How do I create numbered backups?

Question

Some GNU utils (e.g., mv and cp) can create numbered backups (foo.~1~). Others (e.g., wget) cannot. I would like to create numbered backups before running tools which overwrite files by default. Here is a bash function which appears to do what I need:

backup(){                       # back up the file, emacs style
  file=$1
  if test -f "${file}"; then
    /bin/mv --backup=numbered "$(mktemp ${file}XXX)" "${file}"
    /bin/rm "${file}"
  fi
}

to be used, e.g., like this:

backup foo
curl http://.... > foo

I wonder if there is a better way.

Answer 1

A small improvement over the OP's solution to make it more generally usefull:

• Works with directories.
• The mktemp argument might need quotes, too.
• I'd && the commands in order not to get an error message from mv if mktemp fails, etc.
• Added option / argument separators -- in case of filenames which start with a dash.

backup(){                       # back up the file, emacs style
  local file=$1
  local tmp
  if test -e "${file}"; then
    tmp=$(mktemp -- "${file}XXX") && /bin/mv --backup=numbered --no-target-dir -- "$tmp" "${file}" && /bin/rm -- "${file}"
  fi
}

BTW, cp supports making a numbered backup with source == target.

> cp -v --backup=t --force a a
'a' -> 'a.~4~'

Answer 2

Still not exactly a one liner, but you get closer with one single call to perl together with its powerful e option (ie. execute the substitution part):

backup(){
    mv "$1" "$(echo $1 |perl -pe '~s|(.*?)(~([0-9]*)~)?$|print "$1~".(${3}+1)."~"|e and exit')"
}

Notice the and exit what prevents perl to print the matching count that otherwise pollutes the name.

You may also want to add 2>/dev/null to the end of the line to keep it quiet when the file does not exist.