Controlling user input


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# 1  
Controlling user input

I'm trying to use a bash script for a psych experiment that involves listening to sound files and responding. If I have something like the code below, how can I make sure that a key press is assigned to RESPONSE only after the second echo statement?

Code:
for i in 1 2 3; do
 echo "Ready?"
 sleep 2
 mpg123 $i.mp3
 echo "Ok now you can respond. Press 1 if you liked it, 2 if you didn't."
 read -n 1 RESPONSE
done

Also, maybe this is the same question, but how can I make sure that if there are accidentally two or more key presses, that the extra presses are ignored instead of getting assigned to RESPONSE on the next iteration(s)?
# 2  
It is pretty easy in C with fseek(stdin, 0, SEEK_END);, but I don't know of any portable (or even any way without explicit keyboard actions initiated by the user at the terminal) way to do that with shell code.

Have you considered writing this in C?

Note also that asking a question (e.g. Ready?) and expecting users not to respond seems like you're asking for unwanted input.
# 3  
Since you specify using bash shell we can flush the input buffer using:

Code:
while read -rs -N 1 -t 0.1 2> /dev/null ; do : ; done

so your script would look like this:

Code:
#!/bin/bash

for i in 1 2 3; do
 echo "Ready?"
 sleep 2
 mpg123 $i.mp3
 echo "Ok now you can respond. Press 1 if you liked it, 2 if you didn't."
 while read -rs -N 1 -t 0.1 2> /dev/null ; do : ; done
 read -n 1 RESPONSE
done

These 2 Users Gave Thanks to Chubler_XL For This Post:
# 4  
Quote:
Originally Posted by Chubler_XL
Since you specify using bash shell we can flush the input buffer using:

Code:
while read -rs -N 1 -t 0.1 2> /dev/null ; do : ; done

so your script would look like this:

Code:
#!/bin/bash

for i in 1 2 3; do
 echo "Ready?"
 sleep 2
 mpg123 $i.mp3
 echo "Ok now you can respond. Press 1 if you liked it, 2 if you didn't."
 while read -rs -N 1 -t 0.1 2> /dev/null ; do : ; done
 read -n 1 RESPONSE
done

Hi Chubler_XL,
That might work with some versions of bash, but it won't work with 3.2.57 (which comes with the most recent release of macOS Mojave) since the read built-in in that version doesn't have a -N option. The loop you suggest above silently throws away the diagnostic saying there is no -N option and gives no indication that whatever was intended to be done by the while loop didn't do anything. The following read then reads the next available character no matter when it was typed.

Since darwin_866 has told us neither what OS is being used nor which version of bash is being used, we don't have any way to know if what you suggested might work in his/her environment.

Cheers,
Don
# 5  
Works great in ubuntu linux with bash 5.0.3(1)-release. Thanks both!
This User Gave Thanks to darwin_886 For This Post:
# 6  
Quote:
Originally Posted by darwin_886
Works great in ubuntu linux with bash 5.0.3(1)-release. Thanks both!
I also tested the script and it worked fine for me on Ubuntu.

Code:
ubuntu:/tmp# read --help
read: read [-ers] [-a array] [-d delim] [-i text] [-n nchars] [-N nchars] [-p prompt] [-t timeout] [-u fd] [name ...]

Seemed to "kinda work" on macOS if I changed the -N to -n but then seemed to have an issue with the -t option so I moved on to other tasks.

Code:
mojave_10_14_6:tmp $ read --help
-bash: read: --: invalid option
read: usage: read [-ers] [-u fd] [-t timeout] [-p prompt] [-a array] [-n nchars] [-d delim] [name ...]

Glad you got it working darwin_886.
# 7  
BASH allows you to use the basic TTY properties to establish timeouts and read minimums:

Code:
#!/bin/bash

# Timeout is 1/10 of a second, minimum read is 0 characters
stty -icanon min 0 time 1

sleep 3 & # Background command

# Eat any characters until 'sleep' quits
while [ -d /proc/"$!" ] ; do read G ; done

wait

printf "%s" "OK, now respond 1 or 2: "

while true
do
        case "$VAR" in
        1|2)    break           ;;
        *)      read VAR        ;;
        esac
done

echo
echo "You responded:  $VAR"

stty sane # Reset terminal to normal

Haven't got it working in KSH yet. Can't convince it to not mess with the TTY.
These 2 Users Gave Thanks to Corona688 For This Post:
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