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  • command pipes: puts vs. return

    From [email protected]@21:1/5 to All on Sun May 22 21:02:24 2022
    Hello,

    I have a question on the internal processing of command pipes in Tcl.
    When you open a command pipe via [open "|..."' r+] and run a tcl script,
    the output is available as long as you use "puts" statements in the
    code. However, if instead you return a value as the last command in the script, that return value is lost.

    Is there a way to capture the returns instead, or both?


    More details:

    I am reading the book The Tcl Programming Language, chapter 16 on pipes,
    i.e., the construct known as [open "|..."].

    So I wrote a simple script and saved it in a file as follows:

    file name: c:/temp/sample.tcl
    file data: puts "Current time: [clock format [clock seconds]]"

    Following the advice there, I can call it like so:

    set chan [open "|[list [info nameofexecutable]] C:/temp/sample.tcl" r+]
    chan configure $chan -buffering line
    gets $chan

    And I can see the the printed output. However, if I change the
    sample.tcl as follows,

    return "Current time: [clock format [clock seconds]]"

    then nothing is read from the channel.

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  • From [email protected]@21:1/5 to mango on Sun May 22 21:32:40 2022
    On 5/22/22 9:24 PM, mango wrote:

    I'm not sure what you expectation is here. The original script opens a pipeline to read the standard output of a process as if it were a file. When the original script writes to the standard output (using puts), then indeed the output is seen. When you
    change the script to omit generating standard output, then you seen nothing. That's sorta what I would expect. Not clear to me what your expectations were.



    Hello,

    I was interested in capturing the return value, but using the spawning mechanism. But I see what you mean. Thank you.

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  • From mango@21:1/5 to [email protected] on Sun May 22 18:24:16 2022
    On Sunday, May 22, 2022 at 6:02:30 PM UTC-7, [email protected] wrote:
    Hello,

    [snip]

    Following the advice there, I can call it like so:

    set chan [open "|[list [info nameofexecutable]] C:/temp/sample.tcl" r+]
    chan configure $chan -buffering line
    gets $chan

    And I can see the the printed output. However, if I change the
    sample.tcl as follows,

    return "Current time: [clock format [clock seconds]]"

    then nothing is read from the channel.

    I'm not sure what you expectation is here. The original script opens a pipeline to read the standard output of a process as if it were a file. When the original script writes to the standard output (using puts), then indeed the output is seen. When you
    change the script to omit generating standard output, then you seen nothing. That's sorta what I would expect. Not clear to me what your expectations were.

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Christian Gollwitzer@21:1/5 to All on Mon May 23 08:04:55 2022
    Am 23.05.22 um 03:02 schrieb [email protected]:
    Hello,

    I have a question on the internal processing of command pipes in Tcl.
    When you open a command pipe via [open "|..."' r+] and run a tcl script,

    What's your reason to run another Tcl script by this kind of "forking"?
    Why do you not simply "source otherscript.tcl" to run it?

    I can think of the following reasons:

    - If it is to achieve true concurrency, then checkout the package Thread.

    - If you just need isolation between both scripts, i.e. not to confuse
    variable names, use namespaces or an object system like TclOO or snit.

    - If it is for partially untrusted user scripts, use slave interpreters.

    Christian

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  • From heinrichmartin@21:1/5 to saito on Sun May 22 23:00:44 2022
    On Monday, May 23, 2022 at 3:32:45 AM UTC+2, saito wrote:
    On 5/22/22 9:24 PM, mango wrote:

    I'm not sure what you expectation is here. The original script opens a pipeline to read the standard output of a process as if it were a file. When the original script writes to the standard output (using puts), then indeed the output is seen. When
    you change the script to omit generating standard output, then you seen nothing. That's sorta what I would expect. Not clear to me what your expectations were.


    Hello,

    I was interested in capturing the return value, but using the spawning mechanism. But I see what you mean. Thank you.

    You've just discovered one difference between interactive and non-interactive tclsh: return values are printed (one per script) in interactive mode only.

    Notes:
    * This also means that string values are generated for all such return values (follow-up "shimmering").
    * Another notable difference is the behavior of the unknown command (follow-up "tcl_interactive").

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