path: root/doc/exec.n

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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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'\" See the file "license.terms" for information on usage and redistribution
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'\" RCS: @(#) $Id: exec.n,v 1.8 2004/04/22 12:48:25 dkf Exp $
'\" 
.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.

.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''), which can be obtained with the
\fBfile attributes $fileName -shortname\fR command.
.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 mount
point called \fBsystem\fR on the machine called \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.  The \fBfile join\fR
command should be used to concatenate path components.
.sp
.RS
Note that there are two general types of Win32 console applications:
.RS
1) CLI -- CommandLine Interface, simple stdio exchange. \fBnetstat.exe\fR for
example.
.br
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 \fBtelnet.exe\fR
from Windows 2000.  These types of applications are not common in a windows
environment, but do exist.
.RE
\fBexec\fR 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 \fBexec\fR wants to communicate over pipes.  The Expect
extension addresses this issue when communication between a TUI application
is desired.
.sp
.RE
.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 standard Windows 95 DOS shell
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
\fBUnix\fR\0\0\0\0\0\0\0
The \fBexec\fR command is fully functional and works as described.

.SH "UNIX EXAMPLES"
Here are some examples of the use of the \fBexec\fR command on Unix.

To execute a simple program and get its result:
.CS
exec uname -a
.CE

To execute a program that can return a non-zero result, you should
wrap the call to \fBexec\fR in \fBcatch\fR and check what the contents
of the global \fBerrorCode\fR variable is if you have an error:
.CS
set status 0
if {[catch {exec grep foo bar.txt} results]} {
   if {[lindex $::errorCode 0] eq "CHILDSTATUS"} {
      set status [lindex $::errorCode 2]
   } else {
      # Some kind of unexpected failure
   }
}
.CE

When translating a command from a Unix shell invokation, care should
be taken over the fact that single quote characters have no special
significance to Tcl.  Thus:
.CS
awk '{sum += $1} END {print sum}' numbers.list
.CE
would be translated into something like:
.CS
exec awk {{sum += $1} END {print sum}} numbers.list
.CE

If you are converting invokations involving shell globbing, you should
remember that Tcl does not handle globbing or expand things into
multiple arguments by default.  Instead you should write things like
this:
.CS
exec ls -l {expand}[glob *.tcl]
.CE

.SH "WINDOWS EXAMPLES"
Here are some examples of the use of the \fBexec\fR command on Windows.

To start an instance of \fInotepad\fR editing a file without waiting
for the user to finish editing the file:
.CS
exec notepad myfile.txt &
.CE

To print a text file using \fInotepad\fR:
.CS
exec notepad /p myfile.txt
.CE

If a program calls other programs, such as is common with compilers,
then you may need to resort to batch files to hide the console windows
that sometimes pop up:
.CS
exec cmp.bat somefile.c -o somefile
.CE
With the file \fIcmp.bat\fR looking something like:
.CS
@gcc %1 %2 %3 %4 %5 %6 %7 %8 %9
.CE

Sometimes you need to be careful, as different programs may have the
same name and be in the path. It can then happen that typing a command
at the DOS prompt finds \fIa different program\fR than the same
command run via \fBexec\fR. This is because of the (documented)
differences in behaviour between \fBexec\fR and DOS batch files.

When in doubt, use the command \fBauto_execok\fR: it will return the
complete path to the program as seen by the \fBexec\fR command.  This
applies especially when you want to run "internal" commands like
\fIdir\fR from a Tcl script (if you just want to list filenames, use
the \fBglob\fR command.)  To do that, use this:
.CS
exec {expand}[auto_execok dir] *.tcl
.CE

.SH "SEE ALSO"
error(n), open(n)

.SH KEYWORDS
execute, pipeline, redirection, subprocess