CODEWITHSUNDEEP
tcl
toxic_boi_8041
toxic_boi_8041

Reputation: 1492

TCL-Getting Log of Exec'd Process

Currently I am firing following command

set pid [exec make &]

set term_id [wait pid]

First command will execute makefile inside TCL and Second Command will wait for first command's makefile operation to complete. First command displays all logs of makefile on stdout. Is it possible to store all logs in variable or file when "&" is given in the last argument of exec using redirection or any other method?

If "&" is not given then we can take the output using,

set log [exec make]

But if "&" is given then command will return process id,

set pid [exec make &]

So is it possible stop the stdout logs and put them in variable?

Upvotes: 2

Views: 851

Answers (1)

Donal Fellows
Donal Fellows

Reputation: 137767

If you are using Tcl 8.6, you can capture the output using:

lassign [chan pipe] reader writer
set pid [exec make >@$writer &]
close $writer

Don't forget to read from the $reader or the subprocess will stall. Be aware that when used in this way, the output will be delivered fully-buffered, though this is more important when doing interactive work. If you want the output echoed to standard out as well, you will need to make your script do that. Here's a simple reader handler.

while {[gets $reader line] >= 0} {
    lappend log $line
    puts $line
}
close $reader

Before Tcl 8.6, your best bet would be to create a subprocess command pipeline:

set reader [open |make]

If you need the PID, this can become a bit more complicated:

set reader [open |[list /bin/sh -c {echo $$; exec make}]]
set pid [gets $reader]

Yes, that's pretty ugly…

[EDIT]: You're using Tk, in Tcl 8.5 (so you need the open |… pipeline form from above), and so you want to keep the event loop going. That's fine. That's exactly what fileevent is for, but you have to think asynchronously.

# This code assumes that you've opened the pipeline already
fileevent $reader readable [list ReadALine $reader]
proc ReadALine {channel} {
    if {[gets $channel line] >= 0} {
        HandleLine $line
    } else {
        # No line could be read; must be at the end
        close $channel
    }
}
proc HandleLine {line} {
    global log
    lappend log $line;     # Or insert it into the GUI or whatever
    puts $line
}

This example does not use non-blocking I/O. That might cause an issue, but probably won't. If it does cause a problem, use this:

fconfigure $reader -blocking 0
fileevent $reader readable [list ReadALine $reader]
proc ReadALine {channel} {
    if {[gets $channel line] >= 0} {
        HandleLine $line
    } elseif {[eof $channel]} {
        close $channel
    }
}
proc HandleLine {line} {
    global log
    lappend log $line
    puts $line
}

More complex and versatile versions are possible, but they're only really necessary once you're dealing with untrusted channels (e.g., public server sockets).

If you'd been using 8.6, you could have used coroutines to make this code look more similar to the straight-line code I used earlier, but they're a feature that is strictly 8.6 (and later, once we do later versions) only as they depend on the stack-free execution engine.

Upvotes: 3

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