When to use pipes and when to redirect?

Question

Although this is a basic stuff, but still i wonder. Consider these two examples.

Case 1:

[root@dhcppc0 practice]# cat file1 > wc -l
cat: invalid option -- l
Try `cat --help' for more information.

Case 2:

[root@dhcppc0 practice]# cat file1 | wc -l
7

I wonder, why doesn't redirection work in first case? when to use redirect and when to use pipes? I have been Linux for a long time, but still this basic stuff baffles me.

Please help.

Answer 1

The redirection operator is used to stream a command's stdout to a file (>), or to stream a file's contents to stdin for a command (<).

The pipe operator (|) streams the processes' output (stdout) to the next processes' stdin.

The only way to make your first case work would be to use a temporary file, so you would need to use two steps:

cat file1 > tmpfile
wc -l < tmpfile

Answer 2

You use pipes when you want to connect a FD of one executable to a FD of another. You use redirection when you want to connect a FD of an executable to a file.