unix shell, redirect output but keep on stdin

Question

I'd like to have a shell script redirect stdout of a child process in the following manner

1. Redirect stdout to a file
2. Display the output of the process in real time

I know I could do something like

#!/bin/sh

./child > file
cat file

But that would not display stdout in real time. For instance, if the child was

#!/bin/sh

echo 1
sleep 1
echo 2

The user would see "1" and "2" printed at the same time

Answer 1

Use tee:

./child | tee file

tee will copy its standard input to any file on the command line and to standard output as well.

Answer 2

I use: sponge, from moreutils http://linux.die.net/man/1/sponge

you can do something like that:

$ grep something largefile | sponge largefile