Using pipe in bash - is this the expected behaviour?

Login or Register to Ask a Question and Join Our Community

Using pipe in bash - is this the expected behaviour?

Tags

Login to Discuss or Reply to this Discussion in Our Community

Top Forums Shell Programming and Scripting Using pipe in bash - is this the expected behaviour?

05-23-2012

Registered User

295, 6

Join Date: May 2009

Last Activity: 7 May 2020, 5:18 PM EDT

Posts: 295

Thanks Given: 62

Thanked 6 Times in 6 Posts

Using pipe in bash - is this the expected behaviour?

Hi all,

Am trying to convert a script from ksh to bash

. One of the sub is something like below:

Code:

#!/bin/bash

declare -a array01

step_01_test()
{
   local count=0
   ps -ef | grep watch | grep -v grep | awk '{ print $8 }' | while read line
   do
      let count=${count}+1
      array01[${count}]="${line}"
      echo "---> ${array01[*]}"
   done
}

step_02_test()
{
   local count=0
   ps -ef | grep watch | grep -v grep | awk '{ print $8 }' > x.txt
   while read line
   do
      let count=${count}+1
      array01[${count}]="${line}"
      echo "---> ${array01[*]}"
   done < x.txt
   rm x.txt
}

step_01_test
echo "===> ${array01[*]}"
echo
echo "......"
echo
step_02_test
echo "===> ${array01[*]}"

exit 0

The output of running the above script is as below:

Code:

./array_01.bash
---> [watchdog/0]
---> [watchdog/0] [watchdog/1]
---> [watchdog/0] [watchdog/1] [watchdog/2]
---> [watchdog/0] [watchdog/1] [watchdog/2] [watchdog/3]
---> [watchdog/0] [watchdog/1] [watchdog/2] [watchdog/3] [watchdog/4]
---> [watchdog/0] [watchdog/1] [watchdog/2] [watchdog/3] [watchdog/4] [watchdog/5]
---> [watchdog/0] [watchdog/1] [watchdog/2] [watchdog/3] [watchdog/4] [watchdog/5] [watchdog/6]
---> [watchdog/0] [watchdog/1] [watchdog/2] [watchdog/3] [watchdog/4] [watchdog/5] [watchdog/6] [watchdog/7]
===>

......

---> [watchdog/0]
---> [watchdog/0] [watchdog/1]
---> [watchdog/0] [watchdog/1] [watchdog/2]
---> [watchdog/0] [watchdog/1] [watchdog/2] [watchdog/3]
---> [watchdog/0] [watchdog/1] [watchdog/2] [watchdog/3] [watchdog/4]
---> [watchdog/0] [watchdog/1] [watchdog/2] [watchdog/3] [watchdog/4] [watchdog/5]
---> [watchdog/0] [watchdog/1] [watchdog/2] [watchdog/3] [watchdog/4] [watchdog/5] [watchdog/6]
---> [watchdog/0] [watchdog/1] [watchdog/2] [watchdog/3] [watchdog/4] [watchdog/5] [watchdog/6] [watchdog/7]
===> [watchdog/0] [watchdog/1] [watchdog/2] [watchdog/3] [watchdog/4] [watchdog/5] [watchdog/6] [watchdog/7]

As you can see, am not getting the contents of the arrays if I use a pipe with the while loop.

Is this the expected behavior for bash

... Am kinda hoping I can avoid having to create a temporary file just like how am doing it on step_02_test.

What am wanting to accomplish is to be able to put into the array the output of the ps command having to redirect to a file and read from it. Maybe I don't need to use a while loop?

BTW also tried the following:

Code:

step_03_test()
{
   local count=0
   STDOUT=$( ps -ef | grep watch | grep -v grep | awk '{ print $8 }' )
   while read line
   do
      let count=${count}+1
      array01[${count}]="${line}"
      echo "---> ${array01[*]}"
   done <<< "$STDOUT"
}

Unfortunately, the output from step_03_test gives as below:

Code:

---> [watchdog/0] [watchdog/1] [watchdog/2] [watchdog/3] [watchdog/4] [watchdog/5] [watchdog/6] [watchdog/7]
---> [watchdog/0] [watchdog/1] [watchdog/2] [watchdog/3] [watchdog/4] [watchdog/5] [watchdog/6] [watchdog/7]
---> [watchdog/0] [watchdog/1] [watchdog/2] [watchdog/3] [watchdog/4] [watchdog/5] [watchdog/6] [watchdog/7]
---> [watchdog/0] [watchdog/1] [watchdog/2] [watchdog/3] [watchdog/4] [watchdog/5] [watchdog/6] [watchdog/7]
---> [watchdog/0] [watchdog/1] [watchdog/2] [watchdog/3] [watchdog/4] [watchdog/5] [watchdog/6] [watchdog/7]
---> [watchdog/0] [watchdog/1] [watchdog/2] [watchdog/3] [watchdog/4] [watchdog/5] [watchdog/6] [watchdog/7]
---> [watchdog/0] [watchdog/1] [watchdog/2] [watchdog/3] [watchdog/4] [watchdog/5] [watchdog/6] [watchdog/7]
---> [watchdog/0] [watchdog/1] [watchdog/2] [watchdog/3] [watchdog/4] [watchdog/5] [watchdog/6] [watchdog/7]

Feedback/Advise much appreciated. Thanks.

Last edited by newbie_01; 05-23-2012 at 02:22 AM.. Reason: Added more information

newbie_01

View Public Profile for newbie_01

Find all posts by newbie_01

05-23-2012

Registered User

23,310, 4,623

Join Date: Aug 2005

Last Activity: 7 July 2020, 11:47 AM EDT

Location: Saskatchewan

Posts: 23,310

Thanks Given: 1,331

Thanked 4,623 Times in 4,217 Posts

Code:

ps -ef | grep watch | grep -v grep | awk '{ print $8 }' | while read line

This is a KSH-only construct. No ordinary Bourne shell lets you put 'while read' on the end of a pipe. You set variables, but they don't stick around, because they're being set in a subshell, not in your shell.

Cramming it all into backticks will do strange things since it will split wherever it pleases, not on lines.

I'd redirect it into a temp file (/tmp/$$ perhaps) then read from that in the loop.

Code:

while read LINE
do
...
done < inputfile

Corona688

View Public Profile for Corona688

Visit Corona688's homepage!

Find all posts by Corona688

Login or Register to Ask a Question

Previous Thread | Next Thread

10 More Discussions You Might Find Interesting

1. UNIX for Beginners Questions & Answers

[BASH] eval command not expanding variables as expected.

Hi Guys, I wrote a collection of bash functions years ago and now need to use them again but I'm getting some error messages when eval tries to expand the variables names. I recollect that I used the shopt command to set one of the options but I can't quite remember the command that I...

2. UNIX for Beginners Questions & Answers

Bash script does not work as expected

Repeat this text in a file named notes.txt and run the script Before bash is a good language a blank line appears Also, the following notes are displayed incorrectly What is bad? ================================== Title : Note 1 ================================== Category: Computer Date...

3. Shell Programming and Scripting

Bash script unable to disable expected output

I'm trying to understand why a script behaves different when run through a pipe. My OS: Linux myip 3.13.0-92-generic #139-Ubuntu SMP Tue Jun 28 20:42:26 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux This script (myscript.sh): #!/bin/bash echo whoami: whoami echo who: who echo who...

4. UNIX for Advanced & Expert Users

[BASH] Getopts/shift within a function, unexpected behaviour

Hello Gurus :) I'm "currently" (for the last ~2weeks) writing a script to build ffmpeg with some features from scratch. This said, there are quite a few features, libs, to be downloaded, compiled and installed, so figured, writing functions for some default tasks might help. Specialy since...

5. Red Hat

Device-mapper behaviour booting on init=bin/bash

Good morning Recently we needed to change the password from a redhat 6.5 system that no one knew the root password. Starting the system with the init=/bin/bash method took us to the following scenario: system_vg active with only root_lv and tmpfs mounted. our entries at fstab are like...

6. Shell Programming and Scripting

Bash syntax behaviour : [[ vs [

Hello. In the following : RESTORE_FF contain a file name : a_file.tar.gz I am testing in a directory if "a_file.tar.gz" exists and or if any file like "a_file.tar.gz" exists. So "a_file.tar.gz" will give me file exists So "a_file.tar.gz." will give me file exists So...

7. Shell Programming and Scripting

Bash won't exit as expected

Hi there, following code snippet should output nothing, IMHO. But the result is "THE END". #!/bin/bash if true ; thenexit fi | grep "somesearchstring" echo "THE END"using bash 4.1.9(1) Bug or feature? Hagen

8. Shell Programming and Scripting

bash script error with binary operator expected.

Hello, I am not sure, where I am missing in the scirpt, I am trying to grep few users from /etc/passwd file and if exists, I added line to echo as user exist, if not create it. #!/bin/bash for vid in v707 z307 z496 z163 z292 ; do if then echo " $vid User exists " else ...

9. Shell Programming and Scripting

bash variable (set via awk+sed) not working as expected

Hi! Been working on a script and I've been having a problem. I've finally narrowed it down to this variable I'm setting: servername=$(awk -v FS=\/ '{ print $7 } blah.txt | sed 's\/./-/g' | awk -v FS=\- '{print $1}')" This will essentially pare down a line like this: ...

10. Shell Programming and Scripting

Bash script on startup does not respond as expected

Hi, I have a bash script which I have referenced in the rc.local of my fedora linux OS. However it doesnt respond the same as when run in terminal from fedora. The bash script has a series of interactive questions that require user input as shown: #!/bin/bash echo "Do you want to use...

Login or Register to Ask a Question