exec

exec(3tcl)                   Tcl Built-In Commands                  exec(3tcl)



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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:

       -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 separate 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 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
                      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 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.

              Note that the current escape resp. quoting of arguments for
              windows works only with executables using CommandLineToArgv,
              CRT-library or similar, as well as with the windows batch files
              (excepting the newline, see below).  Although it is the common
              escape algorithm, but, in fact, the way how the executable
              parses the command-line (resp. splits it into single arguments)
              is decisive.

              Unfortunately, there is currently no way to supply newline
              character within an argument to the batch files (.cmd or .bat)
              or to the command processor (cmd.exe /c), because this causes
              truncation of command-line (also the argument chain) on the
              first newline character.  But it works properly with an
              executable (using CommandLineToArgv, etc).

              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 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.

              When attempting to execute an application, exec first searches
              for the name as it was specified.  Then, in order, .com, .exe,
              .bat and .cmd 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 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.

       Unix (including Mac OS X)
              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

   WORKING WITH NON-ZERO RESULTS
       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 other error; regenerate it to let caller handle
                      return -options $options -level 0 $results
                  }
              }

       This is more easily written using the try command, as that makes it     │
       simpler to trap specific types of errors. This is done using code like  │
       this:                                                                   │

              try {                                                            │
                  set results [exec grep foo bar.txt]                          │
                  set status 0                                                 │
              } trap CHILDSTATUS {results options} {                           │
                  set status [lindex [dict get $options -errorcode] 2]         │
              }                                                                │

   WORKING WITH QUOTED ARGUMENTS
       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

   WORKING WITH GLOBBING
       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]

   WORKING WITH USER-SUPPLIED SHELL SCRIPT FRAGMENTS
       One useful technique can be to expose to users of a script the ability
       to specify a fragment of shell script to execute that will have some
       data passed in on standard input that was produced by the Tcl program.
       This is a common technique for using the lpr program for printing. By
       far the simplest way of doing this is to pass the user's script to the
       user's shell for processing, as this avoids a lot of complexity with
       parsing other languages.

              set lprScript [get from user...]
              set postscriptData [generate somehow...]

              exec $env(SHELL) -c $lprScript << $postscriptData

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

   WORKING WITH CONSOLE PROGRAMS
       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 %*
       or like another variant using single parameters:
              @gcc %1 %2 %3 %4 %5 %6 %7 %8 %9

   WORKING WITH COMMAND BUILT-INS
       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
       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

   WORKING WITH NATIVE FILENAMES
       Many programs on Windows require filename arguments to be passed in
       with backslashes as pathname separators. This is done with the help of
       the file nativename command. For example, to make a directory (on NTFS)
       encrypted so that only the current user can access it requires use of
       the CIPHER command, like this:

              set secureDir "~/Desktop/Secure Directory"
              file mkdir $secureDir
              exec CIPHER /e /s:[file nativename $secureDir]

SEE ALSO
       error(3tcl), file(3tcl), open(3tcl)

KEYWORDS
       execute, pipeline, redirection, subprocess



Tcl                                   8.5                           exec(3tcl)