#! /bin/sh
Then, mark the script with execute permissions.
$ chmod a+x lockdown
The full script then appears as follows:
#!/bin/sh

# Locks down file permissions.

for filename in *
do
    # Initialize all permissions.
    r=""
    w=""
    x=""

    # Check to preserve existing permissions.

    if [ -r $filename ] 
    then
        r="r"
    fi

    if [ -w $filename ] 
    then
        w="w"
    fi

    if [ -x $filename ] 
    then
        x="x"
    fi

    # Lock down the file permissions.
    chmod u+$r$w$x,g-rwx,o-rwx $filename

done