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bug#6925: strange behavior of mktemp (GNU coreutils) 7.4, Ubuntu 10.04)

From: Eric Blake
Subject: bug#6925: strange behavior of mktemp (GNU coreutils) 7.4, Ubuntu 10.04)
Date: Fri, 27 Aug 2010 10:53:24 -0600
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20100806 Fedora/3.1.2-1.fc13 Mnenhy/0.8.3 Thunderbird/3.1.2

On 08/27/2010 07:08 AM, master atomknuseren wrote:
When I invoke mktemp in a script as:

DIR=$( mktemp||exit 1 )
cd $DIR
echo $?

I receive an error like:
can't cd to /tmp/tmp.68rHO2a3jc

Thanks for the report.  However, this is not a bug.

If you want mktemp to create a directory instead of a file, you must use the -d option. As written, you created a file, which means of course cd will fail because it is not a directory.

My primitive workaround is to recreate  $DIR as in:
DIR=$( mktemp||exit 1 )
rm -r -f $DIR
mkdir -p $DIR

Heavens no. This introduces a data race that exposes you to the very bug that you are trying to avoid by using mktemp in the first place - namely, a window where an attacker can spot the filename you are using and inject a rogue file in it's place in the time where the file does not exist.

Rather, by using DIR=$(mktemp -d), you are guaranteed to have a directory or a failure, without having to call mkdir -p after the fact.

By the way, the ||exit 1 in your command substitution is pointless.

Eric Blake   address@hidden    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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