exec(3tcl) Tcl Built-In Commands exec(3tcl)
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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, inter-
preted 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 mes-
sages to the pipeline's standard error channel as an error
case.
-keepnewline Retains a trailing newline in the pipeline's output. Nor-
mally 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 subpro-
cess(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 stan-
dard output of the preceding command will be piped into
the standard input of the next command.
|& Separates distinct commands in the pipeline. Both stan-
dard output and standard error of the preceding command
will be piped into the standard input of the next com-
mand. 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 in-
put.
> fileName Standard output from the last command is redirected to
the file named fileName, overwriting its previous con-
tents.
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 writ-
ing.
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 re-
turns 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 addi-
tional 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 back-
ground. In this case the exec command will return a list whose ele-
ments 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 redi-
rected, 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-sub-
stitution 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” nota-
tion, does not work. When reading from a socket, a 16-bit DOS
application will hang and a 32-bit application will return imme-
diately with end-of-file. When either type of application
writes to a socket, the information is instead sent to the con-
sole, if one is present, or is discarded.
Note that the current escape resp. quoting of arguments for win-
dows works only with executables using CommandLineToArgv, CRT-
library or similar, as well as with the windows batch files (ex-
cepting the newline, see below). Although it is the common es-
cape algorithm, but, in fact, the way how the executable parses
the command-line (resp. splits it into single arguments) is de-
cisive.
Unfortunately, there is currently no way to supply newline char-
acter within an argument to the batch files (.cmd or .bat) or to
the command processor (cmd.exe /c), because this causes trunca-
tion of command-line (also the argument chain) on the first new-
line character. But it works properly with an executable (using
CommandLineToArgv, etc).
The Tk console text widget does not provide real standard IO ca-
pabilities. 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 dis-
carded.
Either forward or backward slashes are accepted as path separa-
tors for arguments to Tcl commands. When executing an applica-
tion, the path name specified for the application may also con-
tain 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 con-
verted to use the backslash character. If an argument contains
forward slashes as the path separator, it may or may not be rec-
ognized 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 applica-
tions:
[1] CLI — CommandLine Interface, simple stdio ex-
change. 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, de-
tecting 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 de-
tached. This is a designed-in limitation as exec wants to com-
municate 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 direc-
tories 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 exe-
cute 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 re-
turn 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 sig-
nificance 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 multi-
ple 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 espe-
cially 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)
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