shell script respond to keypress - bash

Question

I have a shell script that essentially says something like
while true; do
read -r input
if ["$input" = "a"]; then
echo "hello world"
fi
done
That is all well, and good, but I just realized having to hit ENTER presents a serious problem in this situation. What I need is for the script to respond when a key is pressed, without having to hit enter.
Is there a way to achieve this functionality within a shell script?

Answer 1

read -rsn1
Expect only one letter (and don't wait for submitting) and be silent (don't write that letter back).

Answer 2

so the final working snippet is the following:
#!/bin/bash
while true; do
read -rsn1 input
if [ "$input" = "a" ]; then
echo "hello world"
fi
done

Answer 3

Another way of doing it, in a non blocking way(not sure if its what you want). You can use stty to set the min read time to 0.(bit dangerous if stty sane is not used after)
stty -icanon time 0 min 0
Then just run your loop like normal. No need for -r.
while true; do
read input
if ["$input" = "a"]; then
echo "hello world"
fi
done
IMPORTANT!
After you have finished with non blocking you must remember to set stty back to normal using
stty sane
If you dont you will not be able to see anything on the terminal and it will appear to hang.
You will probably want to inlcude a trap for ctrl-C as if the script is quit before you revert stty back to normal you will not be able to see anything you type and it will appear the terminal has frozen.
trap control_c SIGINT
control_c()
{
stty sane
}
P.S Also you may want to put a sleep statement in your script so you dont use up all your CPU as this will just continuously run as fast as it can.
sleep 0.1
P.S.S It appears that the hanging issue was only when i had used -echo as i used to so is probably not needed. Im going to leave it in the answer though as it is still good to reset stty to its default to avoid future problems.
You can use -echo if you dont want what you have typed to appear on screen.

Answer 4

You can use this getkey function:
getkey() {
old_tty_settings=$(stty -g) # Save old settings.
stty -icanon
Keypress=$(head -c1)
stty "$old_tty_settings" # Restore old settings.
}
It temporarily turns off "canonical mode" in the terminal settings
(stty -icanon) then returns the input of "head" (a shell built-in) with the -c1 option which is returning ONE byte of standard input. If you don't include the "stty -icanon" then the script echoes the letter of the key pressed and then waits for RETURN (not what we want). Both "head" and "stty" are shell built-in commands. It is important to save and restore the old terminal settings after the key-press is received.
Then getkey() can be used in combination with a "case / esac" statement for interactive one-key selection from a list of entries:
example:
case $Keypress in
[Rr]*) Command response for "r" key ;;
[Ww]*) Command response for "w" key ;;
[Qq]*) Quit or escape command ;;
esac
This getkey()/case-esac combination can be used to make many shell scripts interactive. I hope this helps.

Answer 5

How to read a single key press into variable c, and print it out. This prints out the key you pressed instantly, withOUT you having to press Enter first:
read -n1 c && printf "%s" "$c"
Or, with a little more "prettifying" in the output print:
read -n1 c && printf "\nYou Pressed: %s\n" "$c"
Example output of the latter command:
$ read -n1 c && printf "\nYou Pressed: %s\n" "$c"
M
You Pressed: M
To suppress your initial keypress from being echoed to the screen, add the -s option as well, which says from the read --help menu:
-s do not echo input coming from a terminal
Here is the final command:
read -sn1 c && printf "You Pressed: %s\n" "$c"
And a demo:
$ read -sn1 c && printf "You Pressed: %s\n" "$c"
You Pressed: p
You can also optionally separate the -sn1 argument into two arguments (-s -n1) for clarity.
References:
I learned about read -n1 from #pacholik here.
See also:
read_keypress.sh in my eRCaGuy_hello_world repo.
I use this bash cmd in C and C++ system calls to read keys here:
[my answer] Capture characters from standard input without waiting for enter to be pressed
[my answer] Read Key pressings in C ex. Arrow keys, Enter key

Answer 6

I have a way to do this in my project: https://sourceforge.net/p/playshell/code/ci/master/tree/source/keys.sh
It reads a single key everytime key_readonce is called. For special keys, a special parsing loop would run to also be able to parse them.
This is the crucial part of it:
if read -rn 1 -d '' "${T[#]}" "${S[#]}" K; then
KEY[0]=$K
if [[ $K == $'\e' ]]; then
if [[ BASH_VERSINFO -ge 4 ]]; then
T=(-t 0.05)
else
T=(-t 1)
fi
if read -rn 1 -d '' "${T[#]}" "${S[#]}" K; then
case "$K" in
\[)
KEY[1]=$K
local -i I=2
while
read -rn 1 -d '' "${T[#]}" "${S[#]}" "KEY[$I]" && \
[[ ${KEY[I]} != [[:upper:]~] ]]
do
(( ++I ))
done
;;
O)
KEY[1]=$K
read -rn 1 -d '' "${T[#]}" 'KEY[2]'
;;
[[:print:]]|$'\t'|$'\e')
KEY[1]=$K
;;
*)
__V1=$K
;;
esac
fi
fi
utils_implode KEY __V0