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exec(n) Tcl Built-In Commands exec(n)
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NAME
exec - Invoke subprocesses
SYNOPSIS
exec ?switches? arg ?arg ...?
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DESCRIPTION
This command treats its arguments as the specification of one or more subprocesses to exe-
cute. The arguments take the form of a standard shell pipeline where each arg becomes one
word of a command, and each distinct command becomes a subprocess.
If the initial arguments to exec start with - then they are treated as command-line
switches and are not part of the pipeline specification. The following switches are cur-
rently supported:
-ignorestderr
Stops the exec command from treating the output of messages to the pipeline's |
standard error channel as an error case.
-keepnewline Retains a trailing newline in the pipeline's output. Normally a trailing
newline will be deleted.
-- Marks the end of switches. The argument following this one will be treated
as the first arg even if it starts with a -.
If an arg (or pair of args) has one of the forms described below then it is used by exec
to control the flow of input and output among the subprocess(es). Such arguments will not
be passed to the subprocess(es). In forms such as "< fileName", fileName may either be in
a separate argument from "<" or in the same argument with no intervening space (i.e.
"<fileName").
| Separates distinct commands in the pipeline. The standard output of the
preceding command will be piped into the standard input of the next com-
mand.
|& Separates distinct commands in the pipeline. Both standard output and
standard error of the preceding command will be piped into the standard
input of the next command. This form of redirection overrides forms such
as 2> and >&.
< fileName The file named by fileName is opened and used as the standard input for the
first command in the pipeline.
<@ fileId FileId must be the identifier for an open file, such as the return value
from a previous call to open. It is used as the standard input for the
first command in the pipeline. FileId must have been opened for reading.
<< value Value is passed to the first command as its standard input.
> fileName Standard output from the last command is redirected to the file named file-
Name, overwriting its previous contents.
2> fileName Standard error from all commands in the pipeline is redirected to the file
named fileName, overwriting its previous contents.
>& fileName Both standard output from the last command and standard error from all com-
mands are redirected to the file named fileName, overwriting its previous
contents.
>> fileName Standard output from the last command is redirected to the file named file-
Name, appending to it rather than overwriting it.
2>> fileName Standard error from all commands in the pipeline is redirected to the file
named fileName, appending to it rather than overwriting it.
>>& fileName Both standard output from the last command and standard error from all com-
mands are redirected to the file named fileName, appending to it rather
than overwriting it.
>@ fileId FileId must be the identifier for an open file, such as the return value
from a previous call to open. Standard output from the last command is
redirected to fileId's file, which must have been opened for writing.
2>@ fileId FileId must be the identifier for an open file, such as the return value
from a previous call to open. Standard error from all commands in the
pipeline is redirected to fileId's file. The file must have been opened
for writing.
2>@1 Standard error from all commands in the pipeline is redirected to the com-
mand result. This operator is only valid at the end of the command pipe-
line.
>&@ fileId FileId must be the identifier for an open file, such as the return value
from a previous call to open. Both standard output from the last command
and standard error from all commands are redirected to fileId's file. The
file must have been opened for writing.
If standard output has not been redirected then the exec command returns the standard out-
put from the last command in the pipeline, unless "2>@1" was specified, in which case |
standard error is included as well. If any of the commands in the pipeline exit abnor-
mally or are killed or suspended, then exec will return an error and the error message
will include the pipeline's output followed by error messages describing the abnormal ter-
minations; the -errorcode return option will contain additional information about the last
abnormal termination encountered. If any of the commands writes to its standard error
file and that standard error is not redirected and -ignorestderr is not specified, then |
exec will return an error; the error message will include the pipeline's standard output,
followed by messages about abnormal terminations (if any), followed by the standard error
output.
If the last character of the result or error message is a newline then that character is
normally deleted from the result or error message. This is consistent with other Tcl
return values, which do not normally end with newlines. However, if -keepnewline is spec-
ified then the trailing newline is retained.
If standard input is not redirected with "<", "<<" or "<@" then the standard input for the
first command in the pipeline is taken from the application's current standard input.
If the last arg is "&" then the pipeline will be executed in background. In this case the
exec command will return a list whose elements are the process identifiers for all of the
subprocesses in the pipeline. The standard output from the last command in the pipeline
will go to the application's standard output if it has not been redirected, and error out-
put from all of the commands in the pipeline will go to the application's standard error
file unless redirected.
The first word in each command is taken as the command name; tilde-substitution is per-
formed on it, and if the result contains no slashes then the directories in the PATH envi-
ronment variable are searched for an executable by the given name. If the name contains a
slash then it must refer to an executable reachable from the current directory. No "glob"
expansion or other shell-like substitutions are performed on the arguments to commands.
PORTABILITY ISSUES
Windows (all versions)
Reading from or writing to a socket, using the "@ fileId" notation, does not work.
When reading from a socket, a 16-bit DOS application will hang and a 32-bit appli-
cation will return immediately with end-of-file. When either type of application
writes to a socket, the information is instead sent to the console, if one is
present, or is discarded.
The Tk console text widget does not provide real standard IO capabilities. Under
Tk, when redirecting from standard input, all applications will see an immediate
end-of-file; information redirected to standard output or standard error will be
discarded.
Either forward or backward slashes are accepted as path separators for arguments to
Tcl commands. When executing an application, the path name specified for the
application may also contain forward or backward slashes as path separators. Bear
in mind, however, that most Windows applications accept arguments with forward
slashes only as option delimiters and backslashes only in paths. Any arguments to
an application that specify a path name with forward slashes will not automatically
be converted to use the backslash character. If an argument contains forward
slashes as the path separator, it may or may not be recognized as a path name,
depending on the program.
Additionally, when calling a 16-bit DOS or Windows 3.X application, all path names
must use the short, cryptic, path format (e.g., using "applba~1.def" instead of
"applbakery.default"), which can be obtained with the "file attributes fileName
-shortname" command.
Two or more forward or backward slashes in a row in a path refer to a network path.
For example, a simple concatenation of the root directory c:/ with a subdirectory
/windows/system will yield c://windows/system (two slashes together), which refers
to the mount point called system on the machine called windows (and the c:/ is
ignored), and is not equivalent to c:/windows/system, which describes a directory
on the current computer. The file join command should be used to concatenate path
components.
Note that there are two general types of Win32 console applications:
[1] CLI -- CommandLine Interface, simple stdio exchange. netstat.exe for
example.
[2] TUI -- Textmode User Interface, any application that accesses the
console API for doing such things as cursor movement, setting text
color, detecting key presses and mouse movement, etc. An example
would be telnet.exe from Windows 2000. These types of applications
are not common in a windows environment, but do exist.
exec will not work well with TUI applications when a console is not present, as is
done when launching applications under wish. It is desirable to have console
applications hidden and detached. This is a designed-in limitation as exec wants
to communicate over pipes. The Expect extension addresses this issue when communi-
cating with a TUI application.
Windows NT
When attempting to execute an application, exec first searches for the name as it
was specified. Then, in order, .com, .exe, and .bat are appended to the end of the
specified name and it searches for the longer name. If a directory name was not
specified as part of the application name, the following directories are automati-
cally searched in order when attempting to locate the application:
o The directory from which the Tcl executable was loaded.
o The current directory.
o The Windows NT 32-bit system directory.
o The Windows NT 16-bit system directory.
o The Windows NT home directory.
o The directories listed in the path.
In order to execute shell built-in commands like dir and copy, the caller must
prepend the desired command with "cmd.exe /c " because built-in commands are not
implemented using executables.
Windows 9x
When attempting to execute an application, exec first searches for the name as it
was specified. Then, in order, .com, .exe, and .bat are appended to the end of the
specified name and it searches for the longer name. If a directory name was not
specified as part of the application name, the following directories are automati-
cally searched in order when attempting to locate the application:
o The directory from which the Tcl executable was loaded.
o The current directory.
o The Windows 9x system directory.
o The Windows 9x home directory.
o The directories listed in the path.
In order to execute shell built-in commands like dir and copy, the caller must
prepend the desired command with "command.com /c " because built-in commands are
not implemented using executables.
Once a 16-bit DOS application has read standard input from a console and then quit,
all subsequently run 16-bit DOS applications will see the standard input as already
closed. 32-bit applications do not have this problem and will run correctly, even
after a 16-bit DOS application thinks that standard input is closed. There is no
known workaround for this bug at this time.
Redirection between the NUL: device and a 16-bit application does not always work.
When redirecting from NUL:, some applications may hang, others will get an infinite
stream of "0x01" bytes, and some will actually correctly get an immediate end-of-
file; the behavior seems to depend upon something compiled into the application
itself. When redirecting greater than 4K or so to NUL:, some applications will
hang. The above problems do not happen with 32-bit applications.
All DOS 16-bit applications are run synchronously. All standard input from a pipe
to a 16-bit DOS application is collected into a temporary file; the other end of
the pipe must be closed before the 16-bit DOS application begins executing. All
standard output or error from a 16-bit DOS application to a pipe is collected into
temporary files; the application must terminate before the temporary files are
redirected to the next stage of the pipeline. This is due to a workaround for a
Windows 95 bug in the implementation of pipes, and is how the standard Windows 95
DOS shell handles pipes itself.
Certain applications, such as command.com, should not be executed interactively.
Applications which directly access the console window, rather than reading from
their standard input and writing to their standard output may fail, hang Tcl, or
even hang the system if their own private console window is not available to them.
Unix
The exec command is fully functional and works as described.
UNIX EXAMPLES
Here are some examples of the use of the exec command on Unix.
To execute a simple program and get its result:
exec uname -a
To execute a program that can return a non-zero result, you should wrap the call to exec
in catch and check the contents of the -errorcode return option if you have an error:
set status 0
if {[catch {exec grep foo bar.txt} results options]} {
set details [dict get $options -errorcode]
if {[lindex $details 0] eq "CHILDSTATUS"} {
set status [lindex $details 2]
} else {
# Some kind of unexpected failure
}
}
When translating a command from a Unix shell invocation, care should be taken over the
fact that single quote characters have no special significance to Tcl. Thus:
awk '{sum += $1} END {print sum}' numbers.list
would be translated into something like:
exec awk {{sum += $1} END {print sum}} numbers.list
If you are converting invocations involving shell globbing, you should remember that Tcl
does not handle globbing or expand things into multiple arguments by default. Instead you
should write things like this:
exec ls -l {*}[glob *.tcl]
WINDOWS EXAMPLES
Here are some examples of the use of the exec command on Windows.
To start an instance of notepad editing a file without waiting for the user to finish
editing the file:
exec notepad myfile.txt &
To print a text file using notepad:
exec notepad /p myfile.txt
If a program calls other programs, such as is common with compilers, then you may need to
resort to batch files to hide the console windows that sometimes pop up:
exec cmp.bat somefile.c -o somefile
With the file cmp.bat looking something like:
@gcc %1 %2 %3 %4 %5 %6 %7 %8 %9
Sometimes you need to be careful, as different programs may have the same name and be in
the path. It can then happen that typing a command at the DOS prompt finds a different
program than the same command run via exec. This is because of the (documented) differ-
ences in behaviour between exec and DOS batch files.
When in doubt, use the command auto_execok: it will return the complete path to the pro-
gram as seen by the exec command. This applies especially when you want to run "internal"
commands like dir from a Tcl script (if you just want to list filenames, use the glob com-
mand.) To do that, use this:
exec {*}[auto_execok dir] *.tcl
SEE ALSO
error(n), open(n)
KEYWORDS
execute, pipeline, redirection, subprocess
Tcl 8.5 exec(n)
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Under GNU General Public License
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