[SOLVED] how to globally replace backslash with double backslash in a text file using bash?

Issue

I create a file using nano file.txt . I typed in Hello hit enter and then typed mate so the input file looks like this:

Hello
mate

How do I replace the linefeed character in this file with \\n so that I can substitute this value later when I build valid json data, i.e. the expected output would be:

{"output":"Hello\\nmate"}

Right now this is my bash script

#!/bin/bash

expected_output=$(<file.txt)
expected_output=${expected_output//\\/\\\\}
JSON_FMT='{"output":"%s"}'
printf "%s" "$expected_output" >> aaa.txt

the file aaa.txt is being read by a different programming language where I decode the json.

Solution

If your input looks like this:

$ cat file
Hello
mate

and you want to replace all LineFeeds except the last one (assuming it's a valid POSIX text file) with \\n then you could do the following using any awk:

$ awk '{rec=rec sep $0; sep="\\\\n"} END{printf "{\"output\":\"%s\"}\n", rec}' file
{"output":"Hello\\nmate"}

---

Original answer when the OP said their input looked like:

$ cat file
Hello\nmate

Using any sed:

$ sed 's/\\/\\\\/' file
Hello\\nmate

$ sed 's/\(.*\)\\\(.*\)/{"output":"\1\\\\\2"}/' file
{"output":"Hello\\nmate"}

Answered By - Ed Morton

Answer Checked By - Mildred Charles (WPSolving Admin)