bionic (3) blt::bgexec.3tcl.gz
NAME
bgexec - Run programs in the background while handling Tk events. kill - Terminate program or send
signal.
SYNOPSIS
blt::bgexec varName ?switches? program ?arg?...
blt::kill processid ?signal?
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DESCRIPTION
The kill command terminates a processid or under unix sends a signal.
The bgexec command executes a program pipleline using the Tcl event-loop allowing other events to
continue to be serviced. Upon completion it sets the global variable varName with a list of 4 status
values: a text token, the process-id, the exit code, and a text message. Bgexec provides capabilities
similar to the exec command, but with added support for callbacks, output to variables and termination.
When used with no options, the returned value from bgexec is the output from the program. But when the
last arg is an ampersand (&) the program runs detached, and bgexec immediately returns with a list of the
process ids created in the command pipeline. Detached processes can be interrupted and terminated simply
by setting varName.
The valid switches are as follows:
-check num
Interval in ms to poll for the exiting processes. The default is 1000.
-closeonkill millisecs
Force close of stdin/stdout on kill after the given interval. This lets kill finalize processes,
even uninterruptably sleeping ones unable to receive signals. The default is 0 for do not force
close.
-command script
Specifies a command to call upon command completion/termination. Two extra arguments are appended
before the call. The data output from the command, and the status info as set into varName.
-decodeerror encodingName
Specifies the encoding of the stderr channel. This affects only data returned to the Tcl
interpreter. No translation is done on file redirection. For example if data is to be converted
from Unicode for use in Tcl, you would use the "unicode" encoding. The default is that no
tranlation is performed.
-decodeoutput encodingName
Specifies the encoding of the stdout channels. This affects only data returned to the Tcl
interpreter. No translation is done on file redirection. For example if data is to be converted
from Unicode for use in Tcl, you would use the "unicode" encoding. The default is that no
tranlation is performed.
-echo boolean
Indicates if the pipeline's stderr stream should be echoed. Note: this option is deprecated.
-error varName
Specifies that a global variable varName is to be set with the contents of stderr after the
program has completed.
-keepnewline boolean
Specifies that a trailing newline should be retained in the output. If boolean is true, the
trailing newline is truncated from the output of the -onoutput and -output variables. The default
value is true.
-killsignal signal
Specifies the signal to be sent to the program when terminating. This option is available only on
Unix. Signal can either be a number (typically 1-32) or a mnemonic (such as SIGINT). If signal is
the empty string, then no signal is sent. The default signal is 9 (SIGKILL).
-lasterror varName
Specifies a variable varName that is updated whenever data becomes available from standard error
of the program. VarName is a global variable. Unlike the -error option, data is available as soon
as it arrives.
-lastoutput varName
Specifies a variable varName that is updated whenever data becomes available from standard output
of the program. VarName is a global variable. Unlike the -output option, data is available as
soon as it arrives.
-limit numBytes
Limit the size of the returned data to numBytes, terminating the program if exceeded. The limit
applies to both stdout and stderr.
-linebuffered boolean
Specifies that updates should be made on a line-by-line basis. Normally when new data is
available bgexec will set the variable (-lastoutput and -lasterror options) or invoke the command
(-onoutput and -onerror options) delivering all the new data currently available. If boolean is
true, only one line at a time will be delivered. This can be useful when you want to process the
output on a line-by-line basis. The default value is false.
-local boolean
When boolean is true, any unqualified variables or command options are treated as local to the
current namespace. This is mostly useful for non-detaching (no ampersand) commands. Note that
using this flag with a detached command will use variables from the current namespace, not from
the current proc stack-frame.
-onerror command
Specifies the start of a Tcl command that will be executed whenever new data is available from
standard error. The data is appended to the command as an extra argument before it is executed.
-onoutput command
Specifies the start of a Tcl command that will be executed whenever new data is available from
standard output. The data is appended to the command as an extra argument before it is executed.
-output varName
Specifies a global variable varName to be set with the output of the program, upon completion.
-raise boolean
When boolean is true, a non-zero return code from a non-detached command will raise an error (.ie
emulates exec). The default is false an error is generated only if one of the following occurs:
invalid options are given, a redirection error, or process creation failure (eg. executable
program not found). Detached commands, of course, never raise an error on a non-zero return code.
-- This marks the end of the options. The following argument will be considered the name of a
program even if it starts with a dash (-).
USAGE
Invoking bgexec without a trailing ampersand will block and wait for result. However, other Tcl events
continue to be serviced. This prevents Tcl from hanging, eg:
pack [text .t]
set val [blt::bgexec myStatus du -s]
Note that text widget .t continues to respond to events.
CALLBACKS
Here is an example that invokes the Unix du program with a -command callback.
proc Done {data status} { puts "Done($status)\n$data" }
blt::bgexec myStatus -command Done du -s $dir &
When du has completed, the handler Done is called with data and status. Also, the global variable
myStatus is set to contain the program's exit status, eg:
EXITED 26811 0 {child completed normally}
If myStatus is set before du has completed, the process will be killed. Under Unix, this sends a signal
(SIGKILL by default). Under Win32, TerminateProcess is called.
VARIABLE
Here is another example, this time using the -output option to direct output to a variable.
global myStatus myOutput
blt::bgexec myStatus -output myOutput du -s $dir
puts "Disk usage for $dir is $myOutput"
Upon completion, MyOutput will contain the output of the program.
STDERR
Various bgexec options can be used to capture stderr separately from stdout.
global myStatus myOutput myErrs
blt::bgexec myStatus -output myOutput -error myErrs du -s $dir
The -error option is similar to -output in that it sets a variable when the program completes with data
written to stderr.
LOCAL
By default, bgexec treats variable or command options as being in the global namespace. The -local
option can change this to use the current namespace. Thus data can be collected to namespace-local
variables even those inside of procs, eg.
proc Work {} {
blt::bgexec myStatus -local 1 -output val -error err du -s
puts "VAL=$val"
puts "ERR=$err"
}
which collects data to local variables.
For detached processes, -local will cause data to aggregate to namespace variables, ie. outside the proc,
eg.
namespace eval ::Ns {
set pval {}
set perr {}
proc Work {} {
blt::bgexec myStatus -local 1 -output pval -error perr du -s &
}
}
This collects data to ::Ns::pval and stderr to ::Ns::perr. Similarly, proc names (eg -onoutput) will be
relative to the current namespace.
PROGRESS
The -output and -error variables are set only after the program completes. But if a program runs for a
long time, you can gather data as it becomes available using the -onoutput option. As new data becomes
available, this command is executed, with data appended as an argument.
proc GetInfo { data } { puts $data }
blt::bgexec myStatus -onoutput GetInfo du -s $dir
The -onerror option performs a similar function for the stderr data stream.
ERROR HANDLING
Like exec, bgexec returns an error if the exit code of the program is non-zero. To handle this invoke
bgexec from within a catch.
catch { blt::bgexec myStatus -output myOutput du -s $dir }
Detached jobs will generate an error only if the program startup failed. Otherwise the only indication
is the status code set in myStatus.
TKWAIT
By default, bgexec waits for a program to finish and returns the resulting output. To detach a program
simply append an ampersand (&) as the last argument on the command line, eg.
global myStatus myOutput
blt::bgexec myStatus -output myOutput du -s $dir &
Bgexec will then return immediately with the spawned process ids as the result. If needed tkwait can be
used to wait for the program to finish:
global myStatus myOutput
blt::bgexec myStatus -output myOutput du -s $dir &
...
tkwait variable myStatus
Note however that using tkwait can be dangerous. Multiple tkwait/vwait calls must complete in the
reverse order called. The BLT busy command can be used to try and enforce this, but a better alternative
is to just use -command instead.
DIFFERENCES WITH EXEC
Using bgexec without an ampersand will not hang Tcl: events continue to be serviced by the event handler
while the call blocks. Also unlike exec, an error will not be generated if output is appears on stderr.
And output from stderr can be separately managed and collected (without having to redirect to files).
Finally, bgexec ensures that invoked processes get properly cleaned up at termination.
DIFFERENCES WITH FILEEVENT
Since Tk 4.0, a subset of bgexec can be achieved using the fileevent command. The steps for running a
program in the background are:
Execute the program with the open command (using the "|" syntax) and save the file handle.
global fileId
set fileId [open "|du -s $dir" r]
Next register a Tcl code snippet with fileevent to be run whenever output is available on the file
handle. The code snippet will read from the file handle and save the output in a variable.
fileevent fileId readable {
if { [gets $fileId line] < 0 } {
close $fileId
set output $temp
unset fileId temp
} else {
append temp $line
}
}
However, Bgexec is simpler and less error prone than using open + fileevent. You don't have to worry
about non-blocking I/O. Everything is handled for you automatically.
Moreover, bgexec can run programs that fileevent can not. Fileevent assumes that the when stdout is
closed the program has completed. But some programs, like the Unix compress program, reopen stdout,
fooling fileevent into thinking the program has terminated. In the example above, we assume that the
program will write and flush its output line-by-line. However when running another program, your
application can block in the gets command reading a partial line.
Bgexec gives you get back the exit status of the program. It also lets you reliably kill detached
processes and allows you to collect data from both stdout and stderr individually. Finally, since data
collection is handled in C code, bgexec is faster and more efficient.
SEE ALSO
busy, exec, tkwait, vwait
KEYWORDS
exec, background, busy