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

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NAME

       exec - Invoke subprocesses

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:

       -ignorestderr
		    Stops the exec command from treating the output of messages to the pipeline's standard error channel as an error case.	   |

       -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 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 subprocess(es).  In forms such as "< fileName", fileName may either be in a sepa-
       rate 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.

       2>@1	      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.

       >&@ 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, unless |
       "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  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 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 -ignorestderr is not |
       specified, 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 do not normally end with newlines.  However, if -keepnewline is specified  then  the
       trailing newline is retained.

       If  standard input is not redirected with "<", "<<" 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 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.

       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  appli-
	      cation  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"), 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 applications:

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

	      o      The directory from which the Tcl executable was loaded.

	      o      The current directory.

	      o      The Windows NT 32-bit system directory.

	      o      The Windows NT 16-bit system directory.

	      o      The Windows NT home directory.

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

	      o      The directory from which the Tcl executable was loaded.

	      o      The current directory.

	      o      The Windows 9x system directory.

	      o      The Windows 9x home directory.

	      o      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 executables.

	      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.

       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 the contents of the -errorcode
       return option if you have an error:
	      set status 0
	      if {[catch {exec 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 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
       significance 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
       multiple arguments by default.  Instead you should write things like this:
	      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  behav-
       iour 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
       especially 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:
	      exec {*}[auto_execok dir] *.tcl

SEE ALSO

       error(n), open(n)

KEYWORDS

       execute, pipeline, redirection, subprocess

Tcl									8.5								   exec(n)