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Bash script to expand file templates

I am working on pushing all my "dotfiles" into a Git repo. Some contains passwords or other private data. It took me longer than expected to write an expander script in bash. So here it is.

Properties are stored as Java properties.

gittoken=123456789012345ab3bc
backup_destination=emmanuel@devnull.com:./backup

The template uses the $placeholder syntax.

# .gitconfig file
[user]
    name = Emmanuel Bernard
[github]
    user = emmanuelbernard
    token = $gittoken 
[alias]
    co = checkout
    undo = reset --hard
    cb = checkout -b
    br = branch
    cp = cherry-pick
[random]
    complex-backup-link = $backup_destination

The script is used like

./properties-expander.sh <properties file> <template> <destination>

# For example
./properties-expander.sh ~/secret.properties ~/templates/gitconfig ~/.gitconfig

The script itself is fairly compact though some parts were fairly tricky - thanks stackoverflow :) :

#!/bin/bash
#
# Released under the WTFPL license version 2 http://sam.zoy.org/wtfpl/
# Copyright (c) 2012 Emmanuel Bernard
# -----
# Usage: properties-expander.sh <properties file> <template> <destination>
#
# Properties file:
#   PROPERTY1=value1
#   PROPERTY2=value2

#help
if [[ $# -ne 3 ]]; then 
  echo "properties-expander.sh <properties file> <template> <destination>"
  exit 0;
fi

echo Expanding $2 to $3 with properties from $1

#read each line including the last one
while IFS=$'\n' read -r line || [[ -n "$line" ]]; do
    #get the property name before the first =
    property=${line%%=*}
    #get the value after the first =
    value=${line#*=}

    #find a suitable separation character as the value might contain /
    candidates='/:.|@#%^&;,!~abcABC'  # candidates for separation character
    char=$(echo "$candidates" | tr -d "$line")  
    char=${char:0:1}   # choose the first candidate that doesn't appear in the user input

    if [ -z "$char" ]    # this test checks for exhaustion of the candidate character set
    then
        echo "Unusable user input: $line"
        exit 1
    fi

    #add regexp rule
    SEDEXEC="s${char}\$$property${char}$value${char}g;$SEDEXEC"
done < $1

SEDEXEC="sed -e '$SEDEXEC' $2 > $3"
eval $SEDEXEC

For simplicity, I have put the script in a GitHub gist. Please fork, improve and give me feedback :)