About the usage of linux command "xargs"

Question

I have some file like

love.txt   loveyou.txt  

in directory useful; I want to copy this file to directory /tmp.

I use this command:

find ./useful/ -name "love*" | xargs cp /tmp/ 

but is doesn't work, just says:

cp: target `./useful/loveyou.txt' is not a directory 

when I use this command:

 find ./useful/ -name "love*" | xargs -i cp {} /tmp/ 

it works fine,

I want to know why the second works, and more about the usage of -i cp {}.

Answer 1

xargs puts the words coming from the standard input to the end of the argument list of the given command. The first form therefore creates

cp /tmp/ ./useful/love.txt ./useful/loveyou.txt 

Which does not work, because there are more than 2 arguments and the last one is not a directory.

The -i option tells xargs to process one file at a time, though, replacing {} with its name, so it is equivalent to

cp ./useful/love.txt    /tmp/ cp ./useful/loveyou.txt /tmp/ 

Which clearly works well.

Answer 2

When using the xargs -i command, {} is substituted with each element you find. So, in your case, for both "loveyou.txt" and "love.txt", the following command will be run:

cp ./useful/loveyou.txt /tmp/ cp ./useful/love.txt /tmp/ 

if you omit the {}, all the elements you find will automatically be inserted at the end of the command, so, you will execute the nonsensical command:

cp /tmp/ ./useful/loveyou.txt ./useful/love.txt