http://www.purl.org/tcl/home/man/tcl8.5/TclCmd/open.htm
Examples of open include:
set fd0 [open "simpleinput"] ;# open the file for reading set fd1 [open "simpleinput" "r"] ;# open the file for reading set fd2 [open "reldirect/simpleoutput" "w"] ;# open the file for writing set fd3 [open "/full/path/file" "r+"] ;# open the file to be Read and written set fd4 [open "/tmp/output" {WRONLY BINARY}] ;# Since Tcl 8.5 [http://tip.tcl.tk/183]: open in binary mode for writing set fd4b [open "/tmp/output" "wb"] ;# should be equivalent to the fd4 example. set fd5 [open "|simple1" "r"] ;# Read the stdout of simple1 as fd5 set fd6 [open "|simple2" "w"] ;# Writes to fd6 are read by simple2 on its stdin set fd7 [open "|simple1 |& cat" "r"] ;# Results in stdout AND stderr being readable via fd7 set fd8 [open "|simple2" "r+"] ;# Writes to fd8 are read by simple2 on its stdin, # whereas reads will see the stdout of simple2
Are there other useful open examples?
BR - Access mode "wb" doesn't work with any version of Tcl with which I tried it.
It isn't really needed either as we have fconfigure for that.
Vince -- it would only take a few lines of code added to Tcl's core to automatically interpret "b" for binary.
kennykb reports in the chat room that he recalls it is from Tcl 7.3 days or earlier.
schlenk -- The "b" flag is included in TIP #183, so it is available in Tcl 8.5.
LV -- Don't forget to add an example of using -mode (which is now in open rather than fconfigure) for configuring baud rate, etc. on a serial port...
MJ - Above remark may be a bit vague. What's meant is that fconfigure options that are specific for a certain channel type are documented on the man page for the command creating that type of channel. Hence the -mode option is documented in man open.
One nice feature of Tcl is that one can not only open files, but at least on Unix and Windows, one can also open pipelines to other processes.
(does "pipeline" mean "I can write to this and the other process will read it", or does it also mean "I can read from this what the other process writes" ?) Both. The exec and open commands make a remarkable pair. No other common language has anything approaching their portability, maturity, and applicability.
One variation on the fd8 example above which can be useful for debugging is
set fd8 [open "| tee tmp/in | simple2 | tee tmp/out 2>tmp/err" r+]
In addition to running simple2 as an asynchronous child process, this also creates three files tmp/in, tmp/out, and tmp/err to which will be written everything that passes over simple2's stdin, stdout, and stderr respectively. tee here is a Unix utility that copies everything from stdin to stdout, but optionally also writes all of it to the files given as arguments.
Arjen Markus I have experimented a bit with plain Tcl driving another program. As this needs to work on Windows 95 (NT, ...) as well as UNIX in four or five flavours, I wanted to use plain Tcl, not Expect (however much I would appreciate the chance to do something really useful with Expect - apart from Android :-).
I think it is worth a page of its own, but here is a summary:
set inout [open "|myprog" "r+"] fconfigure $inout -buffering line
Buffering might be out of your hands, though, for "real 16-bit commandline applications", which apparently don't have the ability to flush (reliably) except on close.
Neil Madden - here is a real-life example of interacting with a program through a pipe. The program in question is ispell - a UNIX spell-checking utility. I use it to spell-check the contents of a text widget containing LaTeX markup. There are a number of issues to deal with:
The example does not use fileevent, as this would complicate this particular example. The options passed to ispell are -a (which makes it non-interactive) and -t (which makes it recognize TeX input).
set contents [split [$text get 1.0 end] \n] set pipe [open "|ispell -a -t" r+] fconfigure $pipe -blocking 0 -buffering line set ver [gets $pipe] ;# Ignore the initial version line set linenum 1 foreach line $contents { set wordnum 1 foreach word [split $line] { puts $pipe $word ;# Feed word to ispell while {1} { set len [gets $pipe res] if {$len > 0} { # A valid result # do stuff continue } else { if {[fblocked $pipe]} { # No output break } elseif {[eof $pipe]} { # Pipe closed catch {close $pipe} return } # A blank line - skip } } incr wordnum } incr linenum }
Thanks to Kevin Kenny for helping me figure this out.
Note that specifying "a" for the open argument need NOT be the optimal way of opening a file for appending (a violation of Tcl's normal least surprise policy). The "a" works by seeking to the end of file, whereas in some other languages the vanilla append utilizes a special service provided by the OS. The respecive behaviors can be quite different.
DKF - Here's how to tell whether you've got a problem. Open a file in append mode (with "a" or "a+") several times and write a single line to the file each time (I'd suggest writing digits here; that's easy to understand!) Do not explicitly flush or close those file handles. Instead, exit the program; the language's shut-down code should handle the flushing and closing for you. Then look at the contents of the file produced
How to know if the expected behavior happened? Simple. Check if all the values you wrote actually made it to the file. Order does not matter here. If you got all the lines written, that is good. Your language is doing proper (thread- and process-safe) appending, and this is one of the things that must be supportable on platforms that do POSIX. It's also one of the things that tends to get supported early because it is useful for not losing data from system logs...
But what if not all the data makes it? Well, that's really bad because it means that if you had two programs writing to a log file, you might lose some of the log messages. It happens in Tcl because Tcl tries to do a seek() to the end of the file when it is first opened and then presumes that's good enough. Instead, we get a race condition. Using the APPEND flag (which translates into the low-level O_APPEND flag to the PrivoxyWindowOpen() syscall) alters the semantics of writes so that a seek-to-end happens as an atomic part of the write itself, and that ensures that no data is ever lost (though of course apps still need to take care to only ever write complete records/lines to the file.)
Thanks to dkf for this tip. So, instead of doing "a" or "a+", you currently need to
set f [open $theLogFile {WRONLY CREAT APPEND}] ;# instead of "a"
or
set f [open $theLogFile {RDWR CREAT APPEND}] ;# instead of "a+"
to avoid race conditions.
Does Tcl's current (8.4.1.1 and earlier) behavior of "a" have some benefit over the behavior of APPEND? Or should this difference be considered a bug that is fixed? FW seconds this question.
LES on 11 Apr 2005: This page's last update was stamped "1 Sep 2003" when I came here now and changed it. Has that question been answered yet?
Lars H: I'll hazard an answer. Perhaps the benefit of "a" over APPEND is that it is less surprising?! Imagine a program that appends some data to e.g. a list of changes and then seeks to the start to write an updated timestamp. If someone went in and replaced that "a" with {WRONLY CREAT APPEND} then the surprising result would be that all timestamps ended up at the end of the file, because every puts would seek to the end, would it not?
DKF: If you need random access to a file, you shouldn't use "a". You certainly wouldn't be doing this if you were using stdio.
Some additional discussion of the stderr gimmick above is probably appropriate.
In Alpha, there are many real-life examples of communicating with external processes using pipes. The examples include
as well as standard shells. There are several pages dedicated to these issues on the AlphaTcl Wiki at http://alphatcl.sourceforge.net/wikit/ , see notably http://alphatcl.sourceforge.net/wikit/180 which is an overview page. The problems discussed include how to capture stderr, how to filter out prompts and transform an asynchronous interaction into a synchronous one with timeout mechanisms using vwait, how to link a process to a window, how to provide active cells in an Alpha document, a sort of worksheet interface to an external process, how to provide step functionality (stepping through a list of commands, at each step seeing the result in a console), and a check-as-you-type spellchecker (using aspell).
Recall that Alpha is a text editor, just as powerful and configurable as Emacs, but having Tcl as extension language instead of lisp! The AlphaTcl library (http://alphatcl.sf.net ) is nearly 20000 lines of Tcl code, full of gems and useful tricks. AlphaX runs on OSX, AlphaTk (written entirely in TclTk) runs on any platform.
MG just found out (on July 27, 2005) that Tcl 8.4.9 doesn't seem to be able to open hidden files, at least on Windows XP. For example...
2 % set fid [open test w] file9fa4c0 3 % close $fid 4 % file attributes test -hidden 1 5 % set fid [open test w] couldn't open "test": permission denied 6 % file attributes test -hidden 0 7 % set fid [open test w] file9fda20 8 % close $fid
Anyone know if this is intentional behaviour, or whether it's a bug?
WangFan I use "catch {open "|plink -ssh $account@$host" w+} input}", in which "plink" is download from http://www.chiark.greenend.org.uk/~sgtatham/putty/download.html . When pipe is readable, I read the pipe, but I always get NULL. Does anybody know why? The tcl script is shown below:
package require Tk frame .t text .t.log pack .t.log pack .t set account user set host 10.1.1.2 if [catch {open "|plink -ssh $account@$host" w+} input] { .t.log insert end $input\n return 0 } else { fconfigure $input -blocking 0 -buffering line -translation crlf -eofchar {} fileevent $input readable Log } proc Log {} { global input if [eof $input] { Stop } else { gets $input line .t.log insert end $line\n .t.log see end } return } proc Stop {} { global input catch {close $input} return }
Ken: I need to run 2 scripts located in 2 different files. Both scripts have some common input and output variables that are needed. I intend to use 'open' command but realise it is only for files. Is there any way i can script it so that i can use fileevent handle to receive input and output to the other script? Thanks.
Answer: yes. Details are probably better discussed in comp.lang.tcl or the Chat.
See also:
mariuszgrad - 2009-08-17 15:06:22
I tried code like this:
proc uncompress_tgz { bindata } { set cmd "|tar -C -O -xzf -" set fid [ open $cmd "r+" ] fconfigure $fid -translation binary fconfigure $fid -buffering none fconfigure $fid -blocking false puts -nonewline $fid $bindata puts [ read $fid ] close $fid }
Unfortunately it didn't work out. Problem was that there was no output from tar command.
The solution for it was simple:
set fid [ open $cmd "w" ]
It looks that when $fid was opened only for writing (it was my first access to it) and therefor it stuck when i tried to pull out data from it (with read $fid) command. Greetings