Divide an output into multiple variables using shell script - bash

Question

So I have a C program that outputs many numbers. I have to check them all. The problem is, each time I run my program, I need to change seeds. In order to do that, I've been doing it manually and was trying to make a shell script to work around this.
I've tried using sed but couldn't manage to do it.
I'm trying to get the output like this:
a=$(./algorithm < input.txt)
b=$(./algorithm2 < input.txt)
c=$(./algorithm3 < input.txt)
The output of each algorithm program is something like this:
12 13 315
1 2 3 4 5 6 7 8 10 2 8 9 1 0 0 2 3 4 5
So the variable a has all this output, and what I need is
variable a to contain this whole string
and variable a1 to contain only the third number, in this case, 315.
Another example:
2 3 712
1 23 15 12 31 23 3 2 5 6 6 1 2 3 5 51 2 3 21
echo $b should give this output:
2 3 712
1 23 15 12 31 23 3 2 5 6 6 1 2 3 5 51 2 3 21
and echo $b1 should give this output:
712
Thanks!

Answer 1

Not exactly what you are asking, but one way to do this would be to store the results of your algorithm in arrays, and then dereference the item of interest. You'd write something like:
a=( $(./algorithm < input.txt) )
b=( $(./algorithm2 < input.txt) )
c=( $(./algorithm3 < input.txt) )
Notice the extra () that encloses the statements. Now, a, b and c are arrays, and you can access the item of interest like ${a[0]} or $a[1].
For your particular case, since you want the 3rd element, that would have index = 2, hence:
a1=${a[2]}
b1=${b[2]}
c1=${c[2]}

Answer 2

Since you are using the Bash shell (see your tags), you can use Bash arrays to easily access the individual fields in your output strings. For example like so:
#!/bin/bash
# Your lines to gather the output:
# a=$(./algorithm < input.txt)
# b=$(./algorithm2 < input.txt)
# c=$(./algorithm3 < input.txt)
# Just to use your example output strings:
a="$(printf "12 13 315 \n 1 2 3 4 5 6 7 8 10 2 8 9 1 0 0 2 3 4 5")"
b="$(printf "2 3 712 \n 1 23 15 12 31 23 3 2 5 6 6 1 2 3 5 51 2 3 21")"
# Put the output in arrays.
a_array=($a)
b_array=($b)
# You can access the array elements individually.
# The array index starts from 0.
# (The names a1 and b1 for the third elements were your choice.)
a1="${a_array[2]}"
b1="${b_array[2]}"
# Print output strings.
# (The newlines in $a and $b are gobbled by echo, since they are not quoted.)
echo "Output a:" $a
echo "Output b:" $b
# Print third elements.
echo "3rd from a: $a1"
echo "3rd from b: $b1"
This script outputs
Output a: 12 13 315 1 2 3 4 5 6 7 8 10 2 8 9 1 0 0 2 3 4 5
Output b: 2 3 712 1 23 15 12 31 23 3 2 5 6 6 1 2 3 5 51 2 3 21
3rd from a: 315
3rd from b: 712
Explanation:
The trick here is that array constants (literals) in Bash have the form
(<space_separated_list_of_elements>)
for example
(1 2 3 4 a b c nearly_any_string 99)
Any variable that gets such an array assigned, automatically becomes an array variable. In the script above, this is what happens in a_array=($a): Bash expands the $a to the <space_separated_list_of_elements> and reads the whole expression again interpreting it as an array constant.
Individual elements in such arrays can be referenced like variables by using expressions of the form
<array_name>[<idx>]
like a variable name. Therein, <array_name>is the name of the array and <idx> is an integer that references the individual element. For arrays that are represented by array constants, the index counts elements continuously starting from zero. Therefore, in the script, ${a_array[2]} expands to the third element in the array a_array. If the array would have less elements, a_array[2] would be considered unset.
You can output all elements in the array a_array, the corresponding index array, and the number of elements in the array respectively by
echo "${a_array[#]}"
echo "${!a_array[#]}"
echo "${#a_array[#]}"
These commands can be used to track down the fate of the newline: Given the script above, it is still in $a, as can be seen by (watch the quotes)
echo "$a"
which yields
12 13 315
1 2 3 4 5 6 7 8 10 2 8 9 1 0 0 2 3 4 5
But the newline did not make it into the array a_array. This is because Bash considers it as part of the whitespace that separates the third and the fourth element in the array assignment. The same applies if there are no extra spaces around the newline, like here:
12 13 315\n1 2 3 4 5 6 7 8 10 2 8 9 1 0 0 2 3 4 5
I actually assume that the output of your C program comes in this form.

Answer 3

This will store the full string in a[0] and the individual fields in a[1-N]:
$ tmp=$(printf '12 13 315\n1 2 3 4 5 6 7 8 10 2 8 9 1 0 0 2 3 4 5\n')
$ a=( $(printf '_ %s\n' "$tmp") )
$ a[0]="$tmp"
$ echo "${a[0]}"
12 13 315
1 2 3 4 5 6 7 8 10 2 8 9 1 0 0 2 3 4 5
$ echo "${a[3]}"
315
Obviously replace $(printf '12 13 315\n1 2 3 4 5 6 7 8 10 2 8 9 1 0 0 2 3 4 5\n') with $(./algorithm < input.txt) in your real code.