man
3 exec
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. Nor-
mally 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 subpro-
cess(es). In forms such as ``< fileName'' fileName may either be in a
separate argument from ``<'' or in the same argument with no interven-
ing space (i.e. ``<fileName'').
| Separates distinct commands in the pipeline. The stan-
dard output of the preceding command will be piped into
the standard input of the next command.
|& Separates distinct commands in the pipeline. Both stan-
dard output and standard error of the preceding command
will be piped into the standard input of the next com-
mand. 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 in-
put.
> fileName Standard output from the last command is redirected to
the file named fileName, overwriting its previous con-
tents.
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 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 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 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 writ-
ing.
>&@ 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 re-
turns 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 back-
ground. In this case the exec command will return a list whose ele-
ments 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-sub-
stitution 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 ``@ fileId'' no- |
tation, does not work. When reading from a socket, a 16-bit DOS |
application will hang and a 32-bit application will return imme- |
diately with end-of-file. When either type of application |
writes to a socket, the information is instead sent to the con- |
sole, if one is present, or is discarded. |
The Tk console text widget does not provide real standard IO ca- |
pabilities. 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 dis- |
carded. |
Either forward or backward slashes are accepted as path separa- |
tors for arguments to Tcl commands. When executing an applica- |
tion, the path name specified for the application may also con- |
tain 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 con- |
verted to use the backslash character. If an argument contains |
forward slashes as the path separator, it may or may not be rec- |
ognized as a path name, depending on the program. |
Additionally, when calling a 16-bit DOS or Windows 3.X applica- |
tion, all path names must use the short, cryptic, path format |
(e.g., using ``applba~1.def'' instead of ``applbakery.de- |
fault''). |
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 ig- |
nored), and is not equivalent to c:/windows/system, which de- |
scribes 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 for the longer name. If a directory name was not spec- |
ified 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 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 spec- |
ified 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 '' 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 applica- |
tions will see the standard input as already closed. 32-bit ap- |
plications 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 applica- |
tions 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 be- |
fore the 16-bit DOS application begins executing. All standard |
output or error from a 16-bit DOS application to a pipe is col- |
lected into temporary files; the application must terminate be- |
fore 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 exe- |
cuted 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 spec- |
ified 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 '' 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 stan- |
dard 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. Ad- |
ditionally, 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-redi- |
rection for 32-bit applications, the 32-bit application must in- |
stead 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 |
``@ 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 exec(n)