NAME

mktemp — make temporary file name (unique)

SYNOPSIS

mktemp

[ -d][ -q][ -t prefix][ -u] template ...

mktemp

[ -d][ -q][ -u] -t prefix

DESCRIPTION

The mktemp utility takes each of the given file name templates and overwrites a portion of it to create a file name. This file name is unique and suitable for use by the application. The template may be any file name with some number of ‘ Xs’ appended to it, for example /tmp/temp.XXXX. The trailing ‘ Xs’ are replaced with the current process number and/or a unique letter combination. The number of unique file names mktemp can return depends on the number of ‘ Xs’ provided; six ‘ Xs’ will result in mktemp selecting 1 of 56800235584 (62 ** 6) possible file names.

If mktemp can successfully generate a unique file name, the file is created with mode 0600 (unless the -u flag is given) and the filename is printed to standard output.

If the -t prefix option is given, mktemp will generate a template string based on the prefix and the TMPDIR environment variable if set. The default location if TMPDIR is not set is /tmp. Care should be taken to ensure that it is appropriate to use an environment variable potentially supplied by the user.

If no arguments are passed or if only the -d flag is passed mktemp behaves as if -t tmp was supplied.

Any number of temporary files may be created in a single invocation, including one based on the internal template resulting from the -t flag.

The mktemp utility is provided to allow shell scripts to safely use temporary files. Traditionally, many shell scripts take the name of the program with the pid as a suffix and use that as a temporary file name. This kind of naming scheme is predictable and the race condition it creates is easy for an attacker to win. A safer, though still inferior, approach is to make a temporary directory using the same naming scheme. While this does allow one to guarantee that a temporary file will not be subverted, it still allows a simple denial of service attack. For these reasons it is suggested that mktemp be used instead.

OPTIONS

The available options are as follows:

-d

Make a directory instead of a file.

-q

Fail silently if an error occurs. This is useful if a script does not want error output to go to standard error.

-t prefix

Generate a template (using the supplied prefix and TMPDIR if set) to create a filename template.

-u

Operate in “unsafe” mode. The temp file will be unlinked before mktemp exits. This is slightly better than mktemp(3) but still introduces a race condition. Use of this option is not encouraged.

EXIT STATUS

The mktemp utility exits 0 on success, and >0 if an error occurs.

EXAMPLES

The following sh(1) fragment illustrates a simple use of mktemp where the script should quit if it cannot get a safe temporary file.

tempfoo=`basename $0` 
TMPFILE=`mktemp /tmp/${tempfoo}.XXXXXX` || exit 1 
echo "program output" >> $TMPFILE

To allow the use of $TMPDIR:

tempfoo=`basename $0` 
TMPFILE=`mktemp -t ${tempfoo}` || exit 1 
echo "program output" >> $TMPFILE

In this case, we want the script to catch the error itself.

tempfoo=`basename $0` 
TMPFILE=`mktemp -q /tmp/${tempfoo}.XXXXXX` 
if [ $? -ne 0 ]; then 
 echo "$0: Can't create temp file, exiting..." 
 exit 1 
fi

SEE ALSO

mkdtemp(3), mkstemp(3), mktemp(3), environ(7)

HISTORY

A mktemp utility appeared in OpenBSD 2.1. This implementation was written independently based on the OpenBSD man page, and first appeared in FreeBSD 2.2.7. This man page is taken from OpenBSD.