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

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NAME

       exec - Invoke subprocess(es)

SYNOPSIS

       exec ?switches? arg ?arg ...?
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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	specifica-
       tion.  The following switches are currently supported:

       -keepnewline Retains a trailing newline in the pipeline's output.  Normally 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 subprocess(es).  In forms such as ``< fileName'' fileName may either be in a sep-
       arate argument from ``<'' or in the same argument with no intervening space (i.e. ``<fileName'').

       |	      Separates  distinct  commands in the pipeline.  The standard output of the preceding command will be piped into the standard
		      input of the next command.

       |&	      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 >&.

       < 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 contents.

       2> fileName    Standard error from all commands in the pipeline is redirected to the file named fileName,  overwriting  its  previous  con-
		      tents.

       >& 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 over-
		      writing 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 writing.

       >&@ 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  addi-
       tional  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 background.  In this case the exec 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.

       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.

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 console, 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 discarded.	   |

	      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.															   |

	      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'').									   |

	      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.		   |

       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 specified as |
	      part of the application name, the following directories are automatically searched in order when attempting to locate  the  applica- |
	      tion:																   |

		     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  specified	as |
	      part  of	the application name, the following directories are automatically searched in order when attempting to locate the applica- |
	      tion:																   |

		     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  com- |
	      mand.																   |

	      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.		   |

	      Redirection  between  the  NUL: device and a 16-bit application does not always work.  When redirecting from NUL:, 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 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 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. 							   |

	      Certain applications, such as command.com, 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 sys- |
	      tem 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 ALSO
																	   |
       error(n), open(n)															   |

KEYWORDS
																	   |
       execute, pipeline, redirection, subprocess												   |

Tcl									7.6								   exec(n)