NAME¶
exec - Invoke subprocesses
SYNOPSIS¶
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:
- -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.
- >&@ 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
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''), 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.
- 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.
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 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.
- Macintosh
- 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:
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 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:
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
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:
eval [list exec] [auto_execok dir] [list *.tcl]
SEE ALSO¶
error(3tcl), open(3tcl)
KEYWORDS¶
execute, pipeline, redirection, subprocess