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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 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.

       >&gt;& 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 ver-                                                       |
       sions)                                                  |               |
              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.                                               |

       Macin-                                                                  |
       tosh                                                               |    |
              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 ALSO |

       error(n), open(n)                                                       |


KEYWORDS |

       execute, pipeline, redirection, subprocess                              |

Tcl                                   7.6                              exec(n)

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