In place grep - why would anyone want to do such a thing!?

igrep.sh

#!/bin/bash
#
# WARNING: YOU WILL LOSE THE FILE IF YOU SCREW UP BE CAREFUL
# If UNSAFE is set to YES, no backup will occur; you better not screw up!
# grep args or pattern because your file will be gone :>
#
# - AG
set -Eu

declare -r UNSAFE=YES
declare TMPDIR="/dev/shm"

if [ $# -lt 2 ]; then
  echo "--- grepi: inline replacing grep wrapper"
  echo "Usage:"
  echo '  grepi [flags] <pattern> <filename>'
  exit 1
fi

# Override environment TMPDIR by default

if [ ! -d "$TMPDIR" ]; then
  # Pick out any tmpfs mounts that do not have an explicit owner or perm mode
  # By default, the mode for tmpfs is 1777, same as traditional /tmp mounts
  # Typical systems have /run/lock and /dev/shm, both are suitable
  TMPDIR="$(mount | grep -P 'tmpfs(?!.*(mode|[^s]uid=))' | awk '{print $3}' | head -1)" 
  if [ "$TMPDIR" == "" ]; then
    TMPDIR="/tmp"
  fi
fi

# Last arg is the filename
declare -r file="${*: -1}"
# All the rest are grep parameters
declare -a args="${@:1:$#-1}"

if [ "${UNSAFE:-}" != "YES" ]; then
  declare -r basefile="$(basename "$file")"
  declare -r backup="$(realpath "$TMPDIR/$file")"
  echo "Backing file original=$file backup=$backup'"
  cp "$file" "$TMPDIR/"
  echo
  echo "To restore:"
  echo "  mv $backup $file"
  echo
fi

echo "Invoking grep in-place on file=$file"
echo "  $args"
ls -l "$file"
(rm $file; grep ${args[@]} > "$file") < "$file"
ls -l "$file"