qdiff

#!/bin/sh
# Last updated 2016.12.14
PROGCMD=$(basename "$0")
die_() { echo "$PROGCMD: $1" 1>&2; exit $2; }
die() { [ -f "$outfile" ] && rm -f "$outfile"; die_ "$@"; }

if (for i in "$@"; do
    printf "%s" "$i" | grep -E '^(-h|-help|--help)$' && exit 0
    done; exit 1) >/dev/null || [ $# -eq 0 ]; then
  echo "Usage: $PROGCMD [ARGS...]
Run \`diff -u ARGS' and view its output using the program indicated by
environment variable \$DIFF_VIEWER.  Default to \`vim' or \`vi' if
\$DIFF_VIEWER is not set." 1>&2
  exit 2
fi

# If stdout isn't a terminal then just run diff raw.
tty -s <&1 || exec diff -u "$@"

# Set default viewer if none was specified.
for i in "$DIFF_VIEWER" vim vi; do
  if (which "$(echo "$i" | sed 's,[ \t].*,,')" >/dev/null 2>&1); then
    viewer="$i"
    break
  fi
done
[ -n "$viewer" ] || die_ "\$DIFF_VIEWER not found; try \`$PROGCMD --help'" 2

# Run diff, write its output to a temp file, then run the viewer.
outfile=$(mktemp "/tmp/${PROGCMD}-XXXXXX.diff" 2>&1)
[ $? -eq 0 ] || die_ "could not create temporary file: $outfile" 2
diffout=$(diff -u "$@" 2>&1)
status=$?
[ $status -eq 0 ] || [ $status -eq 1 ] || die "$diffout" $status
printf "%s" "$diffout" > "$outfile" || die "could not write to $outfile" 2
chmod 400 "$outfile"

# Don't quote $viewer b/c it will break if, for example, viewer="emacs -nw".
$viewer "$outfile"
rm -f "$outfile"
exit $status