bash Cookbook

Using Secure Temporary Files

Problem

You need to create a temporary file or directory, but are aware of the security implications of using a predictable name.

Solution

The easy and “usually good enough” solution is to just use $RANDOM inline in your script. For example:

# Make sure $TMP is set to something
[ -n "$TMP" ] || TMP='/tmp'

# Make a "good enough" random temp directory
until [ -n "$temp_dir" -a ! -d "$temp_dir" ]; do
    temp_dir="/tmp/meaningful_prefix.${RANDOM}${RANDOM}${RANDOM}"
done
mkdir -p -m 0700 $temp_dir
  || { echo "FATAL: Failed to create temp dir '$temp_dir': $?"; exit 100 }
  # Make a "good enough" random temp file
  until [ -n "$temp_file" -a ! -e "$temp_file" ]; do
      temp_file="/tmp/meaningful_prefix.${RANDOM}${RANDOM}${RANDOM}"
done
touch $temp_file && chmod 0600 $temp_file
  || { echo "FATAL: Failed to create temp file '$temp_file': $?"; exit 101 }

Even better, use both a random temporary directory and a random filename!

# cookbook filename: make_temp

# Make a "good enough" random temp directory
until [ -n "$temp_dir" -a ! -d "$temp_dir" ]; do
    temp_dir="/tmp/meaningful_prefix.${RANDOM}${RANDOM}${RANDOM}"
done
mkdir -p -m 0700 $temp_dir \
  || { echo "FATAL: Failed to create temp dir '$temp_dir': $?"; exit 100 }

# Make a "good enough" random temp file in the temp dir
temp_file="$temp_dir/meaningful_prefix.${RANDOM}${RANDOM}${RANDOM}"
touch $temp_file && chmod 0600 $temp_file \
  || { echo "FATAL: Failed to create temp file '$temp_file': $?"; exit 101 }

No matter how you do it, don’t ...