Friday, May 04, 2007

Getting more from streams in Bash

A handy trick in Bash is to split a stream to process it in more than one way without saving a temporary copy:

# Make 3 a copy of 1 (stdout)
exec 3>&1
result="$(command to generate stream \
    | tee /dev/fd/3 \
    | command to process stream)"
# Now work with $result
# The original stream also went to the console

The actual case I worked with this morning was a very simple demonstration:


exec 3>&1

echo -e "Bob is my friend.\nFred is my friend, too." \
    | tee /dev/fd/3 \
    | cut -f1 -d' ' >&2

Which simply prints:

Bob is my friend.
Fred is my friend.

to stdout and to stderr it prints:

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