The most fascinating thing about Unix is how the following works together:
- the way Unix treats streams
- the paradigm "everything is a file"
- the paradigm "do one thing and do it well"
Let's start with the most basic program: cat. cat inputs a "stream" and outputs a "stream", nothing else. Find it out like this:
poochy:~ # cat Hello, I am typing a line Hello, I am typing a line
After typing cat with no parameters, you can input a line (followed by ENTER). cat does nothing but output the line again. You end your input with CTRL_D. Standard for your input is the keyboard, standard for a program's output is the console.
How can this be useful?
This can be extremely useful. E.g. take the paradigm "everything is a file". So you can replace your input stream by a file using a parameter:
outputs the content of readme.txt to the console. So you can display a file.
Or you can redirect cat's output stream to a file using the > operator:
cat > myfile.txt
Allows you to (over)write directly the file myfile.txt.
Or you can redirect the output of cat to the input of another command using the | (pipe) operator:
cat myfile.txt | cat
is equivalent to cat myfile.txt.
How can this pipe be useful?
There is a command to search a string in a stream (yes, or in a file, because everything is a file), called grep.
takes an input stream and echos only those lines containing the string "needle":
poochy:/usr # grep needle haystack hay more hay even more hay needle needle again hay some more hay
Now if we combine cat and grep, we get
cat * | grep needle
"*" stands for "all files". This command searches in all files for the string "needle" and outputs the files that contain it. Just FYI, this is equivalent to
grep "needle" *
because, as said, you can hand over files to grep.