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Catching a carriage return in bash
23rd of October 2006
I'm not a bash expert. Now I need some help with some bash syntax.
I copied a function called get_key which takes a 1 character length string from the stty input and assigns it to a variable. It's nifty because I can prompt something like this:
1 - Task XYZ
2 - Task F19
3 - Task 123
q - quit
So far so good. What I now want to do is to introduce a possible default. If you just hit enter the variable becomes a carriage return I think. If that's the case I have some other stuff that will kick in and find choose a default value. Here's the code:
get_key Q; # assigns one character to variable Q
if [ "$Q" = "\n" ]; then
echo "Q was blank!";
else
echo "|$Q|"; # for debugging what Q is
fi
The problem is that when I run this it never prints "Q was blank!". Here's the output when I just hit the Enter key when it runs:
|
|
Is the syntax not right? How do I compare if a variable is just a linkbreak/carriage return?
UPDATE
It's quite clear that my get_key function must do something strange. This little script will print "is" when you run it:
if [ "$Q" = "\n" ]; then
echo "is";
fi
The get_key function is defined like this:
{
[ -t 0 ] && { ## Check whether input is coming from a terminal
[ -z "$_STTY" ] && {
_STTY=$(stty -g) ## Store the current settings for later restoration
}
## By default, the minimum number of keys that needs to be entered is 1
## This can be changed by setting the dd_min variable
## If the TMOUT variable is set greater than 0, the time-out is set to
## $TMOUT seconds
if [ ${TMOUT:--1} -ge 0 ]
then
_TMOUT=$TMOUT
stty -echo -icanon time $(( $_TMOUT * 10 )) min ${dd_min:-1}
else
stty -echo -icanon min ${dd_min:-1}
fi
}
## Read a key from the keyboard, using dd with a block size (bs) of 1.
## A period is appended, or command substitution will swallow a newline
_KEY=$(dd bs=1 count=1 2>/dev/null; echo .)
_KEY=${_KEY%?} ## Remove the period
## If a variable has been given on the command line, assign the result to it
[ -n "$1" ] &&
## quoting
case $_KEY in
"'") eval "$1=\"'\"" ;;
*) eval "$1='$_KEY'" ;;
esac
[ -t 0 ] && stty "$_STTY" ## reset terminal
[ -n "$_KEY" ] ## Succeed if a key has been entered (not timed out)
}
UPDATE2
Thank you Ivo! That solved the problem! :)
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Comment
#!/bin/bash
m() {
cat << EOP
Select task:
1 - Task XYZ
2 - Task F19
3 - Task 123
q - quit
EOP
}
k="1"
until [[ $k == [qQ] ]]; do
m
echo "enter 1, 2, 3, q or just hit Enter"
read -n 1 k
case $k in
1) echo "doing task XYZ" ;;
2) echo "doing task F19" ;;
3) echo "doing task 123" ;;
'') echo "Q was blank";;
q) echo "quitting now" && exit 0 ;;
*) echo "invalid key" ;;
esac
done
In case anyone's looking for the answer:
$ x="
"
$ if [ "$x" = $'\n' ]; then echo "newline"; fi
newline