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

______________________________________________________________________________

NAME
       exec - Invoke subprocess(es)

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 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
		      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 communication between a TUI application is desired.   │

       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 │
	      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.     │

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

Tcl				      7.6			       exec(n)
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