;+
; NAME:
;  nthline
;
; PURPOSE: (one line)
;  Return the nth line in a file
;
; DESCRIPTION:
;  Return the nth line in a file
;
; CATEGORY:
;  File IO
;  
; CALLING SEQUENCE:
;  line = nthline(filename, linenum)
;
; INPUTS:
;  filename - file from which to get lines
;  linenum - the line number to return (long or array of long)
;
; OPTIONAL INPUT PARAMETERS:
;  None
;  
; KEYWORD INPUT PARAMETERS:
;  /VERBOSE -- If set, print diagnostics to screen.
;
; KEYWORD OUTPUT PARAMETERS:
;  NSTARS -- Returns the number of successfully-retrieved lines.
;
; OUTPUTS:
;  If input is a scalar, then output is a single string.
;  If input is a vector, then output is an array of strings.
;  If a line is not found (linenum out of range), returned value is ''.
;  
; COMMON BLOCKS:
;  None
;
; SIDE EFFECTS:
;  None
;
; PROCEDURES:
;  spawned call to sed
;
; RESTRICTIONS:
;
; EXAMPLE:
;  print, nthline(102,[430L, 1000L])
; 
; MODIFICATION HISTORY:
;  Written   2006 Apr 23 Leslie Young SwRI
;-
function nthline, filename, linenum, maxline=maxline

    nline = n_elements(linenum)
    lines = strarr(nline)

    if not keyword_set(maxline) then maxline = file_lines(filename)

    for i = 0L, nline-1 do begin
        
        n = linenum[i]
        if n ge 0 and i le maxline then begin

            nstr = strtrim(string(linenum[i]+1),2)

            ; build the unix command to spawn
            cmd = 'sed -e{' + nstr + 'P} -n ' + filename
;            print, cmd
            spawn, cmd, result
;            help, result
            lines[i] = result
        end

    end

    return, lines
       
end