[SOLVED] Resolve variable in bash

Issue

I have a file contains a few lines, and in some lines there is a variable like this:

The-first-line
The-second-${VARIABLE}-line
The-third-line

In a bash scrip, I want to read the lines and resolve the variable wherever exists.

VARIABLE=Awesome
LINES=$(echo $(cat file.txt))
for i in $LINES
do :
  echo "$i"
done

The output is same as the input file (unresolved variables) but I want something like this:

The-first-line
The-second-Awesome-line
The-third-line

Thanks in advance for any help!

Solution

You can try the following (with a recent enough version of bash that supports namerefs):

while IFS= read -r line; do
  while [[ "$line" =~ (.*)\$\{([^}]+)\}(.*) ]]; do
    declare -n var="${BASH_REMATCH[2]}"
    printf -v line '%s%s%s' "${BASH_REMATCH[1]}" "$var" "${BASH_REMATCH[3]}"
  done
  printf '%s\n' "$line"
done < file.txt

In the innermost loop we iterate as long as there is a ${VARIABLE} variable reference, that we replace by the variable's value, thanks to BASH_REMATCH, the var nameref and the -v option of printf.

Warning: if you have a variable named, e.g., VARIABLE and which value is literally ${VARIABLE}, this script will enter an infinite loop.

Answered By - Renaud Pacalet

Answer Checked By - Marie Seifert (WPSolving Admin)