Provided by: tcl8.6-doc_8.6.14+dfsg-1build1_all 
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. The result of the
command is the standard output of the final subprocess in the pipeline, interpreted using
the system encoding; to use any other encoding (especially including binary data), the
pipeline must be opened, configured and read explicitly.
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).
Argument quoting
The arguments of the exec command are mapped to the arguments of the called
program. Additional quote characters (") are automatically added around
arguments if expected. Special characters are escaped by inserting backslash
characters.
The MS-Windows environment does execute programs mentioned in the arguments
and called batch files (conspec) replace environment variables, which may
have side effects (vulnerabilities) or break any already existing quoting
(for example, if the environment variable contains a special character like
a "). Examples are:
% exec my-echo.cmd {test&whoami}
test
mylogin
% exec my-echo.cmd "ENV X:%X%"
ENV X: CONTENT OF X
The following formatting is automatically performed on any argument item to
avoid subprogram execution: Any special character argument containing a
special character (&, |, ^, <, >, !, (, ), (, %) is automatically enclosed
in quotes ("). Any data quote is escaped by insertion of backslash
characters.
The automatic resolving of environment variables using "%var%" is critical,
but has more use than danger and is not escaped.
TCL 8.6.10 refined this quoting by adding quoting for data quotes and
individual quoting of "%". This may break present scripts which rely on the
replacement functionality of environment variables. Thus, the individual
quoting of "%" was removed in TCL 8.6.14, as environment variables are seen
more helpful than a problem. A solution with command parameters is
envisaged for a future release of TCL.
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.
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 32-bit system directory.
• The Windows 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