redirection from a file to command

[kaaliakahn]

Hi there,

I am always confused about the redirection operator <

Lets say i have a file input.txt that contains one word "hello" without the quotes

when i do the following why don't i see any output?

$echo < input.txt



Secondly, i am slightly confused between input redirection < and pipe | operator. Sometimes they seem to do the same thing. For example i can achieve the output from the above command as follows

$ cat input.txt | xargs echo


Can somebody clarify with the help of examples?

Thanks a lot

[hazel]

The echo command needs an argument (i.e. it has to be told what to echo). There are some commands which can take data from standard input and therefore from a file but echo is not one of them. So echo hello will work and so will echo `cat input.txt` but input.txt<echo will not.

A redirection operator is used between a program and a file; it causes one of the standard channels (standard input, standard output or standard error) to be replaced by that file. A pipe can only be used between programs; it links the standard output of the first program to the standard input of the second. xargs is a program, which is why your pipe example works.

[kaaliakahn]

so nice of you