exec (n)
Invoke subprocess(es)
SYNOPSIS
exec ?switches? arg ?arg ...?
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 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 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 >&.
<\0fileName
The file named by fileName is opened and used as the standard
input for the first command in the pipeline.
<@\0fileId
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.
<<\0value
Value is passed to the first command as its standard input.
>\0fileName
Standard output from the last command is redirected to the file named
fileName, overwriting its previous contents.
2>\0fileName
Standard error from all commands in the pipeline is redirected to the
file named fileName, overwriting its previous contents.
>&\0fileName
Both standard output from the last command and standard error from all
commands are redirected to the file named fileName, overwriting
its previous contents.
>>\0fileName
Standard output from the last command is
redirected to the file named fileName, appending to it rather
than overwriting it.
2>>\0fileName
Standard error from all commands in the pipeline is
redirected to the file named fileName, appending to it rather
than overwriting it.
>>&\0fileName
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.
>@\0fileId
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>@\0fileId
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.
>&@\0fileId
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 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 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 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 substitutions
are performed on the arguments to commands.
PORTABILITY ISSUES
Windows (all versions)
Reading from or writing to a socket, using the ``@\0fileId''
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
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'').
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
directory /system on the machine windows (and the c:/ is
ignored), and is not equivalent to c:/windows/system, which describes
a directory on the current computer.
Windows NT
Windows 95
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 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\0'' to the desired command.
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 Windows 95 command line interpreter
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.
Windows 3.X
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 automatically searched in
order when attempting to locate the application:
The directory from which the Tcl executable was loaded.
The current directory.
The Windows 3.X system directory.
The Windows 3.X 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\0'' to the desired command.
16-bit and 32-bit DOS and Windows applications may be executed. However,
redirection and piping of standard IO only works with 16-bit DOS
applications. 32-bit applications always see standard input as already
closed, and any standard output or error is discarded, no matter where in the
pipeline the application occurs or what redirection symbols are used by the
caller. Additionally, for 16-bit applications, standard error is always
sent to the same place as standard output; it cannot be redirected to a
separate location. In order to achieve pseudo-redirection for 32-bit
applications, the 32-bit application must instead be written to take command
line arguments that specify the files that it should read from and write to
and open those files itself.
All applications, both 16-bit and 32-bit, run synchronously; each application
runs to completion before the next one in the pipeline starts. Temporary files
are used to simulate piping between applications. The exec
command cannot be used to start an application in the background.
When standard input is redirected from an open file using the
``@\0fileId'' notation, the open file is completely read up to its
end. This is slightly different than under Windows 95 or NT, where the child
application consumes from the open file only as much as it wants.
Redirecting to an open file is supported as normal.
Macintosh
The exec command is not implemented and does not exist under Macintosh.
Unix\0\0\0\0\0\0\0
The exec command is fully functional and works as described.
SEE ALSO
KEYWORDS
execute, pipeline, redirection, subprocess
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