Question

A fun little bash teaser to which I’d love an explanation.

Two loop constructs I would have though are identical, clearly are not. Seems there’s some difference in piping vs redirecting when doing a while loop.

Input File

Given this sample file called values.txt with this content:

1
2
3
4
5
6

Piping to while

$ value=0; cat values.txt | while read var; do value=`expr $value + $var`; done
$ echo $value
0

Redirecting to while

$ value=0; while read var; do value=`expr $value + $var`; done < values.txt
$ echo $value
21

To be brief, clearly in the first version each iteration of the while loop executes effectively as () and in the second each iteration iterates as {}

The question is not the difference between () and {}. My question is what causes this difference in behavior for while loops?

Is there a logical reason they should behave differently or was it just a bad choice made early on that couldn’t be changed for compatibility reasons? Is it ever possible to pipe to while and get {} behavior?

Answer 1

This is a known problem and is well explained here:
http://mywiki.wooledge.org/BashFAQ/024

To quote the most explanatory part:

Different shells exhibit different behaviors in this situation:

• BourneShell creates a subshell when the input or output of anything
  (loops, case etc..) but a simple command is redirected, either by
  using a pipeline or by a redirection operator (‘<‘, ‘>’).
• BASH creates a new process only if the loop is part of a pipeline.
• KornShell
  creates it only if the loop is part of a pipeline, but not if the loop
  is the last part of it.
• POSIX specifies the bash behaviour, but as
  an extension allows any or all of the parts of the pipeline to run
  without a subshell (thus permitting the KornShell behaviour, as well).

As for the last question: yes, it is possible in certain shells, and in bash only if you have bash >=4.2, and preceed your code with disabling job control and enabling lastpipe option with the following code: set +m; shopt -s lastpipe