Shell script pipe (STDIN/STDOUT/STDERR)

[Bjarne Dein]

I've seen your website and like it - and thought that somebody here might have the answere to my question.

Let's say I've made a (ksh) script named: "ShowPipes.sh" and run the script in the 3 following ways:

$ echo "This is a test" | ./ShowPipes.sh -f /tmp/pipes.txt
$ ./ShowPipes.sh -f /tmp/pipes.txt
$ ./ShowPipes.sh

How do I test within the script if the first case is true - and catch the piped test "This is a test"?

The last 2 way I know, but the problem is, when fx. catching the left input by

LeftInput=`cat`

it hangs, if there is nothing, so I have to get a way how to "kill" that part, if nothing comes in - and "how do we do that"???

PS. And let me just say, I have made a "few" scripts, and as fare as I can see, there is no "simple" way - or...


Kind Regards

Bjarne Dein System Administrator - Unix

Scandinavian IT Group A/S Direct: +45 3232 4610 Mobile: +45 2322 4610 mailto:Bjarne.Dein@SAS.dk

[Justo Exposito]

Hi Bjarne,

I think that you want to check the number of parameters received, isn't?
If yes you can do by checking the shell variable $# this tell you how many parameters received, there are other shell variables interesting like that:
$* all the parameters
$1 the first parameter
$2 the second parameter
shift command skip to the next parameter

Hope this help you,

Justo.

[Justo Exposito]

Hi Again,

You can put your script here and I try to help you better.

Welcome to the forum.

Regards,

Justo.

[Bjarne Dein]

Ok - here is one of my trials!!!

The big point is NOT, what is comming after the command - like "killme.sh -f /tmp/file.txt", but what might be piped in to it or not!

So the real question is - is it somehow possible to check for pipe-in, like "cat /tmp/file.txt | killme.sh".

If there is some, you can just do:
STDIN1=`cat`
and that's done, but if nothing is comming in, it will HANG!!! ... till EOF ups!!!

Kind Regards

Bjarne

[John Palmer · Accepted solution]

Hi,

The shell is pretty limited in its ability to check the status of a file descriptor. However, you can differentiate between the examples you supplied by checking if standard input is from a terminal
if [[ -t 0 ]];
then
although this won't work for a cron job for instance.

Another possibility is to use the read command to try to read data from standard input.
while read RECORD
do

done

Regards,
John

[Deepak Extross]

This works for me:

#!/usr/bin/sh
if [ -t 0 ]
then
echo "No piped input"
else
read data
echo "Piped input is $data"
fi
echo "Command Line parameters are $*"