diffCousin

#!/bin/sh

if [[ "$1" == "-h"  || ! "$3" ]]; then
    cat <<EOF
Usage:
    diffCousin [path] [otherDir]
Find a corresponding file in the same relative position in another dir, and diff.
For example, say you have ~/myStuff/code/foo/bar/spam/eggs/stuff.py, and also a 
backup or other near-copy of myStuff/ under ~/spares/myStuff2.
To diff the versions of stuff.py: 
    diffCousin ~/myStuff ~/spares/myStuff2 code/foo/bar/spam/eggs/stuff.py
    
This will check under the directories given by the first two arguments, for a file
at the relative path given by the third argument. Assuming they both exist, they'll
be handed to \$DIFFER (current set to '$DIFFER').

Note: The third argument usually won't autocomplete while typing the command.
EOF
    exit
fi

if ! [ -d "$1" ]; then
    echo "First directory not found: '$1'."
    exit
fi
if ! [ -d "$2" ]; then
    echo "First directory not found: '$2'."
    exit
fi

file1="$1/$3"
if ! [ -e "$file1" ]; then
    echo "File not found under first directory: '$file1'."
    exit
fi
file2="$2/$3"
if ! [ -e "$file2" ]; then
    echo "File not found under second directory: '$file2'."
    exit
fi

[ $DIFFER ] || DIFFER="diff"
echo "Running $DIFFER \"$file1\" \"$file2\"."
$differ "$file1" "$file2"