Ubuntu Manpage: bgexec - Run programs in the background while handling Tk events. kill

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?
_________________________________________________________________

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.

KEYWORDS

       exec, background, busy