5

While trying the cat "$@" trick to read from either standard input or given files, it turned out that pipe and process substitution handle a missing trailing newline differently:

printf %s foo > test.txt

unset REPLY
while read
do
    :
done < <(cat test.txt)
echo "$REPLY" # Prints foo

unset REPLY
cat test.txt | while read
do
    :
done
echo "$REPLY" # Prints nothing!

Is this by design? Is this "feature" documented anywhere?

D'oh! @fered had the right idea - It's just another example of how variable values are lost because piped commands are run in a subshell.

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  • Only in ATT ksh (ksh88 and ksh93, but not pdksh) and in zsh does the right-hand side of a pipe run in the parent shell. Commented Oct 26, 2011 at 7:28

1 Answer 1

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11

Variables in a pipe never make it out of the pipe alive :)
Process substitution redirects the data to a file descriptor. Behind the scenes, that process is not the same as a | pipe. The following works, because it is all within the same pipe. 

unset REPLY
cat test.txt | { 
  while read ;do : ;done
  echo "$REPLY" 
} # Prints foo!
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