mktemp(1) - OpenBSD manual pages

NAME

mktemp — make temporary filename (unique)

SYNOPSIS

mktemp [-dqtu] [-p directory] [template]

The mktemp utility takes the given filename template and overwrites a portion of it to create a unique filename. The template may be any filename with at least six ‘Xs’ appended to it, for example /tmp/tfile.XXXXXXXXXX. If no template is specified a default of tmp.XXXXXXXXXX is used and the -t flag is implied (see below).

The trailing ‘Xs’ are replaced with a unique digit and letter combination. The name chosen depends both on the number of ‘Xs’ in the template and the number of collisions with pre-existing files. The number of unique filenames mktemp can return depends on the number of ‘Xs’ provided; ten ‘Xs’ will result in mktemp testing roughly 26 ** 10 combinations.

If mktemp can successfully generate a unique filename, the file (or directory) is created with file permissions such that it is only readable and writable by its owner (unless the -u flag is given) and the filename is printed to standard output.

mktemp 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 filename. 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.

The options are as follows:

-d

Make a directory instead of a file.

-p directory

Use the specified directory as a prefix when generating the temporary filename. The directory will be overridden by the user's TMPDIR environment variable if it is set. This option implies the -t flag (see below).

-q

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

-t

Generate a path rooted in a temporary directory. This directory is chosen as follows:

If the user's TMPDIR environment variable is set, the directory contained therein is used.
Otherwise, if the -p flag was given the specified directory is used.
If none of the above apply, /tmp is used.

In this mode, the template (if specified) should be a directory component (as opposed to a full path) and thus should not contain any forward slashes.

-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.

The mktemp utility exits with a value of 0 on success or 1 on failure.

ENVIRONMENT

TMPDIR: directory in which to place the temporary file when in -t mode

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.

TMPFILE=`mktemp /tmp/example.XXXXXXXXXX` || exit 1
echo "program output" >> $TMPFILE

The same fragment with support for a user's TMPDIR environment variable can be written as follows.

TMPFILE=`mktemp -t example.XXXXXXXXXX` || exit 1
echo "program output" >> $TMPFILE

This can be further simplified if we don't care about the actual name of the temporary file. In this case the -t flag is implied.

TMPFILE=`mktemp` || exit 1
echo "program output" >> $TMPFILE

In some cases, it may be desirable to use a default temporary directory other than /tmp. In this example the temporary file will be created in /extra/tmp unless the user's TMPDIR environment variable specifies otherwise.

TMPFILE=`mktemp -p /extra/tmp example.XXXXXXXXXX` || exit 1
echo "program output" >> $TMPFILE

In other cases, we want the script to catch the error. For instance, if we attempt to create two temporary files and the second one fails we need to remove the first before exiting.

TMP1=`mktemp -t example.1.XXXXXXXXXX` || exit 1
TMP2=`mktemp -t example.2.XXXXXXXXXX`
if [ $? -ne 0 ]; then
	rm -f $TMP1
	exit 1
fi

Or perhaps you don't want to exit if mktemp is unable to create the file. In this case you can protect that part of the script thusly.

TMPFILE=`mktemp -q -t example.XXXXXXXXXX` && {
	# Safe to use $TMPFILE in this block
	echo data > $TMPFILE
	...
	rm -f $TMPFILE
}

DIAGNOSTICS

One of the following error messages may be displayed if mktemp does not succeed and the -q option was not specified:

insufficient number of Xs in template: The specified template contained fewer than six ‘Xs’ at the end.
template must not contain directory separators in -t mode: The template contained one or more directory components and the -t option was specified.
cannot make temp dir: mktemp was unable to create the temporary directory for any of the reasons specified by mkdtemp(3).
cannot make temp file: mktemp was unable to create the temporary file for any of the reasons specified by mkstemp(3).
cannot allocate memory: mktemp was unable to allocate memory for any of the reasons specified by malloc(3).

HISTORY

The mktemp utility first appeared in OpenBSD 2.1.

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