UNIX : Substitute value of variable inside script - unix

Question

I may not have got the title perfect for this question, but I am wondering if there is a way to do the following :
Basically, I have a text file with some key value pairs and also a statement (in the same text file) which will be extracted by a shell script and which needs to also simultaneously substitute the A, B, C in the STATEMENT variable .
To make things simple, let me provide an example.
Here is my text file :
File : values.txt
A=1
B=2
C=3
STATEMENT=apple A orange B grape C
Also, I have a shell script which extracts these values and the statement from the text file and uses the STATEMENT variable as a parameter to another script it calls, something like:
Script : first_script.sh
A=`cat values.txt | grep -w '^A' | cut -d'=' -f2`
B=`cat values.txt | grep -w '^B' | cut -d'=' -f2`
C=`cat values.txt | grep -w '^C' | cut -d'=' -f2`
STATEMENT=`cat values.txt | grep -w 'STATEMENT' | cut -d'=' -f2`
second_script.sh $STATEMENT
As you can see, second_script is called from within first_script and it uses the STATEMENT variable, so what I expect to see with the second_script call is :
second_script.sh apple 1 orange 2 grape 3
Note that "A", "B" and "C" should get substituted to their values 1, 2 and 3.
However, what I get is still :
second_script.sh apple A orange B grape C
which is what I don't want.
How do I make sure that A, B and C get substituted to 1,2 and 3 respectively when second_script is called from first_script ?
Sorry to make it confusing.

Answer 1

You can substitute values in a string like this:
STATEMENT=${STATEMENT/A/$A} # replace the letter A with the value of $A
STATEMENT=${STATEMENT/B/$B}
STATEMENT=${STATEMENT/C/$C}
second_script.sh "$STATEMENT"
If you want to match word boundaries use sed:
STATEMENT=$(sed -e "s/\bA\b/$A/g" -e "s/\bB\b/$B/g" -e "s/\bC\b/$C/g" <<< "$STATEMENT")
Also, you don't need to use cat in your script. You can do it like this:
A=`grep -w '^A' values.txt | cut -d'=' -f2`
Or, using awk:
A=$(awk -F= '/^A\y/{print $2}' values.txt)
Alternatively:
Are you able to change the values.txt file?
It would be easier if you could change it to:
A=1
B=2
C=3
STATEMENT="apple $A orange $B grape $C"
Then in your script you could simply import the file like this:
. values.txt
second_script.sh "$STATEMENT"

Answer 2

How about this:
#!/bin/bash
tail -1 values.txt | {
IFS='=' read name statement
./second_script.sh $(echo "$statement" | sed -r $(
head --lines=-1 values.txt | while IFS='=' read name value
do
printf "%s s/\<%q\>/%q/g " "-e" "$name" "$value"
done))
}
I don't like hard coded solutions, so this will read the values.txt file and evaluate all variables (all but last line) in the statement (last line) and give this to the second_script.sh.
I'd be careful about names and values containing special characters like spaces etc. Run thorough test if you plan on using these!