playing with head in ubuntu

Question

Hi while I am playing with Ubuntu head command of CLI, I came to a situation.There what ever I am giving as input thats echoing automatically.

The command and its play mentioned in the image.

so could somebody explain me whats happening in there ? .

Note: here h.txt is a general text file I made with raw information.

Answer 1

With the - option you're reading from "standard input". From the manpage of head(1):

when FILE is -, read standard input.

Your keyboard is the standard input in this case, as your shell doesn't provide another to head here. Everytime you provide a line, it will just process it up to 10 times as it's outputting the first 10 lines by default. As your shell will also print the input you provide, you'll see all lines twice.

When you are providing a stream to the standard input to head, then your keyboard input will not be grabbed. E.g.:

echo blaat | head -

will make echo output "blaat" to standard output and your shell will connect that to standard input of head (as expressed by the |).

Note: Without any arguments given head will also read from standard input.

A more useful (random) example for the use in connecting up standard input with head is this:

grep error /var/log/syslog | grep -iv apache | head

Will print the first 10 lines matching error, but not apache from /var/log/syslog.