How can a node in a Bash pipeline peek at, but not consume, its input stream?

Question

How might a node in a Bash pipeline only peek at, but not consume, its input stream?

For example, how might I modify the following script so that it outputs "print" instead of nothing? In particular, how might I modify or replace grep --quiet print so that it only peeks at, but does not consume, its input?

printf "%s\n" a b print c |
  if grep --quiet print
  then
    grep print | cat
  fi

Output:

Desired output:

print

Ideally, I seek a program peek that is similar to grep, but that doesn't consume its input. peek print would return error code 0 if it finds "print" in the input and if not, returns a non-zero error code.

Here's how peek would work in my example:

printf "%s\n" a b print c |
  if peek print
  then
    grep print | cat
  fi

Output:

print

The reason I want to do this instead of simply filtering the results with grep print is to avoid processing empty search results that might cause an error.

Answer 1

A straight forward approach I can think of is to use a temporary storage for output. It can be done with minimal modification of your code like this:

atmp=$(mktemp "/tmp/XXXXXX")

printf "%s\n" a b print c | tee $atmp | 
  if grep --quiet print
  then
    grep print $atmp 
  fi  
rm $atmp #cleanup

For small volumes of output that may be acceptable.

Answer 2

Another approach similar to @Tagwint's answer is to store the result of grep print in a shell variable and then print the contents of that variable only if there are search results:

printf "%s\n" a print b print c |
 if result=$(grep print)
 then
   echo "$result";
 fi

Output:

print
print

This would be acceptable where the search results are not too large.

Answer 3

I also solved the problem in this specific example using fionread which @mosvy implemented in his answer to How to check if a pipe is empty and run a command on the data if it isn't?.

printf "%s\n" a b print c | grep print | ./fionread echo found

Output:

found

printf "%s\n" a b c | grep print | ./fionread echo found

Output:

Note that gcc easily compiles fionread.c on FreeBSD 11.3:

$ gcc -o fionread fionread.c
$ echo test | grep test | ./fionread echo found
found
$ echo | grep test | ./fionread echo found
$

Answer 4

While not a standard Unix utility or Bash script, I decided to solve the problem in this specific example using ifne from moreutils:

printf "%s\n" a b print c | grep print | ifne echo found

Output:

found

printf "%s\n" a b c | grep print | ifne echo found

Output:

@Nick Wirth referred to lfne in his answer to How to check if a pipe is empty and run a command on the data if it isn't?.

Thank you to @mosvy for referring me to this existing question.