How to make search, enter text and replace a line in bash? - bash

Question

I've been trying to write a bash code (experimenting) where bash will ask (Enter Text) and the user will enter a certain text lets say (I am new) and the text (I am new) will be moved to a line called User=I am new.
So, here is a cfg file named xyz.cfg
User
Name
Address
Country
and here is the bash script called test and with only echo $Input > $file the bash replace the whole file with the entered text but I want to keep the config as it is and add the Enter Text: value to the line User but this doesn't help, the code below,
#! /bin/sh
file=/root/xyz.cfg
echo "Enter text:"
read Input
echo $Input > $file **(confused here)** **(it replaces the whole cfg)**
Another line I thought of is here, but it doesn't work. I am not good with these so I hope it's solved; I tried searching the web with zero luck.
(doesn't work)
#! /bin/sh
file=/root/xyz.cfg
echo "Enter text:"
read Input
echo $Input > sed '/User/a' xyz.cfg
(doesn't work)
#! /bin/sh
file=/root/xyz.cfg
echo "Enter text:"
read Input
echo $Input > sed '/User/a' $file
Edit:
When a someone is asked for Enter Text: while running the bash script ./test and the someone typed Entered Text from bash promot the entered $Input (Entered Text from bash promot) should move to xyz.cfg and add the line to the Variable User:
Finally, the xyz.cfg should look like
User: Entered Text from bash prompt
Name
Address
Country
Before it looked like this,
User:
Name
Address
Country
Edit2: This GIF might explain what I need.

Answer 1

This should do the trick:
#!/bin/bash
FILE=/tmp/test.cfg
echo -n "Enter text: "
read INPUT
sed -i "s/User:/& $INPUT/" "$FILE"
This uses sed to do an in-place search / replace (-i) which appends the entered text after the first occurence of User: on each line within $FILE.
& refers to the matched portion of the text - it's essentially saying "replace <thing> with <thing> <entered text>."
If you need to prompt for text each time a matching User: line is encountered, you can use the following approach:
#!/bin/bash
FILE=/tmp/test.cfg
ORIGINAL=$(cat "$FILE")
truncate -s 0 "$FILE"
while read LINE; do
if `grep -q 'User:' <<< "$LINE"`; then
echo -n "Enter text: "
read INPUT < /dev/tty
sed "s/User:/& $INPUT/" <<< "$LINE" >> "$FILE"
else
echo "$LINE" >> "$FILE"
fi
done <<< "$ORIGINAL"
This second approach does the following:
Reads the entire file contents into a variable;
Removes the existing file contents;
Loops over each line of the original content, and:
If the line contains User:, prompt for and append the custom text;
Append the line to the file.