How to safely escape a variable string (user input) in bash?

Question

I have a very simple chat-like tool that runs within a GNU screen session. The screen window is split, the top part is running tail -f file.txt and the bottom part is running a script with the following content:

#!/bin/bash
while : ; do
        read -p "Message: " msg
        ctime=$(date +"%H:%M:%S")
        echo "[${ctime}] User: ${msg}" >> file.txt
done

Very simple, but gets the job done with the requirements I have. There's only one problem: When I press the ESC or any of the arrow keys, it inserts an escape-sequence, like ^[[D for example. And this messes up the file, resulting in terrible output.

So my question is simple: How can I escape the input from read so it's safe to write to the file?

I've tried echo "[${ctime}] User: ${msg}" | strings >> file.txt which made it a lot better, there were no big mess-ups anymore (e.g. nothing was overwritten or wrongly put out), but things are still not perfect (e.g. entering te^[[Dst would turn into te\n[Dst (the \n being an actual new line)).

Answer 1

How about a slightly different approach? Rather than remove the escape characters and sequences, you can allow users to use them to edit the input line with read -e.

If you want, you can take this even further by recording chat message history, like this:

...
read -e -p "Message: " msg
history -s "$msg"
...

With this, if someone makes a typo in a message, they can hit up-arrow, use left- and right-arrow to edit and fix the typo, then hit return to send the corrected message.

Answer 2

Strip any non-printing character from the message string:

#!/bin/bash
while read -p 'Message: ' message; do
        printf '[%s] User: %s\n' "$( date +%T )"  "${message//[^[:print:]]/}"
done >>file.txt

The parameter substitution ${message//[^[:print:]]/} would expand to the value of the variable message with any non-printable character removed. In the POSIX locale, the printable characters are the alphanumerical characters, the punctuation characters, and space ([[:alnum:][:punct:] ], basically).