theoretical question about bash and text

generalmiaow

so here's an issue I've finding it hard to get my head around, and google isn't helping

I suppose I'd best point to it by saying that

sort `ls`

causes sort to output the contents of all files and alphabetise it

whereas

ls | sort

causes sort to alphabetise the directory contents outputted by ls. I know sort takes a file as input usually, but when a the output of ls (a string?) is piped, it deals with that as if it were the output of a file given as an argument

Am I missing something here? Is this just the difference between "echo" and "cat", or is this something to do with the way bash's own invisible pipes work (like wildcards operating before any program sees them, etc)

Is this the same reason for this

shanet@generalmiaow:~$ echo `cat foo`
Now is the time for all good men to come to the aid of the party
shanet@generalmiaow:~$ cat foo | echo

shanet@generalmiaow:~$ cat foo | cat
Now is the time for all good men to come to the aid of the party
shanet@generalmiaow:~$

I have been trying to figure this out for ages. I think it's something very fundamental but I can't find a good introduction to this notion

PhantomBeaker

Ok, this is essentially a question about backticks vs pipes.

With backticks, it's basically saying "Run the command between the backticks and take it's output. Pretend I said that instead."

so

ping `hostname`

would look up the output of what happens when "hostname" is run, and it would ping it. I.e. the output of hostname is given as a command-line parameter to ping.

With pipes, using the '|' operator, it hooks up the output of the first program into the input of the next program. If you want more information on that, google either "RUTE" for a very good *nix tutorial (redhat linux heavy, but the commandline stuff will work anywhere), or google "redirection bash".

Hint, the reason why "cat foo | echo" didn't work is because echo doesn't listen on its standard input (which has caused me no end of pain on occasion until I remembered that)

Hope that helps,
Aoife

generalmiaow

Thanks Aoife, having a look at that tutorial now and it's great.