exec(n) Tcl Built-In Commands exec(n)_________________________________________________________________NAME
exec - 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 fol-
lowing 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 pre-
ceding command will be piped into the stan-
dard input of the next command.
|& Separates distinct commands in the
pipeline. Both standard output and stan-
dard error of the preceding command will be
piped into the standard input of the next
command. This form of redirection over-
rides 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.
Tcl 7.6 1
exec(n) Tcl Built-In Commands exec(n)
<@ fileId FileId must be the identifier for an open
file, such as the return value from a pre-
vious 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 fileName,
overwriting its previous contents.
2> fileName Standard error from all commands in the
pipeline is redirected to the file named
fileName, overwriting its previous con-
tents.
>& 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
fileName, appending to it rather than over-
writing 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 pre-
vious call to open. Standard output from
the last command is redirected to fileId's
file, which must have been opened for writ-
ing.
2>@ fileId FileId must be the identifier for an open
file, such as the return value from a pre-
vious 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
Tcl 7.6 2
exec(n) Tcl Built-In Commands exec(n)
previous call to open. Both standard out-
put 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, fol-
lowed 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 stan-
dard input.
If the last arg is ``&'' then the pipeline will be exe-
cuted 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 redi-
rected, 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 sub-
stitutions are performed on the arguments to commands.
Tcl 7.6 3
exec(n) Tcl Built-In Commands exec(n)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 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 redirect- |
ing from standard input, all applications will see |
an immediate end-of-file; information redirected to |
standard output or standard error will be dis- |
carded. |
Either forward or backward slashes are accepted as |
path separators for arguments to Tcl commands. |
When executing an application, the path name speci- |
fied 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 sepa- |
rator, 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://win- |
dows/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 ||
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 |
Tcl 7.6 4
exec(n) Tcl Built-In Commands exec(n)
for the longer name. If a directory name was not |
specified as part of the application name, the fol- |
lowing 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 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 fol- |
lowing 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 applica- |
tions do not have this problem and will run cor- |
rectly 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- |
Tcl 7.6 5
exec(n) Tcl Built-In Commands exec(n)
file; the behavior seems to depend upon something |
compiled into the application itself. When redi- |
recting greater than 4K or so to NUL:, some appli- |
cations will hang. The above problems do not hap- |
pen 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 stan- |
dard 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 Win- |
dows 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 fol- |
lowing 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 '' 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 applica- |
tions. 32-bit applications always see standard |
Tcl 7.6 6
exec(n) Tcl Built-In Commands exec(n)
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 redi- |
rected 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 syn- |
chronously; each application runs to completion |
before the next one in the pipeline starts. Tempo- |
rary 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 ``@ fileId'' 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 ||
The exec command is fully functional and works as |
described. |
SEE ALSO |
open(n)KEYWORDS
execute, pipeline, redirection, subprocess
Tcl 7.6 7