How to track a log file in TCL

Question

Say there is a file log.txt and some kind of log is being appended to it permanently.

I want to track that file in the TCL environment.

I have tried this but it didn't worked.

set log [open log.txt a]

for { } { true } { update; after 1000 } {

    # expected to get here the appended part
    read $log

    seek $log 0 end

}

Is it possible to read the modified file by the same file handle log, or I have to close and re-open the file log.txt ?

Is there a kind of equivalent of Linux command tail -f in TCL ?

Answer 1

Just use tail. It knows more that you about how to handle complicated cases (you can look at its source).

In one of my projects, I have something like this to monitor a trace file produced by a proprietary tool:

set fd [open [list | tail --follow=name --retry --lines 0 $opt(trace) 2>@1]]
chan event $fd readable [list FollowTrace $fd]

proc FollowTrace fd {
  if {[gets $fd line] < 0} {
    set code [catch {close $fd} err]
    if {$code == 0} {
      set ::result 0
    } else {
      puts stderr $err
      set ::result 1
    }
    return
  }

  switch -regexp -matchvar parts -- $line {
    {Tm_Session::Open.*FileName=([^,]+)} {
       TryMakeLock [FullPathname [lindex $parts 1]]
     }
     {Tm_Session::Close.*FileName=([^,]+)} {
        StartUpload [lindex $parts 1]
     }
  }
}

The general idea is that you spawn tail on a given file then enter event loop and process tail's output line-by-line.