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exec(n)			     Tcl Built-In Commands		       exec(n)

______________________________________________________________________________

NAME
       exec - Invoke subprocesses

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.  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 args) 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
		      input.

       > 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
       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, 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'' │
	      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  con‐ │
	      sole, if one is present, or is discarded.			       │

	      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  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   ``applbak‐ │
	      ery.default''), which can be obtained with the  file  attributes │
	      $fileName -shortname command.				       │

	      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 mount point called system on the machine called windows (and │
	      the c:/ is ignored), and is not equivalent to c:/windows/system, │
	      which  describes	a directory on the current computer.  The file │
	      join command should be used to concatenate path components.      │

	      Note that there are two general types of Win32 console  applica‐ │
	      tions:							       │
		     1)	 CLI  -- CommandLine Interface, simple stdio exchange. │
		     netstat.exe for example.				       │
		     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 telnet.exe from │
		     Windows 2000.  These types of applications are not common │
		     in a windows environment, but do exist.		       │
	      exec  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  exec  wants  to │
	      communicate  over	 pipes.	  The  Expect extension addresses this │
	      issue when communicating with a TUI application.		       │

       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 shell built-in commands like dir  and	 copy, │
	      the caller must prepend the desired command with ``cmd.exe /c '' │
	      because built-in commands are not implemented using executables. │

       Windows 9x							       │
	      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 9x system directory.			       │
		     The Windows 9x home directory.			       │
		     The directories listed in the path.		       │

	      In order to execute shell built-in commands like dir  and	 copy, │
	      the  caller  must prepend the desired command with ``command.com │
	      /c '' because built-in commands are not implemented  using  exe‐ │
	      cutables.							       │

	      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 │
	      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.							       │

	      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 │
	      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 Windows 95 bug │
	      in the implementation of pipes, and is how the standard  Windows │
	      95 DOS shell 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.					       │

       Macintosh							       │
	      The exec command is not implemented and  does  not  exist	 under │
	      Macintosh.						       │

       Unix								       │
	      The exec command is fully functional and works as described.     │

UNIX EXAMPLES								       │
       Here are some examples of the use of the exec command on Unix.	       │

       To execute a simple program and get its result:			       │
	      exec uname -a						       │

       To execute a program that can return a non-zero result, you should wrap │
       the call to exec in catch and check what the  contents  of  the	global │
       errorCode variable is if you have an error:			       │
	      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			       │
		 }							       │
	      }								       │

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

       If  you are converting invocations involving shell globbing, you should │
       remember that Tcl does not handle globbing or expand things into multi‐ │
       ple arguments by default.  Instead you should write things like this:   │
	      eval [list exec ls -l] [glob *.tcl]			       │

WINDOWS EXAMPLES							       │
       Here are some examples of the use of the exec command on Windows.       │

       To  start an instance of notepad editing a file without waiting for the │
       user to finish editing the file:					       │
	      exec notepad myfile.txt &					       │

       To print a text file using notepad:				       │
	      exec notepad /p myfile.txt				       │

       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:						       │
	      exec cmp.bat somefile.c -o somefile			       │
       With the file cmp.bat looking something like:			       │
	      @gcc %1 %2 %3 %4 %5 %6 %7 %8 %9				       │

       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 a different program than the same  command  run │
       via  exec. This is because of the (documented) differences in behaviour │
       between exec and DOS batch files.				       │

       When in doubt, use the command auto_execok: it will return the complete │
       path  to	 the  program as seen by the exec command.  This applies espe‐ │
       cially when you want to run "internal" commands like  dir  from	a  Tcl │
       script  (if you just want to list filenames, use the glob command.)  To │
       do that, use this:						       │
	      eval [list exec] [auto_execok dir] [list *.tcl]		       │

SEE ALSOerror(n), open(n)KEYWORDS								       │
       execute, pipeline, redirection, subprocess			       │

Tcl				      7.6			       exec(n)
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