Use read as a prompt inside a while loop driven by read?
If I got this right, I think you want to basically loop over lists of values, and then read
another within the loop.
Here's a few options, 1 and 2 are probably the sanest.
1. Emulate arrays with strings
Having 2D arrays would be nice, but not really possible in Bash. If your values don't have whitespace, one workaround to approximate that is to stick each set of three numbers into a string, and split the strings inside the loop:
for x in "1 2 3" "4 5 6"; do
read a b c <<< "$x";
read -p "Enter a number: " d
echo "$a - $b - $c - $d ";
done
Of course you could use some other separator too, e.g. for x in 1:2:3 ...
and IFS=: read a b c <<< "$x"
.
2. Replace the pipe with another redirection to free stdin
Another possibility is to have the read a b c
read from another fd and direct the input to that (this should work in a standard shell):
while read a b c <&3; do
printf "Enter a number: "
read d
echo "$a - $b - $c - $d ";
done 3<<EOF
1 2 3
4 5 6
EOF
And here you can also use a process substitution if you want to get the data from a command: while read a b c <&3; ...done 3< <(echo $'1 2 3\n4 5 6')
(process substitution is a bash/ksh/zsh feature)
3. Take user input from stderr instead
Or, the other way around, using a pipe like in your example, but have the user input read
from stderr
(fd 2) instead of stdin
where the pipe comes from:
echo $'1 2 3\n4 5 6' |
while read a b c; do
read -u 2 -p "Enter a number: " d
echo "$a - $b - $c - $d ";
done
Reading from stderr
is a bit odd, but actually often works in an interactive session. (You could also explicitly open /dev/tty
, assuming you want to actually bypass any redirections, that's what stuff like less
uses to get the user's input even when the data is piped to it.)
Though using stderr
like that might not work in all cases, and if you're using some external command instead of read
, you'd at least need to add a bunch of redirections to the command.
Also, see Why is my variable local in one 'while read' loop, but not in another seemingly similar loop? for some issues regarding ... | while
.
4. Slice parts of an array as needed
I suppose you could also approximate a 2D-ish array by copying slices of a regular one-dimensional one:
data=(1 2 3
4 5 6)
n=3
for ((i=0; i < "${#data[@]}"; i += n)); do
a=( "${data[@]:i:n}" )
read -p "Enter a number: " d
echo "${a[0]} - ${a[1]} - ${a[2]} - $d "
done
You could also assign ${a[0]}
etc. to a
, b
etc if you want names for the variables, but Zsh would do that much more nicely.
There is only one /dev/stdin
, read
will read from it anywhere where it is used (by default).
The solution is to use some other file descriptor instead of 1 (/dev/stdin
).
From code equivalent (in bash) to what you posted [1] (look below)
just add 0</dev/tty
(for example) to read from the "real" tty:
while read a b c
do read -p "Enter a number: " d 0</dev/tty # 0<&2 is also valid
echo "$a -> $b -> $c and ++> $d"
done <<<"$(echo -e '1 2 3\n4 5 6')"
On execution:
$ ./script
Enter a number: 789
1 -> 2 -> 3 and ++> 789
Enter a number: 333
4 -> 5 -> 6 and ++> 333
Other alternative is to use 0<&2
(which might seem odd, but is valid).
Note that the read from /dev/tty
(also 0<&2
) will bypass the stdin of the script, this will not read the values from the echo:
$ echo -e "33\n44" | ./script
Other solutions
What is needed is to redirect one input to some other fd (file descriptor).
Valid in ksh, bash and zsh:
while read -u 7 a b c
do printf "Enter a number: "
read d
echo "$a -> $b -> $c and ++> $d"
done 7<<<"$(echo -e '1 2 3\n4 5 6')"
Or, with exec:
exec 7<<<"$(echo -e '1 2 3\n4 5 6')"
while read -u 7 a b c
do printf "Enter a number: "
read d
echo "$a -> $b -> $c and ++> $d"
done
exec 7>&-
A solution that works in sh (<<<
doesn't work):
exec 7<<-\_EOT_
1 2 3
4 5 6
_EOT_
while read a b c <&7
do printf "Enter a number: "
read d
echo "$a -> $b -> $c and ++> $d"
done
exec 7>&-
But this is probably easier to understand:
while read a b c 0<&7
do printf "Enter a number: "
read d
echo "$a -> $b -> $c and ++> $d"
done 7<<-\_EOT_
1 2 3
4 5 6
_EOT_
1 Simpler code
Your code is:
echo -e "1 2 3\n4 5 6" |\
while read a b c;
do
echo "$a -> $b -> $c";
echo "Enter a number: ";
read d ;
echo "This number is $d" ;
done
A simplified code (in bash) is:
while read a b c
do #0</dev/tty
read -p "Enter a number: " d ;
echo "$a -> $b -> $c and ++> $d";
done <<<"$(echo -e '1 2 3\n4 5 6')"
Which, if executed, prints:
$ ./script
1 -> 2 -> 3 and ++> 4 5 6
Which is just showing that the var d is being read from the same /dev/stdin
.
With zsh
, you can write it instead:
for a b c (
1 2 3
4 5 6
'more complex' $'\n\n' '*** values ***'
) {
read 'd?Enter a number: '
do-something-with $a $b $c $d
}
For 2D arrays, see also the ksh93
shell:
a=(
(1 2 3)
(4 5 6)
('more complex' $'\n\n' '*** values ***')
)
for i in "${!a[@]}"; do
read 'd?Enter a number: '
do-something-with "${a[i][0]}" "${a[i][1]}" "${a[i][2]}" "$d"
done