Finding number of lines of a data file using command line - command-line

Question

There is a conventional way to read each line one by one and check iostat hits nonzero or negative value at every reading. However, I would like to call system(command) routine and
use wc -l command to count the number of and then want to allocate the dimension of the array where I want to put the data. For the example, I am printing the number of lines in both ways:
Program Test_reading_lines
integer:: count,ios, whatever
character(LEN=100):: command
Print*,'Reading number of lines in a standard way'
count=0
open (10, file='DATA_FILE')
Do
read (10,*,iostat=ios) whatever
if (ios/=0) exit
count=count+1
End Do
close(10)
Print*,'Number of lines =', count
Print*,'Reading number of lines using shell command'
command='cat DATA_FILE | wc -l'
call system(command)
Print*,'Number of lines =','< ? >'
End Program Test_reading_lines
Unfortunately, in the latter case, can I assign a variable like count as in the standard case? That is, I want to print a variable instead of '< ? >' in the last print command.

Answer 1

This is not possible in a straightforward way. You could redirect the output of the command to a file, then open it and read it http://compgroups.net/comp.lang.fortran/how-to-get-the-output-of-call-system-in-a-v/216294
Or use some even more sophisticated features of the Unix functions and call its C API (see the first answer in that thread).
The EXECUTE_COMMAND_LINE() also does not have any feature to read the output of the command directly.

Answer 2

If you want to use the Unix command $ wc -l, you could call the Fortran subroutine execute_command_line which is common to many Fortran compilers, gfortran included.
Here is a working example which computes the number of lines, nlines, of a file called style.gnuplot and then uses nlines to append some rows to style.gnuplot by overwriting the last one.
PROGRAM numLines
IMPLICIT NONE
integer, parameter :: n = 100
integer :: i, nLines
real, parameter :: x0 = -3.14, xEnd = 3.14
real :: dx
real, dimension (:), allocatable :: x, fun
allocate(x(0:n)) ! Allocate the x array
allocate(fun(0:n)) ! Allocate the fun array
dx = abs(xEnd-x0)/n
x(0:n) = [(x0+i*dx, i = 0,n)] ! Create the x array
fun(0:n) = [(sin(x0+i*dx), i = 0,n)] ! Create the fun array
open(unit=1,file="plotFunction.dat")
DO i=0,size(x)-1
write(1,*) x(i), ' ', fun(i) ! Save the function to a file to plot
END DO
close(unit=1)
deallocate(x) ! Deallocate the x array
deallocate(fun) ! Deallocate the fun array
open(unit=7, file="style.gnuplot")
write(7,*) "set title 'y = sin(x)' font 'times, 24'"
write(7,*) "set tics font 'times, 20'"
write(7,*) "set key font 'times,20'"
write(7,*) "set grid"
write(7,*) "set key spacing 1.5"
write(7,*) "plot '<cat' u 1:2 w l lw 2 linecolor rgb 'orange' notitle "
close(unit=7)
CALL execute_command_line("wc -l style.gnuplot | cut -f1 -d' ' > nlines.file") ! COunt the lines
open(unit=1,file='nlines.file')
read(1,*) nlines ! Here the number of lines is saved to a variable
close(unit=1)
CALL execute_command_line("rm nlines.file") ! Remove nlines.file
CALL execute_command_line("cat plotFunction.dat | gnuplot -p style.gnuplot") ! Show the plot within the executable
open(unit=7,file="style.gnuplot")
DO i = 1,nLines-1
read(7,*) ! Read the file untile the penultimate row,
END DO ! then append the other rows
write(7,*) "set object rectangle at -3.14,0 size char 1, char 1", &
" fillcolor rgb 'blue' fillstyle solid border lt 2 lw 1.5"
write(7,*) "set object rectangle at 0,0 size char 1, char 1", &
" fillcolor rgb 'blue' fillstyle solid border lt 2 lw 1.5"
write(7,*) "set object rectangle at 3.14,0 size char 1, char 1", &
" fillcolor rgb 'blue' fillstyle solid border lt 2 lw 1.5"
write(7,*) "plot 'plotFunction.dat' u 1:2 w l lw 2 linecolor rgb 'orange' notitle"
close(unit=7)
CALL execute_command_line("gnuplot -p 'style.gnuplot'") ! Load again style.gnulot with the appended lines
END PROGRAM numLines
My code might not be elegant, but it seems to work!