to execute an application, exec first searches
215 for the name as it was specified. Then, in order, .com, .exe,
216 and .bat are appended to the end of the specified name and it
217 searches for the longer name. If a directory name was not spec‐
218 ified as part of the application name, the following directories
219 are automatically searched in order when attempting to locate
220 the application:
221 · The directory from which the Tcl executable was loaded.
223 · The current directory.
225 · The Windows NT 32-bit system directory.
227 · The Windows NT 16-bit system directory.
229 · The Windows NT home directory.
231 · The directories listed in the path.
233 In order to execute shell built-in commands like dir and copy,
235 the caller must prepend the desired command with “cmd.exe /c ”
236 because built-in commands are not implemented using executables.
237 Windows 9x
239 When attempting to execute an application, exec first searches
240 for the name as it was specified. Then, in order, .com, .exe,
241 and .bat are appended to the end of the specified name and it
242 searches for the longer name. If a directory name was not spec‐
243 ified as part of the application name, the following directories
244 are automatically searched in order when attempting to locate
245 the application:
246 · The directory from which the Tcl executable was loaded.
248 · The current directory.
250 · The Windows 9x system directory.
252 · The Windows 9x home directory.
254 · The directories listed in the path.
256 In order to execute shell built-in commands like dir and copy,
258 the caller must prepend the desired command with “command.com
259 /c ” because built-in commands are not implemented using exe‐
260 cutables.
261 Once a 16-bit DOS application has read standard input from a
263 console and then quit, all subsequently run 16-bit DOS applica‐
264 tions will see the standard input as already closed. 32-bit
265 applications do not have this problem and will run correctly,
266 even after a 16-bit DOS application thinks that standard input
267 is closed. There is no known workaround for this bug at this
268 time.
269 Redirection between the NUL: device and a 16-bit application
271 does not always work. When redirecting from NUL:, some applica‐
272 tions may hang, others will get an infinite stream of “0x01”
273 bytes, and some will actually correctly get an immediate end-of-
274 file; the behavior seems to depend upon something compiled into
275 the application itself. When redirecting greater than 4K or so
276 to NUL:, some applications will hang. The above problems do not
277 happen with 32-bit applications.
278 All DOS 16-bit applications are run synchronously. All standard
280 input from a pipe to a 16-bit DOS application is collected into
281 a temporary file; the other end of the pipe must be closed
282 before the 16-bit DOS application begins executing. All stan‐
283 dard output or error from a 16-bit DOS application to a pipe is
284 collected into temporary files; the application must terminate
285 before the temporary files are redirected to the next stage of
286 the pipeline. This is due to a workaround for a Windows 95 bug
287 in the implementation of pipes, and is how the standard Windows
288 95 DOS shell handles pipes itself.
289 Certain applications, such as command.com, should not be exe‐
291 cuted interactively. Applications which directly access the
292 console window, rather than reading from their standard input
293 and writing to their standard output may fail, hang Tcl, or even
294 hang the system if their own private console window is not
295 available to them.
296 Unix
298 The exec command is fully functional and works as described.
299
301 Here are some examples of the use of the exec command on Unix.
302 To execute a simple program and get its result:
304 exec uname -a
305 To execute a program that can return a non-zero result, you should wrap
307 the call to exec in catch and check the contents of the -errorcode
308 return option if you have an error:
309 set status 0
310 if {[catch {exec grep foo bar.txt} results options]} {
311 set details [dict get $options -errorcode]
312 if {[lindex $details 0] eq "CHILDSTATUS"} {
313 set status [lindex $details 2]
314 } else {
315 # Some kind of unexpected failure
316 }
317 }
318 When translating a command from a Unix shell invocation, care should be
320 taken over the fact that single quote characters have no special sig‐
321 nificance to Tcl. Thus:
322 awk '{sum += $1} END {print sum}' numbers.list
323 would be translated into something like:
324 exec awk {{sum += $1} END {print sum}} numbers.list
325 If you are converting invocations involving shell globbing, you should
327 remember that Tcl does not handle globbing or expand things into multi‐
328 ple arguments by default. Instead you should write things like this:
329 exec ls -l {*}[glob *.tcl]
330
332 Here are some examples of the use of the exec command on Windows.
333 To start an instance of notepad editing a file without waiting for the
335 user to finish editing the file:
336 exec notepad myfile.txt &
337 To print a text file using notepad:
339 exec notepad /p myfile.txt
340 If a program calls other programs, such as is common with compilers,
342 then you may need to resort to batch files to hide the console windows
343 that sometimes pop up:
344 exec cmp.bat somefile.c -o somefile
345 With the file cmp.bat looking something like:
346 @gcc %1 %2 %3 %4 %5 %6 %7 %8 %9
347 Sometimes you need to be careful, as different programs may have the
349 same name and be in the path. It can then happen that typing a command
350 at the DOS prompt finds a different program than the same command run
351 via exec. This is because of the (documented) differences in behaviour
352 between exec and DOS batch files.
353 When in doubt, use the command auto_execok: it will return the complete
355 path to the program as seen by the exec command. This applies espe‐
356 cially when you want to run “internal” commands like dir from a Tcl
357 script (if you just want to list filenames, use the glob command.) To
358 do that, use this:
359 exec {*}[auto_execok dir] *.tcl
360
362 error(n), open(n)
363
365 execute, pipeline, redirection, subprocess
366Tcl 8.5 exec(n)