1exec(n) Tcl Built-In Commands exec(n)
2______________________________________________________________________________
6
8 exec - Invoke subprocesses
9
11 exec ?switches? arg ?arg ...?
12_________________________________________________________________
13
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 -keepnewline Retains a trailing newline in the pipeline's output. Nor‐
26 mally a trailing newline will be deleted.
27 -- Marks the end of switches. The argument following this
29 one will be treated as the first arg even if it starts
30 with a -.
31 If an arg (or pair of args) has one of the forms described below then
33 it is used by exec to control the flow of input and output among the
34 subprocess(es). Such arguments will not be passed to the subpro‐
35 cess(es). In forms such as ``< fileName'' fileName may either be in a
36 separate argument from ``<'' or in the same argument with no interven‐
37 ing space (i.e. ``<fileName'').
38 | Separates distinct commands in the pipeline. The stan‐
40 dard output of the preceding command will be piped into
41 the standard input of the next command.
42 |& Separates distinct commands in the pipeline. Both stan‐
44 dard output and standard error of the preceding command
45 will be piped into the standard input of the next com‐
46 mand. This form of redirection overrides forms such as
47 2> and >&.
48 < fileName The file named by fileName is opened and used as the
50 standard input for the first command in the pipeline.
51 <@ fileId FileId must be the identifier for an open file, such as
53 the return value from a previous call to open. It is
54 used as the standard input for the first command in the
55 pipeline. FileId must have been opened for reading.
56 << value Value is passed to the first command as its standard
58 input.
59 > fileName Standard output from the last command is redirected to
61 the file named fileName, overwriting its previous con‐
62 tents.
63 2> fileName Standard error from all commands in the pipeline is
65 redirected to the file named fileName, overwriting its
66 previous contents.
67 >& fileName Both standard output from the last command and standard
69 error from all commands are redirected to the file named
70 fileName, overwriting its previous contents.
71 >> fileName Standard output from the last command is redirected to
73 the file named fileName, appending to it rather than
74 overwriting it.
75 2>> fileName Standard error from all commands in the pipeline is
77 redirected to the file named fileName, appending to it
78 rather than overwriting it.
79 >>& fileName Both standard output from the last command and standard
81 error from all commands are redirected to the file named
82 fileName, appending to it rather than overwriting it.
83 >@ fileId FileId must be the identifier for an open file, such as
85 the return value from a previous call to open. Standard
86 output from the last command is redirected to fileId's
87 file, which must have been opened for writing.
88 2>@ fileId FileId must be the identifier for an open file, such as
90 the return value from a previous call to open. Standard
91 error from all commands in the pipeline is redirected to
92 fileId's file. The file must have been opened for writ‐
93 ing.
94 >&@ fileId FileId must be the identifier for an open file, such as
96 the return value from a previous call to open. Both
97 standard output from the last command and standard error
98 from all commands are redirected to fileId's file. The
99 file must have been opened for writing.
100 If standard output has not been redirected then the exec command
102 returns the standard output from the last command in the pipeline. If
103 any of the commands in the pipeline exit abnormally or are killed or
104 suspended, then exec will return an error and the error message will
105 include the pipeline's output followed by error messages describing the
106 abnormal terminations; the errorCode variable will contain additional
107 information about the last abnormal termination encountered. If any of
108 the commands writes to its standard error file and that standard error
109 isn't redirected, then exec will return an error; the error message
110 will include the pipeline's standard output, followed by messages about
111 abnormal terminations (if any), followed by the standard error output.
112 If the last character of the result or error message is a newline then
114 that character is normally deleted from the result or error message.
115 This is consistent with other Tcl return values, which don't normally
116 end with newlines. However, if -keepnewline is specified then the
117 trailing newline is retained.
118 If standard input isn't redirected with ``<'' or ``<<'' or ``<@'' then
120 the standard input for the first command in the pipeline is taken from
121 the application's current standard input.
122 If the last arg is ``&'' then the pipeline will be executed in back‐
124 ground. In this case the exec command will return a list whose ele‐
125 ments are the process identifiers for all of the subprocesses in the
126 pipeline. The standard output from the last command in the pipeline
127 will go to the application's standard output if it hasn't been redi‐
128 rected, and error output from all of the commands in the pipeline will
129 go to the application's standard error file unless redirected.
130 The first word in each command is taken as the command name; tilde-sub‐
132 stitution is performed on it, and if the result contains no slashes
133 then the directories in the PATH environment variable are searched for
134 an executable by the given name. If the name contains a slash then it
135 must refer to an executable reachable from the current directory. No
136 ``glob'' expansion or other shell-like substitutions are performed on
137 the arguments to commands.
138
141 Windows (all versions) │
142 Reading from or writing to a socket, using the ``@ fileId'' │
143 notation, does not work. When reading from a socket, a 16-bit │
144 DOS application will hang and a 32-bit application will return │
145 immediately with end-of-file. When either type of application │
146 writes to a socket, the information is instead sent to the con‐ │
147 sole, if one is present, or is discarded. │
148 The Tk console text widget does not provide real standard IO │
150 capabilities. Under Tk, when redirecting from standard input, │
151 all applications will see an immediate end-of-file; information │
152 redirected to standard output or standard error will be dis‐ │
153 carded. │
154 Either forward or backward slashes are accepted as path separa‐ │
156 tors for arguments to Tcl commands. When executing an applica‐ │
157 tion, the path name specified for the application may also con‐ │
158 tain forward or backward slashes as path separators. Bear in │
159 mind, however, that most Windows applications accept arguments │
160 with forward slashes only as option delimiters and backslashes │
161 only in paths. Any arguments to an application that specify a │
162 path name with forward slashes will not automatically be con‐ │
163 verted to use the backslash character. If an argument contains │
164 forward slashes as the path separator, it may or may not be rec‐ │
165 ognized as a path name, depending on the program. │
166 Additionally, when calling a 16-bit DOS or Windows 3.X applica‐ │
168 tion, all path names must use the short, cryptic, path format │
169 (e.g., using ``applba~1.def'' instead of ``applbak‐ │
170 ery.default''), which can be obtained with the file attributes │
171 $fileName -shortname command. │
172 Two or more forward or backward slashes in a row in a path refer │
174 to a network path. For example, a simple concatenation of the │
175 root directory c:/ with a subdirectory /windows/system will │
176 yield c://windows/system (two slashes together), which refers to │
177 the mount point called system on the machine called windows (and │
178 the c:/ is ignored), and is not equivalent to c:/windows/system, │
179 which describes a directory on the current computer. The file │
180 join command should be used to concatenate path components. │
181 Note that there are two general types of Win32 console applica‐ │
183 tions: │
184 1) CLI -- CommandLine Interface, simple stdio exchange. │
185 netstat.exe for example. │
186 2) TUI -- Textmode User Interface, any application that │
187 accesses the console API for doing such things as cursor │
188 movement, setting text color, detecting key presses and │
189 mouse movement, etc. An example would be telnet.exe from │
190 Windows 2000. These types of applications are not common │
191 in a windows environment, but do exist. │
192 exec will not work well with TUI applications when a console is │
193 not present, as is done when launching applications under wish. │
194 It is desirable to have console applications hidden and │
195 detached. This is a designed-in limitation as exec wants to │
196 communicate over pipes. The Expect extension addresses this │
197 issue when communicating with a TUI application. │
198 Windows NT │
201 When attempting to execute an application, exec first searches │
202 for the name as it was specified. Then, in order, .com, .exe, │
203 and .bat are appended to the end of the specified name and it │
204 searches for the longer name. If a directory name was not spec‐ │
205 ified as part of the application name, the following directories │
206 are automatically searched in order when attempting to locate │
207 the application: │
208 The directory from which the Tcl executable was loaded. │
210 The current directory. │
211 The Windows NT 32-bit system directory. │
212 The Windows NT 16-bit system directory. │
213 The Windows NT home directory. │
214 The directories listed in the path. │
215 In order to execute shell built-in commands like dir and copy, │
217 the caller must prepend the desired command with ``cmd.exe /c '' │
218 because built-in commands are not implemented using executables. │
219 Windows 9x │
222 When attempting to execute an application, exec first searches │
223 for the name as it was specified. Then, in order, .com, .exe, │
224 and .bat are appended to the end of the specified name and it │
225 searches for the longer name. If a directory name was not spec‐ │
226 ified as part of the application name, the following directories │
227 are automatically searched in order when attempting to locate │
228 the application: │
229 The directory from which the Tcl executable was loaded. │
231 The current directory. │
232 The Windows 9x system directory. │
233 The Windows 9x home directory. │
234 The directories listed in the path. │
235 In order to execute shell built-in commands like dir and copy, │
237 the caller must prepend the desired command with ``command.com │
238 /c '' because built-in commands are not implemented using exe‐ │
239 cutables. │
240 Once a 16-bit DOS application has read standard input from a │
242 console and then quit, all subsequently run 16-bit DOS applica‐ │
243 tions will see the standard input as already closed. 32-bit │
244 applications do not have this problem and will run correctly, │
245 even after a 16-bit DOS application thinks that standard input │
246 is closed. There is no known workaround for this bug at this │
247 time. │
248 Redirection between the NUL: device and a 16-bit application │
250 does not always work. When redirecting from NUL:, some applica‐ │
251 tions may hang, others will get an infinite stream of ``0x01'' │
252 bytes, and some will actually correctly get an immediate end-of- │
253 file; the behavior seems to depend upon something compiled into │
254 the application itself. When redirecting greater than 4K or so │
255 to NUL:, some applications will hang. The above problems do not │
256 happen with 32-bit applications. │
257 All DOS 16-bit applications are run synchronously. All standard │
259 input from a pipe to a 16-bit DOS application is collected into │
260 a temporary file; the other end of the pipe must be closed │
261 before the 16-bit DOS application begins executing. All stan‐ │
262 dard output or error from a 16-bit DOS application to a pipe is │
263 collected into temporary files; the application must terminate │
264 before the temporary files are redirected to the next stage of │
265 the pipeline. This is due to a workaround for a Windows 95 bug │
266 in the implementation of pipes, and is how the standard Windows │
267 95 DOS shell handles pipes itself. │
268 Certain applications, such as command.com, should not be exe‐ │
270 cuted interactively. Applications which directly access the │
271 console window, rather than reading from their standard input │
272 and writing to their standard output may fail, hang Tcl, or even │
273 hang the system if their own private console window is not │
274 available to them. │
275 Macintosh │
277 The exec command is not implemented and does not exist under │
278 Macintosh. │
279 Unix │
281 The exec command is fully functional and works as described. │
282
285 Here are some examples of the use of the exec command on Unix. │
286 To execute a simple program and get its result: │
288 exec uname -a │
289 To execute a program that can return a non-zero result, you should wrap │
291 the call to exec in catch and check what the contents of the global │
292 errorCode variable is if you have an error: │
293 set status 0 │
294 if {[catch {exec grep foo bar.txt} results]} { │
295 if {[lindex $::errorCode 0] eq "CHILDSTATUS"} { │
296 set status [lindex $::errorCode 2] │
297 } else { │
298 # Some kind of unexpected failure │
299 } │
300 } │
301 When translating a command from a Unix shell invocation, care should be │
303 taken over the fact that single quote characters have no special sig‐ │
304 nificance to Tcl. Thus: │
305 awk '{sum += $1} END {print sum}' numbers.list │
306 would be translated into something like: │
307 exec awk {{sum += $1} END {print sum}} numbers.list │
308 If you are converting invocations involving shell globbing, you should │
310 remember that Tcl does not handle globbing or expand things into multi‐ │
311 ple arguments by default. Instead you should write things like this: │
312 eval [list exec ls -l] [glob *.tcl] │
313
315 Here are some examples of the use of the exec command on Windows. │
316 To start an instance of notepad editing a file without waiting for the │
318 user to finish editing the file: │
319 exec notepad myfile.txt & │
320 To print a text file using notepad: │
322 exec notepad /p myfile.txt │
323 If a program calls other programs, such as is common with compilers, │
325 then you may need to resort to batch files to hide the console windows │
326 that sometimes pop up: │
327 exec cmp.bat somefile.c -o somefile │
328 With the file cmp.bat looking something like: │
329 @gcc %1 %2 %3 %4 %5 %6 %7 %8 %9 │
330 Sometimes you need to be careful, as different programs may have the │
332 same name and be in the path. It can then happen that typing a command │
333 at the DOS prompt finds a different program than the same command run │
334 via exec. This is because of the (documented) differences in behaviour │
335 between exec and DOS batch files. │
336 When in doubt, use the command auto_execok: it will return the complete │
338 path to the program as seen by the exec command. This applies espe‐ │
339 cially when you want to run "internal" commands like dir from a Tcl │
340 script (if you just want to list filenames, use the glob command.) To │
341 do that, use this: │
342 eval [list exec] [auto_execok dir] [list *.tcl] │
343
346 error(n), open(n) │
347
350 execute, pipeline, redirection, subprocess │
351Tcl 7.6 exec(n)