TCL - How to print on the screen th messages that where printed during execution of exec command?

Say I have want to execute a script or and executable file开发者_JAVA百科 by printing runtime the output of execution.

When I do:

set log [exec ./executable_file]
puts $log

Then it waits a long time and then prints everything at once. But I want runtime printing. How can I do this?

Not perfect (as it require writing to external file):

set log [exec executable_file | tee log.txt >@stdout]

The output will be displayed immediately, at the same time, saved to 'log.txt'. If you don't care about saving the output:

set log [exec executable_file >@stdout]

Use open "| ..." and asyncronous linewise reading from the returned descriptor, like this:

proc ReadLine fd {
  if {[gets $fd line] < 0} {
    if {[chan eof $fd]} {
      chan close $fd
      set ::forever now
      return
    }
  }
  puts $line
}

set fd [open "| ./executable_file"]
chan configure $fd -blocking no
chan event $fd readable [list ReadLine $fd]

vwait forever

See this wiki page for more involved examples.

In a real program you will probably already have an event loop running so there would be no need for a vwait specific to reading the output of one command.

Also if you need to collect the output, not just [puts] each line after it has been read, you will pobably need to create a global (usually namespaced) variable, initialize it to "", pass its name as another argument to the callback procedure (ReadLine here) and append the line to that variable's value.

May be you can launch another process in background before your executable, like tail -f logfile.txt, and outputting the results of your executable in this logfile.txt ?