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I have a simple bash script and want to count how often a command gets called. Here is a minimal example, the counter is called c and should be 4 at the end:

#!/bin/bash c=0; for a in X Y; do for b in 1 2; do c="$(( ${c} + 1 ))" echo "${a}${b}" done #| xargs -L 1 -P 20 echo echo "count $c" done echo "--" echo "final $c"

Works great without xargs (final=4), but when I uncomment the pipe to xargs it counts nothing (final=0). Why?

Expected output: | This happens with xargs: X1 | X1 X2 | X2 count 2 | count 0 Y1 | Y1 Y2 | Y2 count 4 | count 0 -- | -- final 4 | final 0
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The pipe means that your for loop is happening in a subshell, which doesn't pass $c back to the rest of your program. You'll need to rewrite without the pipe. This StackOverflow question is pretty similar. The <( ) syntax might be your friend here.

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  • In this case, I think >( ) is the form wanted. Something like ... done > >(xargs ... ) Commented May 19, 2014 at 15:55
  • xargs.. < <(for..) did not work, fixed it by writing all results to a file first and then calling xargs. But my question here was why, so thanks a lot! Commented May 19, 2014 at 15:58
  • @Gordon, sorry, didn't see your answer before posting, but it doesn't seem to work here, I only get Y1 X2 Y2 and then it hangs, weird. Commented May 19, 2014 at 16:02

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