Shrinking file size of PDF using a small bash script

#!/bin/sh

# http://www.alfredklomp.com/programming/shrinkpdf
# Licensed under the 3-clause BSD license:
#
# Copyright (c) 2014-2019, Alfred Klomp
# All rights reserved.
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# Redistribution and use in source and binary forms, with or without
# modification, are permitted provided that the following conditions are met:
# 1. Redistributions of source code must retain the above copyright notice,
#    this list of conditions and the following disclaimer.
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#    this list of conditions and the following disclaimer in the documentation
#    and/or other materials provided with the distribution.
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#    may be used to endorse or promote products derived from this software
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# POSSIBILITY OF SUCH DAMAGE.


shrink ()
{
    gs                  \
      -q -dNOPAUSE -dBATCH -dSAFER      \
      -sDEVICE=pdfwrite         \
      -dCompatibilityLevel=1.3      \
      -dPDFSETTINGS=/screen         \
      -dEmbedAllFonts=true          \
      -dSubsetFonts=true            \
      -dAutoRotatePages=/None       \
      -dColorImageDownsampleType=/Bicubic   \
      -dColorImageResolution=$3     \
      -dGrayImageDownsampleType=/Bicubic    \
      -dGrayImageResolution=$3      \
      -dMonoImageDownsampleType=/Subsample  \
      -dMonoImageResolution=$3      \
      -sOutputFile="$2"         \
      "$1"
}

check_smaller ()
{
    # If $1 and $2 are regular files, we can compare file sizes to
    # see if we succeeded in shrinking. If not, we copy $1 over $2:
    if [ ! -f "$1" -o ! -f "$2" ]; then
        return 0;
    fi
    ISIZE="$(echo $(wc -c "$1") | cut -f1 -d\ )"
    OSIZE="$(echo $(wc -c "$2") | cut -f1 -d\ )"
    if [ "$ISIZE" -lt "$OSIZE" ]; then
        echo "Input smaller than output, doing straight copy" >&2
        cp "$1" "$2"
    fi
}

usage ()
{
    echo "Reduces PDF filesize by lossy recompressing with Ghostscript."
    echo "Not guaranteed to succeed, but usually works."
    echo "  Usage: $1 infile [outfile] [resolution_in_dpi]"
}

IFILE="$1"

# Need an input file:
if [ -z "$IFILE" ]; then
    usage "$0"
    exit 1
fi

# Output filename defaults to "-" (stdout) unless given:
if [ ! -z "$2" ]; then
    OFILE="$2"
else
    OFILE="-"
fi

# Output resolution defaults to 72 unless given:
if [ ! -z "$3" ]; then
    res="$3"
else
    res="72"
fi

shrink "$IFILE" "$OFILE" "$res" || exit $?

check_smaller "$IFILE" "$OFILE"