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Shell variable substitution from within a file.
# Just found the amazing builtin 'mapfile', perfect
# for substitution variable inside a file by its values.
mateus@mateus:/tmp$ cat input.txt
a = $a
b = $b
mateus@mateus:/tmp$ echo a=$a, b=$b
a=1, b=2
mateus@mateus:/tmp$ function subst() { eval echo -E "$2"; }
mateus@mateus:/tmp$ mapfile -c 1 -C subst < input.txt
a = 1
b = 2
# Simpler solution
mateus@mateus:/tmp$ EOF=EOF_$RANDOM; eval echo "\"$(cat <<$EOF
a = 1
b = 2
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