Is it possible to get the segment number in an xargs invocation

Question

Xargs can be used to cut up the contents of standard input into manageable pieces and invoke a command on each such piece. But is it possible to know which piece it is ? To give an example:

seq 1 10 | xargs -P 2 -n 2 mycommand

will call

mycommand 1 2 &
mycommand 3 4 &
mycommand 5 6 &
mycommand 7 8 &
mycommand 9 10 &

But I would like to know in my "mycommand" script that

mycommand 1 2

is processing the first piece/segment, and so on. Is it possible to access that information ?

p.s. In the simple example above I can just look at the numbers and tell. But for arbitrary lists how does one access the information without actually injecting piece# in the input stream ?

Answer 1

I only see you can do this if you change your input and add the sequence number:

seq 1 10 | perl -ne '$. % 2 and print (($.+1)/2,"\n"); print' | xargs -n3 ...

It is unclear why you need this, but if your final goal is to keep the output in the same order as the input, it may be easier to use GNU Parallel:

seq 1 10 | parallel -j+0 -n2 -k mycommand 

Watch the intro video for GNU Parallel to learn more: http://www.youtube.com/watch?v=OpaiGYxkSuQ