How to invoke a program and pass it standard input - fortran

Question

How do I invoke a program and pass it standard input? Hypothetical example in Bash:
(echo abc; echo abba) | tr b B
Note that:
I don't have the input in a string (I'm generating it as I iterate)
I don't know how long input is
The input may span multiple lines, as in this example
I've written this in 19 other languages already, the way I usually approach it is to get a file descriptor for the program's standard input, and then write to the file descriptor the same way I would write to standard output.
What I've tried so far: Based on Invoke external program and pass arguments I tried passing it to echo and using the shell to handle the piping. This doesn't work if my input has single quotes in it, and it doesn't work if I don't have my input in a string (which I don't)
Here is my code, currently trying to pull it off by calculating the string that will be printed (it fails right now).

Answer 1

As for single quotation, how about inserting the escape character (\) before each quotation mark (')...? It seems to be working somehow for the "minimal" example:
module utils
implicit none
contains
function esc( inp ) result( ret )
character(*), intent(in) :: inp
character(:), allocatable :: ret
integer :: i
ret = ""
do i = 1, len_trim( inp )
if ( inp( i:i ) == "'" ) then
ret = ret // "\'"
else
ret = ret // inp( i:i )
endif
enddo
endfunction
endmodule
program test
use utils
implicit none
character(100) :: str1, str2
integer i
call getcwd( str1 ) !! to test a directory name containing single quotes
str2 = "ab'ba"
print *, "trim(str1) = ", trim( str1 )
print *, "trim(str2) = ", trim( str2 )
print *
print *, "esc(str1) = ", esc( str1 )
print *, "esc(str2) = ", esc( str2 )
print *
print *, "using raw str1:"
call system( "echo " // trim(str1) // " | tr b B" )
print *
print *, "using raw str2:"
call system( "echo " // trim(str2) // " | tr b B" ) !! error
print *
print *, "using esc( str1 ):"
call system( "echo " // esc(str1) // " | tr b B" )
print *
print *, "using esc( str2 ):"
call system( "echo " // esc(str2) // " | tr b B" )
print *
print *, "using esc( str1 ) and esc( str2 ):"
call system( "(echo " // esc(str1) // "; echo " // esc(str2) // ") | tr b B" )
! pbcopy (copy to pasteboard; mac-only)
! call system( "(echo " // esc(str1) // "; echo " // esc(str2) // ") | pbcopy" )
endprogram
If we run the above program in the directory /foo/baa/x\'b\'z, we obtain
trim(str1) = /foo/baa/x'b'y
trim(str2) = ab'ba
esc(str1) = /foo/baa/x\'b\'y
esc(str2) = ab\'ba
using raw str1:
/foo/baa/xBy
using raw str2:
sh: -c: line 0: unexpected EOF while looking for matching `''
sh: -c: line 1: syntax error: unexpected end of file
using esc( str1 ):
/foo/baa/x'B'y
using esc( str2 ):
aB'Ba
using esc( str1 ) and esc( str2 ):
/foo/baa/x'B'y
aB'Ba

Answer 2

This is what I mean by a minimal example.
call execute_command_line("(echo ""hexxo world"";echo abba)|tr x l")
end
hello world
abba
Is this not doing exactly what you ask, invoking tr and passing standard input?