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Filename : add-shell
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#!/bin/sh -e

if test $# -eq 0
then
	echo usage: $0 shellname [shellname ...]
	exit 1
fi

file=/etc/shells
# I want this to be GUARANTEED to be on the same filesystem as $file
tmpfile=${file}.tmp

set -o noclobber

trap "rm -f $tmpfile" EXIT

if ! awk '{print}' $file > $tmpfile
then
        cat 1>&2 <<EOF
Either another instance of $0 is running, or it was previously interrupted.
Please examine ${tmpfile} to see if it should be moved onto ${file}.
EOF
        exit 1
fi

for i
do
        if ! grep -q "^${i}$" $tmpfile
        then
                echo $i >> $tmpfile
        fi
done

chmod --reference=$file $tmpfile
chown --reference=$file $tmpfile

mv $tmpfile $file

trap "" EXIT
exit 0