Re: mysqldump usage in bash

[Greg Wooledge]

On Thu, Aug 27, 2009 at 06:13:38AM -0700, Arenstar wrote:
>     temp="mysqldump -h $DBSource -u $USER -p$PASS $DB $TABLE --where='$Field
> > $VarStart  AND $Field < $VarEnd' > $TABLE$DumpName"
>     exec $temp

The obvious problem here is that you want the last ">" to be treated as
a redirection operator.  This means the parser has to see it, outside
of a variable, and recognize it as a redirection, rather than a piece
of raw data.  Either the > has to be put on the exec line outside of a
variable, or you to use eval.  But if you use eval, it would change the
meaning of your inner quoted expression.

There is also a more subtle problem you've missed: the --where thing
contains whitespace, so it's going to be word-split in ways that you
simply can't deal with using this sort of approach.

Best bet if you have to retain the putting-commands-into-variables nonsense
is to split it into two parts: a command, which should be stored in an
array, rather than a string; and an output destination.

  command=(mysqldump -h "$DBSource" ... --where="$Field > $VarStart...")
  output=$TABLE$DumpName
  exec "${command[@]}" > "$output"

This accomplishes a few things:

 1) The command can have all the quoted whitespace you want without any
    danger.  You just can't have any pipes, redirections, etc. in it.
 2) The > is seen as a redirection, without problems.
 3) You didn't have to use eval, and endure the headaches eval brings with it.

Now, personally, I'd just skip all the putting-commands-into-variables
nonsense and write it like this:

  exec mysqldump -h "$DBSource" -u "$USER" -p"$PASS" "$DB" "$TABLE" \
    --where="$Field $VarStart AND $Field < $VarEnd" \
    > "$TABLE$DumpName"

I cover these issues in a bit more depth at:
http://mywiki.wooledge.org/BashFAQ/050