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'\" Copyright (c) 1993 The Regents of the University of California.

'\" Copyright (c) 1994-1996 Sun Microsystems, Inc.

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'\" RCS: @(#) $Id: exec.n,v 1.1.1.1 2002-01-16 10:25:24 markom Exp $

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.so man.macros

.TH exec n 7.6 Tcl "Tcl Built-In Commands"

.BS

'\" Note:  do not modify the .SH NAME line immediately below!

.SH NAME

exec \- Invoke subprocess(es)

.SH SYNOPSIS

\fBexec \fR?\fIswitches\fR? \fIarg \fR?\fIarg ...\fR?

.BE

.SH DESCRIPTION

.PP

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 \fIarg\fR becomes one word of a command, and

each distinct command becomes a subprocess.

.PP

If the initial arguments to \fBexec\fR start with \fB\-\fR then

they are treated as command-line switches and are not part

of the pipeline specification.  The following switches are

currently supported:

.TP 13

\fB\-keepnewline\fR

Retains a trailing newline in the pipeline's output.

Normally a trailing newline will be deleted.

.TP 13

\fB\-\|\-\fR

Marks the end of switches.  The argument following this one will

be treated as the first \fIarg\fR even if it starts with a \fB\-\fR.

.PP

If an \fIarg\fR (or pair of \fIarg\fR's) has one of the forms

described below then it is used by \fBexec\fR 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 ``< \fIfileName\fR'' \fIfileName\fR may either be in a

separate argument from ``<'' or in the same argument with no

intervening space (i.e. ``<\fIfileName\fR'').

.TP 15

|

Separates distinct commands in the pipeline.  The standard output

of the preceding command will be piped into the standard input

of the next command.

.TP 15

|&

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 >&.

.TP 15

<\0\fIfileName\fR

The file named by \fIfileName\fR is opened and used as the standard

input for the first command in the pipeline.

.TP 15

<@\0\fIfileId\fR

\fIFileId\fR must be the identifier for an open file, such as the return

value from a previous call to \fBopen\fR.

It is used as the standard input for the first command in the pipeline.

\fIFileId\fR must have been opened for reading.

.TP 15

<<\0\fIvalue\fR

\fIValue\fR is passed to the first command as its standard input.

.TP 15

>\0\fIfileName\fR

Standard output from the last command is redirected to the file named

\fIfileName\fR, overwriting its previous contents.

.TP 15

2>\0\fIfileName\fR

Standard error from all commands in the pipeline is redirected to the

file named \fIfileName\fR, overwriting its previous contents.

.TP 15

>&\0\fIfileName\fR

Both standard output from the last command and standard error from all

commands are redirected to the file named \fIfileName\fR, overwriting

its previous contents.

.TP 15

>>\0\fIfileName\fR

Standard output from the last command is

redirected to the file named \fIfileName\fR, appending to it rather

than overwriting it.

.TP 15

2>>\0\fIfileName\fR

Standard error from all commands in the pipeline is

redirected to the file named \fIfileName\fR, appending to it rather

than overwriting it.

.TP 15

>>&\0\fIfileName\fR

Both standard output from the last command and standard error from

all commands are redirected to the file named \fIfileName\fR,

appending to it rather than overwriting it.

.TP 15

>@\0\fIfileId\fR

\fIFileId\fR must be the identifier for an open file, such as the return

value from a previous call to \fBopen\fR.

Standard output from the last command is redirected to \fIfileId\fR's

file, which must have been opened for writing.

.TP 15

2>@\0\fIfileId\fR

\fIFileId\fR must be the identifier for an open file, such as the return

value from a previous call to \fBopen\fR.

Standard error from all commands in the pipeline is

redirected to \fIfileId\fR's file.

The file must have been opened for writing.

.TP 15

>&@\0\fIfileId\fR

\fIFileId\fR must be the identifier for an open file, such as the return

value from a previous call to \fBopen\fR.

Both standard output from the last command and standard error from

all commands are redirected to \fIfileId\fR's file.

The file must have been opened for writing.

.PP

If standard output has not been redirected then the \fBexec\fR

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 \fBexec\fR will return an error

and the error message will include the pipeline's output followed by

error messages describing the abnormal terminations; the

\fBerrorCode\fR 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 \fBexec\fR 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.

.PP

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 \fB\-keepnewline\fR is specified then the trailing

newline is retained.

.PP

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.

.PP

If the last \fIarg\fR is ``&'' then the pipeline will be

executed in background.

In this case the \fBexec\fR 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.

.PP

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.

.VS

.SH "PORTABILITY ISSUES"

.TP

\fBWindows\fR (all versions)

.

Reading from or writing to a socket, using the ``\fB@\0\fIfileId\fR''

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.

.sp

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.

.sp

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.

.sp

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'').

.sp

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

\fBc:/\fR with a subdirectory \fB/windows/system\fR will yield

\fBc://windows/system\fR (two slashes together), which refers to the

directory \fB/system\fR on the machine \fBwindows\fR (and the \fBc:/\fR is

ignored), and is not equivalent to \fBc:/windows/system\fR, which describes

a directory on the current computer.

.TP

\fBWindows NT\fR

.

When attempting to execute an application, \fBexec\fR first searches for the

name as it was specified.  Then, in order, \fB.com\fR, \fB.exe\fR, and \fB.bat\fR

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:

.sp

.RS

.RS

The directory from which the Tcl executable was loaded.

.br

The current directory.

.br

The Windows NT 32-bit system directory.

.br

The Windows NT 16-bit system directory.

.br

The Windows NT home directory.

.br

The directories listed in the path.

.RE

.sp

In order to execute the shell builtin commands like \fBdir\fR and \fBcopy\fR,

the caller must prepend ``\fBcmd.exe /c\0\fR'' to the desired command.

.sp

.RE

.TP

\fBWindows 95\fR

.

When attempting to execute an application, \fBexec\fR first searches for the

name as it was specified.  Then, in order, \fB.com\fR, \fB.exe\fR, and \fB.bat\fR

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:

.sp

.RS

.RS

The directory from which the Tcl executable was loaded.

.br

The current directory.

.br

The Windows 95 system directory.

.br

The Windows 95 home directory.

.br

The directories listed in the path.

.RE

.sp

In order to execute the shell builtin commands like \fBdir\fR and \fBcopy\fR,

the caller must prepend ``\fBcommand.com /c\0\fR'' to the desired command.

.sp

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.

.sp

Redirection between the \fBNUL:\fR device and a 16-bit application does not

always work.  When redirecting from \fBNUL:\fR, 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 \fBNUL:\fR, some applications will hang.  The above problems do not

happen with 32-bit applications.

.sp

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.

.sp

Certain applications, such as \fBcommand.com\fR, 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.

.RE

.TP

\fBWindows 3.X\fR

.

When attempting to execute an application, \fBexec\fR first searches for the

name as it was specified.  Then, in order, \fB.com\fR, \fB.exe\fR, and \fB.bat\fR

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:

.sp

.RS

.RS

The directory from which the Tcl executable was loaded.

.br

The current directory.

.br

The Windows 3.X system directory.

.br

The Windows 3.X home directory.

.br

The directories listed in the path.

.RE

.sp

In order to execute the shell builtin commands like \fBdir\fR and \fBcopy\fR,

the caller must prepend ``\fBcommand.com /c\0\fR'' to the desired command.

.sp

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.

.sp

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 \fBexec\fR

command cannot be used to start an application in the background.

.sp

When standard input is redirected from an open file using the

``\fB@\0\fIfileId\fR'' 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.

.RE

.TP

\fBMacintosh\fR

The \fBexec\fR command is not implemented and does not exist under Macintosh.

.TP

\fBUnix\fR\0\0\0\0\0\0\0

The \fBexec\fR command is fully functional and works as described.

.SH "SEE ALSO"

open(n)

.VE

.SH KEYWORDS

execute, pipeline, redirection, subprocess