bgexec(n)

💅 bgexec(n) — BLT Built-In Commands 💅

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✨ NAME ✨

bgexec — run programs in the background while handling Tk events. kill — terminate a program or send a signal.

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🌸 SYNOPSIS 🌸

blt::bgexec varName ?switches? program ?arg?...
blt::kill processid ?signal?

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💖 DESCRIPTION 💖

Okay so bestie, the kill command terminates a processid, or under Unix sends a signal. That's the whole vibe there.

The bgexec command executes a program pipeline using the Tcl event-loop, which means other events keep getting serviced while it does its thing. 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 gives you capabilities similar to the exec command, but with added support for callbacks, output to variables, and termination. It's giving upgraded <italic>exec</italic> energy, fr fr.

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. Iconic.

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 translation 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 translation 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 stackframe.
• -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 (i.e. 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 (-).

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🦋 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. Main character energy: it's waiting, but it's not blocking the whole group chat.

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🎀 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. Respect the cleanup era.

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🍓 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. Slay, data collected.

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🧚 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. Separate channels, separate vibes.

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💜 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. Namespace girlies rise up.

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🌈 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. Live updates, bestie.

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🔥 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. Not every plot twist is an actual error, iykyk.

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⭐ 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. Do it the -command way — periodt.

💅 DIFFERENCES WITH EXEC

Okay so this is the part where everyone's like "wait, isn't this just exec with extra steps?" and the answer is no bestie, it is not. Here's the tea 🍵:

• First off, running bgexec without an ampersand will not hang Tcl — the event handler keeps servicing events the whole time the call is blocking. So it blocks, but like, politely. ✨
• Unlike exec, you will not get an error generated if output appears on stderr. She's unbothered. 💅
• And speaking of stderr — its output can be separately managed and collected, without you having to redirect anything to files. Iconic behaviour honestly.
• Last but never least: bgexec makes sure the processes it invoked get properly cleaned up at termination. She does not leave a mess. Periodt. 🎀

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🦋 DIFFERENCES WITH FILEEVENT

Since Tk 4.0, you can achieve a subset of what bgexec does using the fileevent command. So if anyone tries to tell you this is brand new tech, they're lowkey wrong. Here are the steps for running a program in the background the old way:

1. Execute the program with the open command (using the "|" syntax) and save the file handle.

                global fileId
                set fileId [open "|du -s $dir" r]

2. Next, register a Tcl code snippet with fileevent to be run whenever output is available on the file handle. That 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
                }
              }

But here's the thing — Bgexec is simpler and less error prone than using open + fileevent. You don't have to worry about non-blocking I/O at all. Everything is handled for you automatically. 🫶

Moreover — and this is the part that really matters — bgexec can run programs that fileevent can not. Fileevent assumes that when stdout is closed, the program has completed. But some programs, like the Unix compress program, reopen stdout, which fools 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. Yeah. Not cute.

Bgexec gives you back the exit status of the program. It also lets you reliably kill detached processes, and it 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. She's got range. 👑

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🌈 SEE ALSO

• busy
• exec
• tkwait
• vwait

⭐ KEYWORDS

exec, background, busy

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And that's the whole page, bestie! 🎀 This little masterpiece is from BLT 2.5 — the bgexec(n) manual page, to be exact — and yes, every single word of it is here, so you can go forth and background your processes with confidence. No cap, you're gonna slay this. 💖✨🦋