How do I create a temporary file in a secure manner?

There does not appear to be any single command that simply works everywhere. tempfile is not portable. mktemp exists more widely (but still not ubiquitously), but it may require a -c switch to create the file in advance; or it may create the file by default and barf if -c is supplied. Some systems don't have either command (Solaris, POSIX).

The traditional answer has usually been something like this:

   # Do not use!  Race condition!
   trap 'rm -f "$tempfile"; exit 1' 1 2 3 15
   rm -f "$tempfile"
   touch "$tempfile"

The problem with this is: if the file already exists (for example, as a symlink to /etc/passwd), then the script may write things in places they should not be written. Even if you remove the file immediately before using it, you still have a RaceCondition: someone could re-create a malicious symlink in the interval between your shell commands.

The best portable answer is to put your temporary files in your home directory (or some other private directory) where nobody else has write access. Then at least you don't have to worry about malicious users. Simplistic PID-based schemes (or hostname + PID for shared file systems) should be enough to prevent conflicts with your own scripts.

Unfortunately, people don't seem to like that answer. They demand that their temporary files should be in /tmp or /var/tmp. For those people, there is no clean answer, so they must choose a hack they can live with.

Making a temporary directory

In some systems (like Linux):

   # POSIX

   # Set $TMPDIR to "/tmp" only if it didn't have a value previously
   : ${TMPDIR:=/tmp}

   # We can find about $TMPDIR in

   # Create a private temporary directory inside $TMPDIR
   # Remove the temporary directory when the script finishes
   unset temporary_dir
   trap '[ -n "$temporary_dir" ] && rm -rf "$temporary_dir"' EXIT

   umask 077
   temporary_dir=$(mktemp -d "$TMPDIR/XXXXXXXXXXXXXXXXXXXXXXXXXXXXX") || { echo "ERROR creating a temporary file" >&2; exit 1; }
   umask "$save_mask"

And then you can create your particular files inside the temporary folder.

A different suggestion which does not require a mktemp command: a temporary directory can be created that is unlikely to match an existing directory using the RANDOM variable as follows:

   mkdir "$temp_dir"

This will make a directory of the form: /tmp/20445/. To decrease the chance of collision with an existing directory, the RANDOM variable can be used a number of times, or combined with the process ID:

   mkdir -m 700 "$temp_dir" || { echo "failed to create temp_dir" >&2; exit 1; }

This will make a directory of the form /tmp/24953-2875/. This avoids a race condition because the mkdir is atomic, as we see in FAQ #45. It is imperative that you check the result of mkdir; if it fails to create the directory, obviously you cannot proceed.

(Some systems have a 14-character filename limit, so avoid the temptation to string $RANDOM together more than twice. You're relying on the atomicity of mkdir for your security, not the obscurity of your random name. If someone fills up /tmp with hundreds of thousands of random-number files to thwart you, you've got bigger issues.)

Instead of RANDOM, awk can be used to generate a random number in a POSIX compatible way:

   temp_dir=/tmp/$(awk 'BEGIN { srand (); print rand() }')
   mkdir -m 700 "$temp_dir"

Note, however that "srand()" seeds the random number generator using seconds since the epoch which is fairly easy for an adversary to predict and perform a denial of service attack.

Combining the above we can use:

   # POSIX
   temp_dir=${TMPDIR:=/tmp}/$(awk -v p=$$ 'BEGIN { srand(); s = rand(); sub(/^0./, "", s); printf("%X_%X", p, s) }')
   trap '[ -n "$temp_dir" ] && rm -fr "$temp_dir"' EXIT
   mkdir -m 700 "$temp_dir" || { echo '!! unable to create a tempdir' >&2; temp_dir=; exit 1; }

Since the default awk numeric format is %g which equates to 6 digits after the decimal point, we have a 6 digit trail. 999,999 is F423F which gives us one character back, another for the _ and 8 available for a prefixed pid on a 14-char filesystem. An unsigned 32-bit pid will be a maximum of 8 hex digits, so the above should stand a chance in most places one might encounter such an fs. And of course we bail out as greycat mentioned when the creation fails.

Other approaches

I reiterate that the best solution is to use your $HOME directory to hold your private files. If you're implementing a daemon or something, which runs under a user account with no home directory, why not simply make a private directory for your daemon at the same time you're installing the code?

Another not-quite-serious suggestion is to include C code in the script that implements a mktemp(1) command based on the mktemp(3) library function, compile it, and use that in the script. But this has a couple problems:

BashFAQ/062 (last edited 2014-02-19 21:55:18 by 109)