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exec(3tcl) | Tcl Built-In Commands | exec(3tcl) |
NAME¶
exec - Invoke subprocessesSYNOPSIS¶
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:- -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 -.
- |
- 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.
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”),
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:
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.
- [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.
- 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 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.
- 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 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
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 system 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
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 } }
awk '{sum += $1} END {print sum}' numbers.list
exec awk {{sum += $1} END {print sum}} numbers.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 &
exec notepad /p myfile.txt
exec cmp.bat somefile.c -o somefile
@gcc %1 %2 %3 %4 %5 %6 %7 %8 %9
exec {*}[auto_execok dir] *.tcl
SEE ALSO¶
error(3tcl), open(3tcl)KEYWORDS¶
execute, pipeline, redirection, subprocess8.5 | Tcl |