bash script to merge two jars (e.g. for minecraft, commandline)

gistfile1.sh

$ cat ~/merge.sh
#!/bin/bash

function die {
    echo $1
    exit -1
}


[[ $# -eq 2 ]] || die "$0 jar1 jar2"

JAR1="$1"
JAR2="$2"
NOW=$(date +"%Y%m%d-%H%M%S")
TMP=$(mktemp -d tmp.XXXXXXXXXX)

[[ -f $JAR1  ]] || die "jar1 $JAR1 not exists"
[[ -f $JAR2  ]] || die "jar2 $JAR2 not exists"

echo backup of $JAR1 is $JAR1-saved-$NOW
cp -v $JAR1 $JAR1.saved-$NOW

echo creating $TMP
rm -rf $TMP
mkdir -p $TMP

echo extracting $JAR1 and $JAR2 into $TMP
cd $TMP
jar xf ../$JAR1
jar xf ../$JAR2

jar uf ../$JAR1 ./
cd ..

echo cleaning up $TMP
rm -rf $TMP