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File name : add-shell
Content :
#!/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
        REALDIR="$(dirname $(realpath -m $i))/$(basename $i)"
        for j in "$i" "$REALDIR"
        do
                if ! grep -q "^${j}$" $tmpfile
                then
                        echo $j >> $tmpfile
                fi
        done
done

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

mv $tmpfile $file

trap "" EXIT
exit 0