On Saturday, 4 September 2021 at 15:41:51 UTC, eXodiquas wrote:
> Hello everyone,
I created a small little D program that reads in a string from the command line and shuffles the letters of the nouns a bit around. This is pretty straight forward, but what I see now happening is a bit strange, at least for me.
I am reading the args out of the main function arguments.
void main(string[] args) {
args.writeln();
}
this works fine whenever I call the program like ./nounscramble "Hello, my name is Earl!"
the string shows up in args[1]
. But when I call echo "Hello, my name is Earl!" | ./nounscramble
it does not show up in the args array, the only thing showing up is the name of the executable (which is expected).
My question is now, can someone explain what I am doing wrong? Maybe I misunderstood the pipe in Linux systems and it is obvious for someone who knows how this works exactly, or maybe D works differently with pipes and I havn't found the correct documentation.
Thanks in advance. :)
eXodiquas
There are two things happening here. Command line arguments and stdin, standard in.
When part of an actual command line, parameters are expanded as command line arguments, passed to the main
function. When echo
ed, the arguments are passed to the echo
command, which then displays those arguments on stdout, standard out. A pipe is connecting the stdout of the left hand side, and streaming that data to the stdin of the right hand side, after the pipe symbol. These are not command line arguments, but data on the stdin/stdout channels (file descriptors to be more exact with the jargon).
You could write is as echo thing | ./nounscramble commandarg
.
echo
will display thing. Your program with get commandarg
in the array passed to main
, but the stdout data displayed by echo
is never read, and just ends up in a pipeline bitbucket at the end of processing.
Most shells, like bash
allow a different kind of parameter replacement, command arg expansion. Use like ./nounstatement $(echo this that)
, and the shell will manipulate things to replace the stdout from the echo
into word expanded arguments placed in the atring array used by main
.
Cheers