Using read to assign value to bash variable not working


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# 1  
Using read to assign value to bash variable not working

Hi,

I am attempting to assign the output of the following command, to two bash variables, var1 and var2 using "read," but it doesn't seem to be working.

Code:
[root@xxx ~]# openstack hypervisor stats show | awk -F'|' 'NR==14{print $2,$3}'
 vcpus                  92

[root@xxx ~]# echo $?
0

[root@xxx ~]# openstack hypervisor stats show | awk -F'|' 'NR==14{print $2,$3}' | read var1 var2

[root@xxx ~]# echo $var1

[root@xxx ~]# echo $var2

Could I get some guidance around what is happening here?

I'm familiar with using var1=$(awk ...) to assign output to a bash variable, but wanted to see how it's done with read.

Thanks.

Last edited by rbatte1; 02-06-2020 at 06:35 AM..
# 2  
This is because commands within a pipeline are run in subshells so the read statement is setting the variables is a subshell which has not impact on the main shell.

You can achieve what you want using process substitution like this:

Code:
# read var1 var2 < <(openstack hypervisor stats show | awk -F'|' 'NR==14{print $2,$3}')
# echo $var1
vcpus

Edit:
Also in bash 4.* and later you can use a shell option to run the last pipeline command in the current shell.
Note this only work for shells without job control, so in an interactive shell (as opposed to a script) you will also need turn off job control (set +m).

Quote:
lastpipe
If set, and job control is not active, the shell runs the last command of a pipeline
not executed in the background in the current shell environment.
Code:
# echo $BASH_VERSION
5.0.7(1)-release
# shopt -s lastpipe
# set +m

# echo A B C | head -1 | read first second rest
# echo $first
A

# set -m
# shopt -u lastpipe


Last edited by Chubler_XL; 01-30-2020 at 10:25 PM..
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# 3  
Hi Chubler_XL,

Appreciate the clarification.

What is the purpose of two "<" ? Does it mean that each value from awk will be assigned to the bash variable, with whitespace as delimiter?

Thanks.
# 4  
the first < is a standard input redirection and causes input to the read command to come from a file.

The second part is process substitution witch takes the format of <(list). The value list can be a command or a pipeline of commands which is executed and the output is a file name of a named pipe that contains the output of list.
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# 5  
Here are some more alternatives to try:

Use a here string
Code:
read var1 var2 <<< "$(openstack hypervisor stats show | awk -F'|' 'NR==14{print $2,$3}')"

Use a here document
Code:
read var1 var2 << EOB
  $(openstack hypervisor stats show | awk -F'|' 'NR==14{print $2,$3}')
EOB

Use a code block (the variables can be used inside the code block)
Code:
openstack hypervisor stats show | awk -F'|' 'NR==14{print $2,$3}' |
{
  read var1 var2
  echo "$var1"
  echo "$var2"
}

Use the if statement as block and test if the variables are being read (the variables can be used inside the if statement).
Code:
openstack hypervisor stats show | awk -F'|' 'NR==14{print $2,$3}' |
if read var1 var2; then
  echo "$var1"
  echo "$var2"
fi

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