Swap two files reasonably safely

swap

#!/bin/sh
# Swap two files safely.
#
# Under normal circumstances, just do the right thing. Under unusual
# circumstances, give a reasonably helpful error message.
#
# This was an exercise in thinking about how generally reliable functions might
# fail and what to communicate to users.
#
# Copyright Colin Caine. License: GPLv2

USAGE="Usage: `basename $0` file1 file2

  Basically the same as:
    mv file1 \$tmpfile
    mv file2 file1
    mv \$tmpfile file1

  But does something sensible if errors occur.
"

use_error () {
  [ -n "$1" ] && echo -e "$1\n" 2>&1
  echo "$USAGE"
  exit 1
}

descriptive_mv () {
  # $1 and $2 are the paths to two files in the same directory.
  # As of a moment ago, both files existed and the directory was writable by
  # our user (we could make the $tmpfile).
  if ! mv "$1" "$2"; then
    # Something bananas has happened. mv won't necessarily say something
    # sensible, so let's be as helpful as we can.
    echo "Error: mv '$1' '$2' failed!
       This is quite unusual.
       Some possible reasons:
         1) $USER's write access to dir `dirname $1` or execute permission on
            parent directories has been changed during the runtime of this 
            script.
            --> Data should be in '$1'
         2) '$1' has been moved or deleted by something else
         3) You're using a filesystem or kernel with odd properties (FUSE?)
            --> Your data may be in '$1' or '$2'" >&2
    exit 100
  fi
}

file1="$1"
file2="$2"

if [ ! $# = 2 ]; then
  use_error
elif [ ! -e "$file1" ]; then
  use_error "Error: '$file1' does not exist!"
elif [ ! -e "$file2" ]; then
  use_error "Error: '$file2' does not exist!"
else
  if ! tmpfile=$(mktemp $(dirname "$file1")/"$file1".tmp.XXXXXXXXXXX); then
    # The error message from mktemp or whatever should be enough.
    echo "Error: Could not create temporary file!" >&2
    exit 2
  fi
  descriptive_mv "$file1" "$tmpfile"
  if ! mv "$file2" "$file1"; then
    if [ ! -e "$file1" ]; then
      # $USER probably doesn't have permission to delete or rename $file2.
      # $file2 will be unmodified, so just put $file1 back where it was to
      # rollback the swap.
      descriptive_mv "$tmpfile" "$file1"
      echo "Error: mv '$file2' '$file1' failed. 
       Swap rolled back: files unchanged." >&2
      exit 3
    else
      # If $file1 exists, `mv $file2 $file1` failed in a highly unusual way, or
      # $file1 was created again after we moved it the first time.
      echo "Error: mv '$file2' '$file1' failed.
       Tried to move '$tmpfile' back to '$file1', but '$1' already exists!
       Possible reasons:
         1) A new '$file1' was created after this script renamed the original
             --> original '$file1' is in '$tmpfile'
                 '$file2' is where it started
         2) mv failed in a highly unusual way (weird filesystem?)
             --> original '$file1' is in '$tmpfile'
                 original '$file2' might be in '$file1' or '$file2'" >&2
      exit 5
    fi
  fi
  if ! mv "$tmpfile" "$file2"; then
    # Maybe $file1 and $file2 were on different devices and $file1 (now
    # $tmpfile) won't fit onto $file2's device. Maybe something else exciting.
    echo "Error: Couldn't complete final move.
       Succeeded: 'mv '$file1' '$tmpfile;' mv '$file2' '$file1;''
       Failed: 'mv '$tmpfile' '$file2''
       Content of the file originally called '$file1' is probably in '$tmpfile'.
       In extraordinary circumstances, the content may be in '$file2'." >&2
    exit 4
  fi
fi