exec(n) Tcl Built-In Commands exec(n)
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NAME
exec - Invoke subprocess(es)
SYNOPSIS
exec ?switches? arg ?arg ...?
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DESCRIPTION
This command treats its arguments as the specification of one or more subprocesses to execute. 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 currently supported:
-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 arg's) 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 sub-process(es). subprocess(es).
process(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 command.
|& 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 com-
mand in the pipeline.
<@ fileId FileId must be the identifier for an open file, such as the return value from a previ-
ous call to open. It is used as the standard input for the first command in the pipe-line. pipeline.
line. 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 fileName, over-writing overwriting
writing its previous contents.
2> fileName Standard error from all commands in the pipeline is redirected to the file named file-Name, fileName,
Name, overwriting its previous contents.
>& fileName Both standard output from the last command and standard error from all commands are
redirected to the file named fileName, overwriting its previous contents.
>> fileName Standard output from the last command is redirected to the file named fileName,
appending to it rather than overwriting it.
2>> fileName Standard error from all commands in the pipeline is redirected to the file named file-Name, fileName,
Name, appending to it rather than overwriting it.
>>& fileName Both standard output from the last command and standard error from all commands 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 previ-ous previous
ous 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 previ-ous previous
ous 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.
>&@ fileId FileId must be the identifier for an open file, such as the return value from a previ-ous previous
ous 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 output from the
last command in the pipeline. If any of the commands in the pipeline exit abnormally 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 terminations; the errorCode variable 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 isn't redirected, 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
don't normally end with newlines. However, if -keepnewline is specified then the trailing newline is
retained.
If standard input isn't redirected with ``<'' or ``<<'' 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 hasn't been redirected, and error output from all of the commands in the pipe-
line 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 performed on it,
and if the result contains no slashes then the directories in the PATH environment 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 substitu-tions substitutions
tions are performed on the arguments to commands.
PORTABILITY ISSUES |
Windows (all ver- |
sions) | |
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 application will return |
immediately with end-of-file. When either type of application writes to a socket, the infor- |
mation 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; informa- |
tion redirected to standard output or standard error will be discarded. |
Either forward or backward slashes are accepted as path separators for arguments to Tcl com- |
mands. 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 Win- |
dows applications accept arguments with forward slashes only as option delimiters and back- |
slashes 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 ``applbak- |
ery.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 exam- |
ple, 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:/win- |
dows/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 communication between a TUI application is desired. |
Windows |
NT | |
When attempting to execute an application, exec first searches for the name as it was speci- |
fied. 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 appli- |
cation name, the following directories are automatically searched in order when attempting to |
locate the application: |
The directory from which the Tcl executable was loaded. |
The current directory. |
The Windows NT 32-bit system directory. |
The Windows NT 16-bit system directory. |
The Windows NT home directory. |
The directories listed in the path. |
In order to execute the shell builtin commands like dir and copy, the caller must prepend |
``cmd.exe /c '' to the desired command. |
Windows |
95 | |
When attempting to execute an application, exec first searches for the name as it was speci- |
fied. 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 appli- |
cation name, the following directories are automatically searched in order when attempting to |
locate the application: |
The directory from which the Tcl executable was loaded. |
The current directory. |
The Windows 95 system directory. |
The Windows 95 home directory. |
The directories listed in the path. |
In order to execute the shell builtin commands like dir and copy, the caller must prepend |
``command.com /c '' to the desired command. |
Once a 16-bit DOS application has read standard input from a console and then quit, all subse- |
quently 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 applica- |
tion 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 redi- |
recting 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 ter- |
minate 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 stan- |
dard 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 pri- |
vate console window is not available to them. |
Macin- |
tosh | |
The exec command is not implemented and does not exist under Macintosh. |
Unix ||
The exec command is fully functional and works as described. |
SEE ALSO |
error(n), open(n) |
KEYWORDS |
execute, pipeline, redirection, subprocess |
Tcl 7.6 exec(n)
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