topic Re: std in & read problem in Operating System - Linux

std in & read problem

amit singh_3 — Mon, 01 Mar 2004 03:46:06 GMT

hey ,

I am trying following command on HP-UX,
#date |cut -d " " -f 2,3,6 |read var
#echo $var
& it is showing me the value of filtered
date command.

But when I am trying the following command on
Linux ,
#echo $var ,is not showing any things.
why?

Amit singh

Re: std in & read problem

Robert Binkhorst — Mon, 01 Mar 2004 04:24:53 GMT

Hi,

You're using a different shell on Linux (Bash) than on HP-UX (ksh). You can install pdksh on Linux to also get a korn shell (ksh) there, or you could look into bash. Follow this link for more info: http://www.tldp.org/LDP/abs/html/

Anyway, try this one:
# var=`date |cut -d " " -f 2,3,6`
# echo $var

HTH,

Robert

Re: std in & read problem

Mark Grant — Mon, 01 Mar 2004 04:27:10 GMT

I like what you are trying to do but you are going about it the wrong way. "date" has a "format" field which will output the date or bits of the date any how you like.

var=`date +"%d %m %y"`

Will get you what you want.

The reason your thing didn't work on Linux is because thre is no field 6 from "date" on linux .

THe above command will work on HPUX and Linux

Re: std in & read problem

Mark Grant — Mon, 01 Mar 2004 04:30:05 GMT

Ooops. There is a field 6 in the Linux "date".

However, "read" on Linux does not read from a pipe. It should do though *scratches head*, but doesn't.

Re: std in & read problem

Stuart Browne — Mon, 01 Mar 2004 19:00:55 GMT

pdksh behaves in the same manner.

In both bash and ksh, you can also do the var=$(cut -d " " -f2,3,6) method.

What's odd is that while 'cut -d " " -f 2,3,6 | read var' doesn't work, the following does:

cut -d " " -f 2,3,6 | while read var; do echo $var;done

I'm still working on trying to figure *that* one out..

I don't have a HPUX box handy, does anybody have what the IFS is for that? ("echo $IFS | od -x" or thorugh hexdump or something).

Both bash and ksh work with the while in there. My thought is it has something to do with the IFS, but.. need more experimentation.

Re: std in & read problem

Stuart Browne — Mon, 01 Mar 2004 19:19:11 GMT

I poked a SCO OSR box, and it behaves in the same manner as the HP-UX shell mentioned above.

The IFS is the same as the Linux box..

The mystery continues..

Re: std in & read problem

Stuart Browne — Mon, 01 Mar 2004 19:44:46 GMT

Ahh, and here's the kicker!

This has to do with the way bash/pdksh deal with pipelined commands.

From the man page of 'bash':

Each command in a pipeline is executed as a separate process (i.e., in a subshell).

Translation:

As the 'read' is in a pipe, a sub-shell is created. The sub-shell thus has a private environment space, and can't be propergated back *up* the environment tree to make it into your current environment.

So, either using "var=`command`", or "var=$(command)" are the only ways to fill a variable from the output of another command.

*Wheee!* :) love a good mystery ;)

Re: std in & read problem

Stuart Browne — Thu, 11 Mar 2004 00:23:17 GMT

And the continuation of a good mystery, is a solution!

Whee!!

Reading through the advanced bash scripting guide (whoa, head spin), it brings up this lovely solution..

It took me 3-4 reads to understand what the hell was going on, but once figured out, it makes sense..

Anyway!:

read var < <(date | cut -d " " -f 2,3,6) ; echo $var

This introduces the variable 'var' to to the current environment, not to a subshell as it would using |'s.

This is a 'bash' solution, and am unsure if it would work in a 'pdksh' or posix 'ksh' environment.

Hope this helps! :)

(Note: brief explanation..

<(COMMAND)

Basically it says that the output of 'command' will get thrown to a file descriptor. As the above does a '< <(COMMAND)', it says take the output of 'COMMAND', and shove it into the STDIN of the read. Thus you get the effect of a pipe, without creating a subshell.)