How to deal with multiline strings and string interpolation?

Question

Say I want to have a template somewhere which is multiline string:

I have some
text with ${placeholders}
in this and some ${different}
ones ${here} and ${there}

What would be my best way of replacing the placeholders with input from a user? Would here-documents be a good use?

Answer 1

Assuming ^[a] that no \<newline>, nor the characters \, $, or ` are used in the multiline string (or they are properly quoted), a here-document (and variables) is your best option:

#!/bin/sh

placeholders="value one"
different="value two"
here="value three"
there="value four"

cat <<-_EOT_
I have some
text with ${placeholders}
in this and some ${different}
ones ${here} and ${there}.
_EOT_

If executed:

$ sh ./script
I have some
text with value one
in this and some value two
ones value three and value four.

Of course, correctly playing with qouting, even one variable could do:

$ multilinevar='I have some
> text with '"${placeholders}"'
> in this and some '"${different}"'
> ones '"${here}"' and '"${there}"'.'
$ echo "$multilinevar"
I have some
text with value one
in this and some value two
ones value three and value four.

Both solutions could accept multiline variable placeholders.

---

^[a]From the manual:

... the character sequence \<newline> is ignored, and \ must be used to quote the characters \, $, and `. ...

Answer 2

Here's a way to do this in bash. You need a recent version because I rely on associative arrays here

template=$(cat << 'END_TEMPLATE'
I have some
text with ${placeholders}
in this and some ${different}
ones ${here} and ${there}
END_TEMPLATE
)

mapfile -t placeholders < <(grep -Po '(?<=\$\{).+?(?=\})' <<< "$template" | sort -u)

declare -A data
for key in "${placeholders[@]}"; do
    read -p "Enter the '$key' value: " -r data[$key]
done

t=$template
while [[ $t =~ '${'([[:alnum:]_]+)'}' ]]; do
    t=${t//"${BASH_REMATCH[0]}"/"${data[${BASH_REMATCH[1]}]}"}
    #      ^^^^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    #      |                    + the value the user entered
    #      + the placeholder in the text
done

echo "$t"

Answer 3

Given user inputs one, two, three

The following command will replace all instances of a given ${placeholder} with your input:

sed -i 's/${placeholders}/one/g; s/${different}/two/g; s/${here}/three/g' yourTemplateFile

If you use this in a bash script and have the user input in shell variables this would do all the replacements in one command.

Answer 4

Tested with below script and worked fine

echo "enter count of userinput required"
read c
for ((i=1;i<=$c;i++))
do
echo "enter the values"
read input_$i
done

awk -v i="$input_1" -v j="$input_2" -v k="$input_3" -v l="$input_4"   '{gsub(/\${placeholders}/,i,$0);gsub(/\${different}/,j,$0);gsub(/\${here}/,k,$0);gsub(/\${there}/,l,$0);print $0}' filename >>filename_tmp && mv filename_tmp filename