Sockets in Your Shell

Re:Sockets in Your Shell by gricardo99 on Thursday December 03, 2020 @01:42PM (#25292742)

Sure enough, I tested this, and the server up/down is reversed from what it should be:

   > exec 3>/dev/tcp/localhost/4000
     bash: connect: Connection refused
     bash: /dev/tcp/localhost/4000: Connection refused
   > echo $?
    1

But if a localhost port is listening (in another terminal window: > nc -l localhost 4000):

   > exec 3>/dev/tcp/localhost/4000
   > echo $?
     0

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Re:Sockets in Your Shell by dllthomas on Thursday December 03, 2020 @03:25PM (#25294272)

In the quoted code, "server up" is for a zero result - `if` is reversed from what you think it is:

    > if true; then echo yup; fi
    yup
    > true
    > echo $?
    0

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Re:Sockets in Your Shell by gricardo99 on Thursday December 03, 2020 @05:01PM (#25295792)

indeed you are right. Thanks for the reply. I'm too accustomed to seeing code like:

  if [ $? -eq 0 ]; then echo "pass";else echo "fail";fi

But without the expression '[ ... ]' shell/bash does indeed treat zero return code (success) as boolean true. For whatever reason I have not seen much (if any) bash code that simply uses:

  if COMMAND; then ...

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Re:Sockets in Your Shell by fl0wenol on Thursday December 03, 2020 @05:19PM (#25296026)

You really really should use if COMMAND logic, in the event that all you care about is zero or non-zero result. I find it easier to follow for shell control flow.

It also works for shell functions.

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Re:Sockets in Your Shell by dllthomas on Thursday December 03, 2020 @06:44PM (#25297063)

From one perspective,

    if [ $? -eq 0 ]; then ...

is actually an instance of

    if COMMAND; then ...

because [ is itself a command.

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Re:Sockets in Your Shell by gricardo99 on Thursday December 03, 2020 @11:35PM (#25299004)

yep, I think that's the key point I need to drill into my head. The unix/linux idiom where 0 is true/success and non-zero is a failure can be counter-intuitive when combined with "if" (at least for me, hence my mistake reading the code). In most/other computing contexts "if 0" is never taken. I think this might explain why I've seen a lot more bash code that is explicitly written in the form more familiar to "if" usage in other languages (pseudo-code): "if res == 0". But "if COMMAND" is taking care of the interpretation of the return code, so doing the explicit compare is redundant.

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