mktemp
SYNOPSIS
mktemp [-V] | [-dqtu] [-p directory] [template]
DESCRIPTION
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 some number of `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 combination of the
current process number and random letters. The name cho
sen 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 stan
dard 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 rea
sons it is suggested that mktemp be used instead.
The options are as follows:
-V Print the version and exit.
-d Make a directory instead of a file.
-p directory
Use the specified directory as a prefix when gener
ating 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
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.
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 envi
ronment 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 tempo
rary directory other than /tmp. In this example the tem
porary 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 some 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
TMPDIR directory in which to place the temporary file
when in -t mode
SEE ALSO
mkdtemp(3), mkstemp(3), mktemp(3)
HISTORY
The mktemp utility appeared in OpenBSD 2.1.
30 September 2001 MKTEMP(1)
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