print value of environment variable from a file - shell

Question

Problem Description
In my Unix ksh system, I am having an environment variable A on doing
echo ${A}
I am getting the desired value which is hello.
I can check this value in
env | grep ${A}
output: A=hello
or in
printenv | grep ${A}
output: A=hello
Now I have a file file which contains the list of environment variables and I have to fetch the corresponding value.
Now what I tried just for only first value of the file.
env | grep $(cat file | awk 'NR==1{print $1}') --shows nothing
b=$(cat file | awk 'NR==1{print $1}')
env | grep echo $b
b=cat TEMP_ES_HOST_MAP | awk -F"|" 'NR==1{print $2 }'
echo $b
c=eval $b
c=echo $b
Nothing seems to be working.
Thank you

Answer 1

You can use xargs:
awk -F '[$()]+' '{print $1$2}' envfile | xargs printenv
Where:
cat envfile
$(LANG)
$LINES
USER
HISTFILESIZE

Answer 2

If you are looking for the variable named A in the output from env and printenv then using grep ${A} is incorrect (and unsafe/does not work for variables of more than one word).
What you want for that is:
env | grep ^A=
printenv | grep ^A=
So assuming your file looks like this:
VAR1
VAR2
OTHER_VAR
and you want to search for those you could use something like this (assuming you have process substitution):
env | grep -f <(awk '{print "^"$0"="}' varfile)
Assuming you don't have process substitution (or you would rather a simpler solution, I do) you can do this:
env | awk -F = 'NR==FNR{a[$1]++; next} $1 in a' varfile -
Actually this should work too and is even simpler (assuming an awk with ENVIRON):
awk '$1 in ENVIRON {print $1"="ENVIRON[$1]}' varfile