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:
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”).
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.
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 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:
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:
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.
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 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]
}
awk '{sum += $1} END {print sum}' numbers.list
would be translated into something like:
exec awk {{sum += $1} END {print sum}} numbers.list
exec ls -l {*}[glob *.tcl]
set lprScript [get from user...] set postscriptData [generate somehow...] exec $env(SHELL) -c $lprScript << $postscriptData
exec notepad myfile.txt &
To print a text file using notepad:
exec notepad /p myfile.txt
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
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
set secureDir "~/Desktop/Secure Directory" file mkdir $secureDir exec CIPHER /e /s:[file nativename $secureDir]