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doc/exec.n
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'\" Copyright (c) 1993 The Regents of the University of California.
'\" Copyright (c) 1994-1996 Sun Microsystems, Inc.
'\" Copyright (c) 2006 Donal K. Fellows.
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.TH exec n 8.5 Tcl "Tcl Built-In Commands"
.so man.macros
.BS
'\" Note:  do not modify the .SH NAME line immediately below!
.SH NAME
exec \- Invoke subprocesses
.SH SYNOPSIS
\fBexec \fR?\fIswitches\fR? \fIarg \fR?\fIarg ...\fR? ?\fB&\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.
The result of the command is the standard output of the final subprocess in
the pipeline, interpreted using the system \fBencoding\fR; to use any other
encoding (especially including binary data), the pipeline must be
\fBopen\fRed, configured and read explicitly.
.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:
.\" OPTION: -encoding
.TP 13
\fB\-encoding \fIencodingName\fR
.
Specifies the name of the encoding to use to decode the output of the first
subprocess. Defaults to that returned by the \fBencoding system\fR command.
.\" OPTION: -ignorestderr
.TP 13
\fB\-ignorestderr\fR
.
Stops the \fBexec\fR command from treating the output of messages to the
pipeline's standard error channel as an error case.
.\" OPTION: -keepnewline
.TP 13
\fB\-keepnewline\fR
.
Retains a trailing newline in the pipeline's output.
Normally a trailing newline will be deleted.
.\" OPTION: --
.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\fRs) 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
.QW "\fB<\fI fileName\fR" ,
\fIfileName\fR may either be in a separate argument from
.QW \fB<\fR
or in the same argument with no intervening space (i.e.
.QW \fB<\fIfileName\fR ).
.TP 15
\fB|\fR
.
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
\fB|&\fR
.
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
\fB<\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
\fB<@\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
\fB<<\0\fIvalue\fR
.
\fIValue\fR is encoded using the system encoding, newlines
replaced by platform-specific line ending sequences, and then
passed to the first command as its standard input.
.TP 15
\fB>\0\fIfileName\fR
.
Standard output from the last command is redirected to the file named
\fIfileName\fR, overwriting its previous contents.
.TP 15
\fB2>\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
\fB>&\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
\fB>>\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
\fB2>>\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
\fB>>&\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
\fB>@\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
\fB2>@\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
\fB2>@1\0\fR
.
Standard error from all commands in the pipeline is redirected to the
command result.  This operator is only valid at the end of the command
pipeline.
.TP 15
\fB>&@\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, unless
.QW 2>@1
was specified, in which case standard error is included as well.
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
\fB\-errorcode\fR return option 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 is not redirected
and \fB\-ignorestderr\fR is not specified,
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 do not
normally end with newlines.
However, if \fB\-keepnewline\fR is specified then the trailing
newline is retained.
.PP
If standard input is not redirected with
.QW < ,
.QW <<
or
.QW <@
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
.QW &
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 has not 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;
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
.QW 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
.QW \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.
.RS
.PP
Note that the current escape resp. quoting of arguments for windows works only
with executables using CommandLineToArgv, CRT-library or similar, as well as
with the windows batch files (excepting the newline, see below).
Although it is the common escape algorithm, but, in fact, the way how the
executable parses the command-line (resp. splits it into single arguments)
is decisive.
.PP
Unfortunately, there is currently no way to supply newline character within
an argument to the batch files (\fB.cmd\fR or \fB.bat\fR) or to the command
processor (\fBcmd.exe /c\fR), because this causes truncation of command-line
(also the argument chain) on the first newline character.
But it works properly with an executable (using CommandLineToArgv, etc).
.PP
\fBArgument quoting\fR
.RS
The arguments of the \fBexec\fR command are mapped to the arguments of the called
program. Additional quote characters (\fB"\fR) are automatically  added around
arguments if expected. Special characters are escaped by inserting backslash
characters.
.PP
The MS-Windows environment does execute programs mentioned in the arguments and
called batch files (conspec) replace environment variables, which may have side
effects (vulnerabilities) or break any already existing quoting (for example,
if the environment variable contains a special character like a \fB"\fR).
Examples are:
.CS
% exec my-echo.cmd {test&whoami}
  test
  mylogin
% exec my-echo.cmd "ENV X:%X%"
  ENV X: CONTENT OF X
.CE
The following formatting is automatically performed on any
argument item to avoid subprogram execution:
Any special character argument containing a special character (\fB&\fR, \fB|\fR,
\fB^\fR, \fB<\fR, \fB>\fR, \fB!\fR, \fB(\fR, \fB)\fR, \fB(\fR, \fB%\fR)
is automatically enclosed in quotes (\fB"\fR). Any data quote is escaped by
insertion of backslash characters.
.PP
The automatic resolving of environment variables using "\fB%var%\fR" is critical,
but has more use than danger and is not escaped.
.RE
.PP
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.
.PP
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.
.PP
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.
.PP
Note that there are two general types of Win32 console applications:
.RS
.IP [1]
CLI \(em CommandLine Interface, simple stdio exchange. \fBnetstat.exe\fR for
example.
.IP [2]
TUI \(em 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
.PP
\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 communicating with a TUI application.
.PP
When attempting to execute an application, \fBexec\fR searches directories
in the same manner as described for the \fBauto_execok\fR command. However,
with respect to file extensions, unlike \fBauto_execok\fR, \fBexec\fR only
searches for
the name as it was specified and then in order,
\fB.com\fR, \fB.exe\fR, \fB.bat\fR and \fB.cmd\fR. The \fBPATHEXT\fR
environment variable is disregarded as are any file associations.
In order to execute shell built-in commands like \fBdir\fR and \fBcopy\fR,
other extensions in \fBPATHEXT\fR, or registered application paths,
make use of \fBauto_execok\fR to retrieve the command to be passed to
\fBexec\fR.
.RE
.TP
\fBUnix\fR (including macOS)
.
The \fBexec\fR command is fully functional and works as described.
.SH "UNIX EXAMPLES"
.PP
Here are some examples of the use of the \fBexec\fR command on Unix.
To execute a simple program and get its result:
.PP
.CS
\fBexec\fR uname -a
.CE
.SS "WORKING WITH NON-ZERO RESULTS"
.PP
To execute a program that can return a non-zero result, you should
wrap the call to \fBexec\fR in \fBcatch\fR and check the contents
of the \fB\-errorcode\fR return option if you have an error:
.PP
.CS
set status 0
if {[catch {\fBexec\fR grep foo bar.txt} results options]} {
    set details [dict get $options -errorcode]
    if {[lindex $details 0] eq "CHILDSTATUS"} {
        set status [lindex $details 2]
    } else {
        # Some other error; regenerate it to let caller handle
        return -options $options -level 0 $results
    }
}
.CE
.PP
This is more easily written using the \fBtry\fR command, as that makes
it simpler to trap specific types of errors. This is
done using code like this:
.PP
.CS
try {
    set results [\fBexec\fR grep foo bar.txt]
    set status 0
} trap CHILDSTATUS {results options} {
    set status [lindex [dict get $options -errorcode] 2]
}
.CE
.SS "WORKING WITH QUOTED ARGUMENTS"
.PP
When translating a command from a Unix shell invocation, care should
be taken over the fact that single quote characters have no special
significance to Tcl.  Thus:
.PP
.CS
awk '{sum += $1} END {print sum}' numbers.list
.CE
.PP
would be translated into something like:
.PP
.CS
\fBexec\fR awk {{sum += $1} END {print sum}} numbers.list
.CE
.SS "WORKING WITH GLOBBING"
.PP
If you are converting invocations 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:
.PP
.CS
\fBexec\fR ls -l {*}[glob *.tcl]
.CE
.SS "WORKING WITH USER-SUPPLIED SHELL SCRIPT FRAGMENTS"
.PP
One useful technique can be to expose to users of a script the ability
to specify a fragment of shell script to execute that will have some
data passed in on standard input that was produced by the Tcl program.
This is a common technique for using the \fIlpr\fR program for
printing. By far the simplest way of doing this is to pass the user's
script to the user's shell for processing, as this avoids a lot of
complexity with parsing other languages.
.PP
.CS
set lprScript [\fIget from user...\fR]
set postscriptData [\fIgenerate somehow...\fR]

\fBexec\fR $env(SHELL) -c $lprScript << $postscriptData
.CE
.SH "WINDOWS EXAMPLES"
.PP
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:
.PP
.CS
\fBexec\fR notepad myfile.txt &
.CE
.PP
To print a text file using \fInotepad\fR:
.PP
.CS
\fBexec\fR notepad /p myfile.txt
.CE
.SS "WORKING WITH CONSOLE PROGRAMS"
.PP
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:
.PP
.CS
\fBexec\fR cmp.bat somefile.c -o somefile
.CE
.PP
With the file \fIcmp.bat\fR looking something like:
.PP
.CS
@gcc %*
.CE
.PP
or like another variant using single parameters:
.PP
.CS
@gcc %1 %2 %3 %4 %5 %6 %7 %8 %9
.CE
.SS "WORKING WITH COMMAND BUILT-INS"
.PP
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.
.PP
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
.QW 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:
.PP
.CS
\fBexec\fR {*}[auto_execok dir] *.tcl
.CE
.SS "WORKING WITH NATIVE FILENAMES"
.PP
Many programs on Windows require filename arguments to be passed in with
backslashes as pathname separators. This is done with the help of the
\fBfile nativename\fR command. For example, to make a directory (on NTFS)
encrypted so that only the current user can access it requires use of
the \fICIPHER\fR command, like this:
.PP
.CS
set secureDir [file join [file home] Desktop/SecureDirectory]
file mkdir $secureDir
\fBexec\fR CIPHER /e /s:[file nativename $secureDir]
.CE
.SH "SEE ALSO"
error(n), file(n), open(n)
.SH KEYWORDS
execute, pipeline, redirection, subprocess
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