how to redirect bash variable to an executable?

Question

I have an executable, say it's called a.out. It takes two lines of input after a prompt --

> ./a.out 
> give me input-1: 0 0 10
> give me input-2: 10 10 5
> this is the output: 20 20 20

I can store the inputs in a file (input.txt) and redirect it to a.out, the file looks like this --

0 0 10
10 10 5

and I can call a.out like --

> ./a.out < input.txt
> give me input-1: 0 0 10 give me input-2: 10 10 5
> this is the output: 20 20 20

Now I want to store multiple inputs in that file and redirect into a.out. The file will look like this with 2 inputs --

and I am writing a bash script like --

exec 5< input.txt
while read line1 <&5; do
      read line2 <&5;
      ./a.out < `printf "$line1\n$line2"` ;
done

It does not work, how do I do that?

It might help to describe more specifically what about your final attempt doesn't work exactly. (From a general high quality question perspective if nothing else.) — Etan Reisner
– Etan Reisner, Commented Sep 22, 2014 at 17:18
Much safer to use printf '%s\n' "$line1" "$line2"; that way you don't introduce bugs if anything in line1 or line2 reads as a format string. — Charles Duffy
– Charles Duffy, Commented Sep 24, 2014 at 16:30

chepner · Accepted Answer · 2014-09-24 16:34:00Z

5

< requires a file name containing content, not the content itself. You probably just want to use a pipe:

exec 5< input.txt
while read line1 <&5; do
    read line2 <&5
    printf "%s\n%s\n" "$line1" "$line2" | ./a.out
done

or a process substitution:

exec 5< input.txt
while read line1 <&5; do
    read line2 <&5
    ./a.out < <(printf "%s\n%s\n" "$line1" "$line2")
done

You don't need to use a separate file descriptor, though. Just redirect standard input to the loop:

while read line1; do
    read line2
    printf "%s\n%s\n" "$line1" "$line2" | ./a.out
done < input.txt

You might also use a here document (but note the indentation):

while read line1; do
    read line2
    ./a.out <<EOF
$line1
$line2
EOF
done < input.txt

or a here string:

while read line1; do
    read line2
    # ./a.out <<< $'$line1\n$line2\n'
    ./a.out <<<"$line1
$line2"
done < input.txt

The newline can be included using the special $'...' quoting, which can specify a newline with \n', or the string can simply have an embedded newline.

If you are using bash 4 or later, you can use the -t option to detect the end of the input, so that a.out can read directly from the file.

# read -t 0 doesn't consume any input; it just exits successfully if there
# is input available.
while read -t 0; do
    ./a.out
done < input.txt

edited Sep 24, 2014 at 16:34

answered Sep 22, 2014 at 17:15

chepner

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6 Comments

Etan Reisner Over a year ago

A HERESTRING would also work (without needing a sub-shell).

chepner Over a year ago

I was just about to add a here string, but then decided to just add a here document, since embedding a newline in the here string requires either Yet Another New Syntax ($'...\n...') or something that looks almost exactly like a here doc anyway.

chepner Over a year ago

OK, I caved on the here string, since I realized process substitution was probably also worth mentioning.

chepner Over a year ago

-t would be my preferred option.

Charles Duffy Over a year ago

Copying printf "$line1\n$line2\n" from the OP is bad juju -- if anything in either of those lines is a format string...

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