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 2
errors. This is done using code like this: 2
try { 2
set results [exec grep foo bar.txt] 2
set status 0 2
} trap CHILDSTATUS {results options} { 2
set status [lindex [dict get $options -errorcode] 2] 2
} 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)